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diff --git a/share/doc/gcc/C_002b_002b-Dialect-Options.html b/share/doc/gcc/C_002b_002b-Dialect-Options.html new file mode 100644 index 0000000..4944fbd --- /dev/null +++ b/share/doc/gcc/C_002b_002b-Dialect-Options.html @@ -0,0 +1,2069 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Using the GNU Compiler Collection (GCC): C++ Dialect Options</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): C++ Dialect Options"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): C++ Dialect Options"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Indices.html#Indices" rel="index" title="Indices"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Invoking-GCC.html#Invoking-GCC" rel="up" title="Invoking GCC"> +<link href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options" rel="next" title="Objective-C and Objective-C++ Dialect Options"> +<link href="C-Dialect-Options.html#C-Dialect-Options" rel="previous" title="C Dialect Options"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en_US" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="C_002b_002b-Dialect-Options"></a> +<div class="header"> +<p> +Next: <a href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options" accesskey="n" rel="next">Objective-C and Objective-C++ Dialect Options</a>, Previous: <a href="C-Dialect-Options.html#C-Dialect-Options" accesskey="p" rel="previous">C Dialect Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Options-Controlling-C_002b_002b-Dialect"></a> +<h3 class="section">3.5 Options Controlling C++ Dialect</h3> + +<a name="index-compiler-options_002c-C_002b_002b"></a> +<a name="index-C_002b_002b-options_002c-command_002dline"></a> +<a name="index-options_002c-C_002b_002b"></a> +<p>This section describes the command-line options that are only meaningful +for C++ programs. You can also use most of the GNU compiler options +regardless of what language your program is in. For example, you +might compile a file <samp>firstClass.C</samp> like this: +</p> +<div class="smallexample"> +<pre class="smallexample">g++ -g -fstrict-enums -O -c firstClass.C +</pre></div> + +<p>In this example, only <samp>-fstrict-enums</samp> is an option meant +only for C++ programs; you can use the other options with any +language supported by GCC. +</p> +<p>Some options for compiling C programs, such as <samp>-std</samp>, are also +relevant for C++ programs. +See <a href="C-Dialect-Options.html#C-Dialect-Options">Options Controlling C Dialect</a>. +</p> +<p>Here is a list of options that are <em>only</em> for compiling C++ programs: +</p> +<dl compact="compact"> +<dd> +<a name="index-fabi_002dversion"></a> +</dd> +<dt><code>-fabi-version=<var>n</var></code></dt> +<dd><p>Use version <var>n</var> of the C++ ABI. The default is version 0. +</p> +<p>Version 0 refers to the version conforming most closely to +the C++ ABI specification. Therefore, the ABI obtained using version 0 +will change in different versions of G++ as ABI bugs are fixed. +</p> +<p>Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. +</p> +<p>Version 2 is the version of the C++ ABI that first appeared in G++ +3.4, and was the default through G++ 4.9. +</p> +<p>Version 3 corrects an error in mangling a constant address as a +template argument. +</p> +<p>Version 4, which first appeared in G++ 4.5, implements a standard +mangling for vector types. +</p> +<p>Version 5, which first appeared in G++ 4.6, corrects the mangling of +attribute const/volatile on function pointer types, decltype of a +plain decl, and use of a function parameter in the declaration of +another parameter. +</p> +<p>Version 6, which first appeared in G++ 4.7, corrects the promotion +behavior of C++11 scoped enums and the mangling of template argument +packs, const/static_cast, prefix ++ and –, and a class scope function +used as a template argument. +</p> +<p>Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a +builtin type and corrects the mangling of lambdas in default argument +scope. +</p> +<p>Version 8, which first appeared in G++ 4.9, corrects the substitution +behavior of function types with function-cv-qualifiers. +</p> +<p>Version 9, which first appeared in G++ 5.2, corrects the alignment of +<code>nullptr_t</code>. +</p> +<p>Version 10, which first appeared in G++ 6.1, adds mangling of +attributes that affect type identity, such as ia32 calling convention +attributes (e.g. ‘<samp>stdcall</samp>’). +</p> +<p>Version 11, which first appeared in G++ 7, corrects the mangling of +sizeof... expressions and operator names. For multiple entities with +the same name within a function, that are declared in different scopes, +the mangling now changes starting with the twelfth occurrence. It also +implies <samp>-fnew-inheriting-ctors</samp>. +</p> +<p>Version 12, which first appeared in G++ 8, corrects the calling +conventions for empty classes on the x86_64 target and for classes +with only deleted copy/move constructors. It accidentally changes the +calling convention for classes with a deleted copy constructor and a +trivial move constructor. +</p> +<p>Version 13, which first appeared in G++ 8.2, fixes the accidental +change in version 12. +</p> +<p>Version 14, which first appeared in G++ 10, corrects the mangling of +the nullptr expression. +</p> +<p>Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI +tag regression. +</p> +<p>Version 16, which first appeared in G++ 11, changes the mangling of +<code>__alignof__</code> to be distinct from that of <code>alignof</code>, and +dependent operator names. +</p> +<p>Version 17, which first appeared in G++ 12, fixes layout of classes +that inherit from aggregate classes with default member initializers +in C++14 and up. +</p> +<p>Version 18, which first appeard in G++ 13, fixes manglings of lambdas +that have additional context. +</p> +<p>See also <samp>-Wabi</samp>. +</p> +<a name="index-fabi_002dcompat_002dversion"></a> +</dd> +<dt><code>-fabi-compat-version=<var>n</var></code></dt> +<dd><p>On targets that support strong aliases, G++ +works around mangling changes by creating an alias with the correct +mangled name when defining a symbol with an incorrect mangled name. +This switch specifies which ABI version to use for the alias. +</p> +<p>With <samp>-fabi-version=0</samp> (the default), this defaults to 13 (GCC 8.2 +compatibility). If another ABI version is explicitly selected, this +defaults to 0. For compatibility with GCC versions 3.2 through 4.9, +use <samp>-fabi-compat-version=2</samp>. +</p> +<p>If this option is not provided but <samp>-Wabi=<var>n</var></samp> is, that +version is used for compatibility aliases. If this option is provided +along with <samp>-Wabi</samp> (without the version), the version from this +option is used for the warning. +</p> +<a name="index-fno_002daccess_002dcontrol"></a> +<a name="index-faccess_002dcontrol"></a> +</dd> +<dt><code>-fno-access-control</code></dt> +<dd><p>Turn off all access checking. This switch is mainly useful for working +around bugs in the access control code. +</p> +<a name="index-faligned_002dnew"></a> +</dd> +<dt><code>-faligned-new</code></dt> +<dd><p>Enable support for C++17 <code>new</code> of types that require more +alignment than <code>void* ::operator new(std::size_t)</code> provides. A +numeric argument such as <code>-faligned-new=32</code> can be used to +specify how much alignment (in bytes) is provided by that function, +but few users will need to override the default of +<code>alignof(std::max_align_t)</code>. +</p> +<p>This flag is enabled by default for <samp>-std=c++17</samp>. +</p> +<a name="index-fchar8_005ft"></a> +<a name="index-fno_002dchar8_005ft"></a> +</dd> +<dt><code>-fchar8_t</code></dt> +<dt><code>-fno-char8_t</code></dt> +<dd><p>Enable support for <code>char8_t</code> as adopted for C++20. This includes +the addition of a new <code>char8_t</code> fundamental type, changes to the +types of UTF-8 string and character literals, new signatures for +user-defined literals, associated standard library updates, and new +<code>__cpp_char8_t</code> and <code>__cpp_lib_char8_t</code> feature test macros. +</p> +<p>This option enables functions to be overloaded for ordinary and UTF-8 +strings: +</p> +<div class="smallexample"> +<pre class="smallexample">int f(const char *); // #1 +int f(const char8_t *); // #2 +int v1 = f("text"); // Calls #1 +int v2 = f(u8"text"); // Calls #2 +</pre></div> + +<p>and introduces new signatures for user-defined literals: +</p> +<div class="smallexample"> +<pre class="smallexample">int operator""_udl1(char8_t); +int v3 = u8'x'_udl1; +int operator""_udl2(const char8_t*, std::size_t); +int v4 = u8"text"_udl2; +template<typename T, T...> int operator""_udl3(); +int v5 = u8"text"_udl3; +</pre></div> + +<p>The change to the types of UTF-8 string and character literals introduces +incompatibilities with ISO C++11 and later standards. For example, the +following code is well-formed under ISO C++11, but is ill-formed when +<samp>-fchar8_t</samp> is specified. +</p> +<div class="smallexample"> +<pre class="smallexample">const char *cp = u8"xx";// error: invalid conversion from + // `const char8_t*' to `const char*' +int f(const char*); +auto v = f(u8"xx"); // error: invalid conversion from + // `const char8_t*' to `const char*' +std::string s{u8"xx"}; // error: no matching function for call to + // `std::basic_string<char>::basic_string()' +using namespace std::literals; +s = u8"xx"s; // error: conversion from + // `basic_string<char8_t>' to non-scalar + // type `basic_string<char>' requested +</pre></div> + +<a name="index-fcheck_002dnew"></a> +</dd> +<dt><code>-fcheck-new</code></dt> +<dd><p>Check that the pointer returned by <code>operator new</code> is non-null +before attempting to modify the storage allocated. This check is +normally unnecessary because the C++ standard specifies that +<code>operator new</code> only returns <code>0</code> if it is declared +<code>throw()</code>, in which case the compiler always checks the +return value even without this option. In all other cases, when +<code>operator new</code> has a non-empty exception specification, memory +exhaustion is signalled by throwing <code>std::bad_alloc</code>. See also +‘<samp>new (nothrow)</samp>’. +</p> +<a name="index-fconcepts"></a> +<a name="index-fconcepts_002dts"></a> +</dd> +<dt><code>-fconcepts</code></dt> +<dt><code>-fconcepts-ts</code></dt> +<dd><p>Enable support for the C++ Concepts feature for constraining template +arguments. With <samp>-std=c++20</samp> and above, Concepts are part of +the language standard, so <samp>-fconcepts</samp> defaults to on. +</p> +<p>Some constructs that were allowed by the earlier C++ Extensions for +Concepts Technical Specification, ISO 19217 (2015), but didn’t make it +into the standard, can additionally be enabled by +<samp>-fconcepts-ts</samp>. +</p> +<a name="index-fconstexpr_002ddepth"></a> +</dd> +<dt><code>-fconstexpr-depth=<var>n</var></code></dt> +<dd><p>Set the maximum nested evaluation depth for C++11 constexpr functions +to <var>n</var>. A limit is needed to detect endless recursion during +constant expression evaluation. The minimum specified by the standard +is 512. +</p> +<a name="index-fconstexpr_002dcache_002ddepth"></a> +</dd> +<dt><code>-fconstexpr-cache-depth=<var>n</var></code></dt> +<dd><p>Set the maximum level of nested evaluation depth for C++11 constexpr +functions that will be cached to <var>n</var>. This is a heuristic that +trades off compilation speed (when the cache avoids repeated +calculations) against memory consumption (when the cache grows very +large from highly recursive evaluations). The default is 8. Very few +users are likely to want to adjust it, but if your code does heavy +constexpr calculations you might want to experiment to find which +value works best for you. +</p> +<a name="index-fconstexpr_002dfp_002dexcept"></a> +</dd> +<dt><code>-fconstexpr-fp-except</code></dt> +<dd><p>Annex F of the C standard specifies that IEC559 floating point +exceptions encountered at compile time should not stop compilation. +C++ compilers have historically not followed this guidance, instead +treating floating point division by zero as non-constant even though +it has a well defined value. This flag tells the compiler to give +Annex F priority over other rules saying that a particular operation +is undefined. +</p> +<div class="smallexample"> +<pre class="smallexample">constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except +</pre></div> + +<a name="index-fconstexpr_002dloop_002dlimit"></a> +</dd> +<dt><code>-fconstexpr-loop-limit=<var>n</var></code></dt> +<dd><p>Set the maximum number of iterations for a loop in C++14 constexpr functions +to <var>n</var>. A limit is needed to detect infinite loops during +constant expression evaluation. The default is 262144 (1<<18). +</p> +<a name="index-fconstexpr_002dops_002dlimit"></a> +</dd> +<dt><code>-fconstexpr-ops-limit=<var>n</var></code></dt> +<dd><p>Set the maximum number of operations during a single constexpr evaluation. +Even when number of iterations of a single loop is limited with the above limit, +if there are several nested loops and each of them has many iterations but still +smaller than the above limit, or if in a body of some loop or even outside +of a loop too many expressions need to be evaluated, the resulting constexpr +evaluation might take too long. +The default is 33554432 (1<<25). +</p> +<a name="index-fcontracts"></a> +</dd> +<dt><code>-fcontracts</code></dt> +<dd><p>Enable experimental support for the C++ Contracts feature, as briefly +added to and then removed from the C++20 working paper (N4820). The +implementation also includes proposed enhancements from papers P1290, +P1332, and P1429. This functionality is intended mostly for those +interested in experimentation towards refining the feature to get it +into shape for a future C++ standard. +</p> +<p>On violation of a checked contract, the violation handler is called. +Users can replace the violation handler by defining +</p><div class="smallexample"> +<pre class="smallexample">void +handle_contract_violation (const std::experimental::contract_violation&); +</pre></div> + +<p>There are different sets of additional flags that can be used together +to specify which contracts will be checked and how, for N4820 +contracts, P1332 contracts, or P1429 contracts; these sets cannot be +used together. +</p> +<dl compact="compact"> +<dd><a name="index-fcontract_002dmode"></a> +</dd> +<dt><code>-fcontract-mode=[on|off]</code></dt> +<dd><p>Control whether any contracts have any semantics at all. Defaults to on. +</p> +<a name="index-fcontract_002dassumption_002dmode"></a> +</dd> +<dt><code>-fcontract-assumption-mode=[on|off]</code></dt> +<dd><p>[N4820] Control whether contracts with level ‘<samp>axiom</samp>’ +should have the assume semantic. Defaults to on. +</p> +<a name="index-fcontract_002dbuild_002dlevel"></a> +</dd> +<dt><code>-fcontract-build-level=[off|default|audit]</code></dt> +<dd><p>[N4820] Specify which level of contracts to generate checks +for. Defaults to ‘<samp>default</samp>’. +</p> +<a name="index-fcontract_002dcontinuation_002dmode"></a> +</dd> +<dt><code>-fcontract-continuation-mode=[on|off]</code></dt> +<dd><p>[N4820] Control whether to allow the program to continue executing +after a contract violation. That is, do checked contracts have the +‘<samp>maybe</samp>’ semantic described below rather than the ‘<samp>never</samp>’ +semantic. Defaults to off. +</p> +<a name="index-fcontract_002drole"></a> +</dd> +<dt><code>-fcontract-role=<name>:<default>,<audit>,<axiom></code></dt> +<dd><p>[P1332] Specify the concrete semantics for each contract level +of a particular contract role. +</p> +</dd> +<dt><code>-fcontract-semantic=[default|audit|axiom]:<semantic></code></dt> +<dd><p>[P1429] Specify the concrete semantic for a particular +contract level. +</p> +<a name="index-fcontract_002dstrict_002ddeclarations"></a> +</dd> +<dt><code>-fcontract-strict-declarations=[on|off]</code></dt> +<dd><p>Control whether to reject adding contracts to a function after its +first declaration. Defaults to off. +</p></dd> +</dl> + +<p>The possible concrete semantics for that can be specified with +‘<samp>-fcontract-role</samp>’ or ‘<samp>-fcontract-semantic</samp>’ are: +</p> +<dl compact="compact"> +<dt><code>ignore</code></dt> +<dd><p>This contract has no effect. +</p> +</dd> +<dt><code>assume</code></dt> +<dd><p>This contract is treated like C++23 <code>[[assume]]</code>. +</p> +</dd> +<dt><code>check_never_continue</code></dt> +<dt><code>never</code></dt> +<dt><code>abort</code></dt> +<dd><p>This contract is checked. If it fails, the violation handler is +called. If the handler returns, <code>std::terminate</code> is called. +</p> +</dd> +<dt><code>check_maybe_continue</code></dt> +<dt><code>maybe</code></dt> +<dd><p>This contract is checked. If it fails, the violation handler is +called. If the handler returns, execution continues normally. +</p></dd> +</dl> + +<a name="index-fcoroutines"></a> +</dd> +<dt><code>-fcoroutines</code></dt> +<dd><p>Enable support for the C++ coroutines extension (experimental). +</p> +<a name="index-fno_002delide_002dconstructors"></a> +<a name="index-felide_002dconstructors"></a> +</dd> +<dt><code>-fno-elide-constructors</code></dt> +<dd><p>The C++ standard allows an implementation to omit creating a temporary +that is only used to initialize another object of the same type. +Specifying this option disables that optimization, and forces G++ to +call the copy constructor in all cases. This option also causes G++ +to call trivial member functions which otherwise would be expanded inline. +</p> +<p>In C++17, the compiler is required to omit these temporaries, but this +option still affects trivial member functions. +</p> +<a name="index-fno_002denforce_002deh_002dspecs"></a> +<a name="index-fenforce_002deh_002dspecs"></a> +</dd> +<dt><code>-fno-enforce-eh-specs</code></dt> +<dd><p>Don’t generate code to check for violation of exception specifications +at run time. This option violates the C++ standard, but may be useful +for reducing code size in production builds, much like defining +<code>NDEBUG</code>. This does not give user code permission to throw +exceptions in violation of the exception specifications; the compiler +still optimizes based on the specifications, so throwing an +unexpected exception results in undefined behavior at run time. +</p> +<a name="index-fextern_002dtls_002dinit"></a> +<a name="index-fno_002dextern_002dtls_002dinit"></a> +</dd> +<dt><code>-fextern-tls-init</code></dt> +<dt><code>-fno-extern-tls-init</code></dt> +<dd><p>The C++11 and OpenMP standards allow <code>thread_local</code> and +<code>threadprivate</code> variables to have dynamic (runtime) +initialization. To support this, any use of such a variable goes +through a wrapper function that performs any necessary initialization. +When the use and definition of the variable are in the same +translation unit, this overhead can be optimized away, but when the +use is in a different translation unit there is significant overhead +even if the variable doesn’t actually need dynamic initialization. If +the programmer can be sure that no use of the variable in a +non-defining TU needs to trigger dynamic initialization (either +because the variable is statically initialized, or a use of the +variable in the defining TU will be executed before any uses in +another TU), they can avoid this overhead with the +<samp>-fno-extern-tls-init</samp> option. +</p> +<p>On targets that support symbol aliases, the default is +<samp>-fextern-tls-init</samp>. On targets that do not support symbol +aliases, the default is <samp>-fno-extern-tls-init</samp>. +</p> +<a name="index-ffold_002dsimple_002dinlines"></a> +<a name="index-fno_002dfold_002dsimple_002dinlines"></a> +</dd> +<dt><code>-ffold-simple-inlines</code></dt> +<dt><code>-fno-fold-simple-inlines</code></dt> +<dd><p>Permit the C++ frontend to fold calls to <code>std::move</code>, <code>std::forward</code>, +<code>std::addressof</code> and <code>std::as_const</code>. In contrast to inlining, this +means no debug information will be generated for such calls. Since these +functions are rarely interesting to debug, this flag is enabled by default +unless <samp>-fno-inline</samp> is active. +</p> +<a name="index-fno_002dgnu_002dkeywords"></a> +<a name="index-fgnu_002dkeywords"></a> +</dd> +<dt><code>-fno-gnu-keywords</code></dt> +<dd><p>Do not recognize <code>typeof</code> as a keyword, so that code can use this +word as an identifier. You can use the keyword <code>__typeof__</code> instead. +This option is implied by the strict ISO C++ dialects: <samp>-ansi</samp>, +<samp>-std=c++98</samp>, <samp>-std=c++11</samp>, etc. +</p> +<a name="index-fimplicit_002dconstexpr"></a> +</dd> +<dt><code>-fimplicit-constexpr</code></dt> +<dd><p>Make inline functions implicitly constexpr, if they satisfy the +requirements for a constexpr function. This option can be used in +C++14 mode or later. This can result in initialization changing from +dynamic to static and other optimizations. +</p> +<a name="index-fno_002dimplicit_002dtemplates"></a> +<a name="index-fimplicit_002dtemplates"></a> +</dd> +<dt><code>-fno-implicit-templates</code></dt> +<dd><p>Never emit code for non-inline templates that are instantiated +implicitly (i.e. by use); only emit code for explicit instantiations. +If you use this option, you must take care to structure your code to +include all the necessary explicit instantiations to avoid getting +undefined symbols at link time. +See <a href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</a>, for more information. +</p> +<a name="index-fno_002dimplicit_002dinline_002dtemplates"></a> +<a name="index-fimplicit_002dinline_002dtemplates"></a> +</dd> +<dt><code>-fno-implicit-inline-templates</code></dt> +<dd><p>Don’t emit code for implicit instantiations of inline templates, either. +The default is to handle inlines differently so that compiles with and +without optimization need the same set of explicit instantiations. +</p> +<a name="index-fno_002dimplement_002dinlines"></a> +<a name="index-fimplement_002dinlines"></a> +</dd> +<dt><code>-fno-implement-inlines</code></dt> +<dd><p>To save space, do not emit out-of-line copies of inline functions +controlled by <code>#pragma implementation</code>. This causes linker +errors if these functions are not inlined everywhere they are called. +</p> +<a name="index-fmodules_002dts"></a> +<a name="index-fno_002dmodules_002dts"></a> +</dd> +<dt><code>-fmodules-ts</code></dt> +<dt><code>-fno-modules-ts</code></dt> +<dd><p>Enable support for C++20 modules (see <a href="C_002b_002b-Modules.html#C_002b_002b-Modules">C++ Modules</a>). The +<samp>-fno-modules-ts</samp> is usually not needed, as that is the +default. Even though this is a C++20 feature, it is not currently +implicitly enabled by selecting that standard version. +</p> +<a name="index-fmodule_002dheader"></a> +</dd> +<dt><code>-fmodule-header</code></dt> +<dt><code>-fmodule-header=user</code></dt> +<dt><code>-fmodule-header=system</code></dt> +<dd><p>Compile a header file to create an importable header unit. +</p> +<a name="index-fmodule_002dimplicit_002dinline"></a> +</dd> +<dt><code>-fmodule-implicit-inline</code></dt> +<dd><p>Member functions defined in their class definitions are not implicitly +inline for modular code. This is different to traditional C++ +behavior, for good reasons. However, it may result in a difficulty +during code porting. This option makes such function definitions +implicitly inline. It does however generate an ABI incompatibility, +so you must use it everywhere or nowhere. (Such definitions outside +of a named module remain implicitly inline, regardless.) +</p> +<a name="index-fno_002dmodule_002dlazy"></a> +<a name="index-fmodule_002dlazy"></a> +</dd> +<dt><code>-fno-module-lazy</code></dt> +<dd><p>Disable lazy module importing and module mapper creation. +</p> +<a name="index-CXX_005fMODULE_005fMAPPER-environment-variable"></a> +<a name="index-fmodule_002dmapper"></a> +</dd> +<dt><code>-fmodule-mapper=<span class="roman">[</span><var>hostname</var><span class="roman">]</span>:<var>port</var><span class="roman">[</span>?<var>ident</var><span class="roman">]</span></code></dt> +<dt><code>-fmodule-mapper=|<var>program</var><span class="roman">[</span>?<var>ident</var><span class="roman">]</span> <var>args...</var></code></dt> +<dt><code>-fmodule-mapper==<var>socket</var><span class="roman">[</span>?<var>ident</var><span class="roman">]</span></code></dt> +<dt><code>-fmodule-mapper=<><span class="roman">[</span><var>inout</var><span class="roman">]</span><span class="roman">[</span>?<var>ident</var><span class="roman">]</span></code></dt> +<dt><code>-fmodule-mapper=<<var>in</var>><var>out</var><span class="roman">[</span>?<var>ident</var><span class="roman">]</span></code></dt> +<dt><code>-fmodule-mapper=<var>file</var><span class="roman">[</span>?<var>ident</var><span class="roman">]</span></code></dt> +<dd><p>An oracle to query for module name to filename mappings. If +unspecified the <code>CXX_MODULE_MAPPER</code> environment variable is used, +and if that is unset, an in-process default is provided. +</p> +<a name="index-fmodule_002donly"></a> +</dd> +<dt><code>-fmodule-only</code></dt> +<dd><p>Only emit the Compiled Module Interface, inhibiting any object file. +</p> +<a name="index-fms_002dextensions-1"></a> +</dd> +<dt><code>-fms-extensions</code></dt> +<dd><p>Disable Wpedantic warnings about constructs used in MFC, such as implicit +int and getting a pointer to member function via non-standard syntax. +</p> +<a name="index-fnew_002dinheriting_002dctors"></a> +</dd> +<dt><code>-fnew-inheriting-ctors</code></dt> +<dd><p>Enable the P0136 adjustment to the semantics of C++11 constructor +inheritance. This is part of C++17 but also considered to be a Defect +Report against C++11 and C++14. This flag is enabled by default +unless <samp>-fabi-version=10</samp> or lower is specified. +</p> +<a name="index-fnew_002dttp_002dmatching"></a> +</dd> +<dt><code>-fnew-ttp-matching</code></dt> +<dd><p>Enable the P0522 resolution to Core issue 150, template template +parameters and default arguments: this allows a template with default +template arguments as an argument for a template template parameter +with fewer template parameters. This flag is enabled by default for +<samp>-std=c++17</samp>. +</p> +<a name="index-fno_002dnonansi_002dbuiltins"></a> +<a name="index-fnonansi_002dbuiltins"></a> +</dd> +<dt><code>-fno-nonansi-builtins</code></dt> +<dd><p>Disable built-in declarations of functions that are not mandated by +ANSI/ISO C. These include <code>ffs</code>, <code>alloca</code>, <code>_exit</code>, +<code>index</code>, <code>bzero</code>, <code>conjf</code>, and other related functions. +</p> +<a name="index-fnothrow_002dopt"></a> +</dd> +<dt><code>-fnothrow-opt</code></dt> +<dd><p>Treat a <code>throw()</code> exception specification as if it were a +<code>noexcept</code> specification to reduce or eliminate the text size +overhead relative to a function with no exception specification. If +the function has local variables of types with non-trivial +destructors, the exception specification actually makes the +function smaller because the EH cleanups for those variables can be +optimized away. The semantic effect is that an exception thrown out of +a function with such an exception specification results in a call +to <code>terminate</code> rather than <code>unexpected</code>. +</p> +<a name="index-fno_002doperator_002dnames"></a> +<a name="index-foperator_002dnames"></a> +</dd> +<dt><code>-fno-operator-names</code></dt> +<dd><p>Do not treat the operator name keywords <code>and</code>, <code>bitand</code>, +<code>bitor</code>, <code>compl</code>, <code>not</code>, <code>or</code> and <code>xor</code> as +synonyms as keywords. +</p> +<a name="index-fno_002doptional_002ddiags"></a> +<a name="index-foptional_002ddiags"></a> +</dd> +<dt><code>-fno-optional-diags</code></dt> +<dd><p>Disable diagnostics that the standard says a compiler does not need to +issue. Currently, the only such diagnostic issued by G++ is the one for +a name having multiple meanings within a class. +</p> +<a name="index-fpermissive"></a> +</dd> +<dt><code>-fpermissive</code></dt> +<dd><p>Downgrade some diagnostics about nonconformant code from errors to +warnings. Thus, using <samp>-fpermissive</samp> allows some +nonconforming code to compile. +</p> +<a name="index-fno_002dpretty_002dtemplates"></a> +<a name="index-fpretty_002dtemplates"></a> +</dd> +<dt><code>-fno-pretty-templates</code></dt> +<dd><p>When an error message refers to a specialization of a function +template, the compiler normally prints the signature of the +template followed by the template arguments and any typedefs or +typenames in the signature (e.g. <code>void f(T) [with T = int]</code> +rather than <code>void f(int)</code>) so that it’s clear which template is +involved. When an error message refers to a specialization of a class +template, the compiler omits any template arguments that match +the default template arguments for that template. If either of these +behaviors make it harder to understand the error message rather than +easier, you can use <samp>-fno-pretty-templates</samp> to disable them. +</p> +<a name="index-fno_002drtti"></a> +<a name="index-frtti"></a> +</dd> +<dt><code>-fno-rtti</code></dt> +<dd><p>Disable generation of information about every class with virtual +functions for use by the C++ run-time type identification features +(<code>dynamic_cast</code> and <code>typeid</code>). If you don’t use those parts +of the language, you can save some space by using this flag. Note that +exception handling uses the same information, but G++ generates it as +needed. The <code>dynamic_cast</code> operator can still be used for casts that +do not require run-time type information, i.e. casts to <code>void *</code> or to +unambiguous base classes. +</p> +<p>Mixing code compiled with <samp>-frtti</samp> with that compiled with +<samp>-fno-rtti</samp> may not work. For example, programs may +fail to link if a class compiled with <samp>-fno-rtti</samp> is used as a base +for a class compiled with <samp>-frtti</samp>. +</p> +<a name="index-fsized_002ddeallocation"></a> +</dd> +<dt><code>-fsized-deallocation</code></dt> +<dd><p>Enable the built-in global declarations +</p><div class="smallexample"> +<pre class="smallexample">void operator delete (void *, std::size_t) noexcept; +void operator delete[] (void *, std::size_t) noexcept; +</pre></div> +<p>as introduced in C++14. This is useful for user-defined replacement +deallocation functions that, for example, use the size of the object +to make deallocation faster. Enabled by default under +<samp>-std=c++14</samp> and above. The flag <samp>-Wsized-deallocation</samp> +warns about places that might want to add a definition. +</p> +<a name="index-fstrict_002denums"></a> +</dd> +<dt><code>-fstrict-enums</code></dt> +<dd><p>Allow the compiler to optimize using the assumption that a value of +enumerated type can only be one of the values of the enumeration (as +defined in the C++ standard; basically, a value that can be +represented in the minimum number of bits needed to represent all the +enumerators). This assumption may not be valid if the program uses a +cast to convert an arbitrary integer value to the enumerated type. +</p> +<a name="index-fstrong_002deval_002dorder"></a> +</dd> +<dt><code>-fstrong-eval-order</code></dt> +<dd><p>Evaluate member access, array subscripting, and shift expressions in +left-to-right order, and evaluate assignment in right-to-left order, +as adopted for C++17. Enabled by default with <samp>-std=c++17</samp>. +<samp>-fstrong-eval-order=some</samp> enables just the ordering of member +access and shift expressions, and is the default without +<samp>-std=c++17</samp>. +</p> +<a name="index-ftemplate_002dbacktrace_002dlimit"></a> +</dd> +<dt><code>-ftemplate-backtrace-limit=<var>n</var></code></dt> +<dd><p>Set the maximum number of template instantiation notes for a single +warning or error to <var>n</var>. The default value is 10. +</p> +<a name="index-ftemplate_002ddepth"></a> +</dd> +<dt><code>-ftemplate-depth=<var>n</var></code></dt> +<dd><p>Set the maximum instantiation depth for template classes to <var>n</var>. +A limit on the template instantiation depth is needed to detect +endless recursions during template class instantiation. ANSI/ISO C++ +conforming programs must not rely on a maximum depth greater than 17 +(changed to 1024 in C++11). The default value is 900, as the compiler +can run out of stack space before hitting 1024 in some situations. +</p> +<a name="index-fno_002dthreadsafe_002dstatics"></a> +<a name="index-fthreadsafe_002dstatics"></a> +</dd> +<dt><code>-fno-threadsafe-statics</code></dt> +<dd><p>Do not emit the extra code to use the routines specified in the C++ +ABI for thread-safe initialization of local statics. You can use this +option to reduce code size slightly in code that doesn’t need to be +thread-safe. +</p> +<a name="index-fuse_002dcxa_002datexit"></a> +</dd> +<dt><code>-fuse-cxa-atexit</code></dt> +<dd><p>Register destructors for objects with static storage duration with the +<code>__cxa_atexit</code> function rather than the <code>atexit</code> function. +This option is required for fully standards-compliant handling of static +destructors, but only works if your C library supports +<code>__cxa_atexit</code>. +</p> +<a name="index-fno_002duse_002dcxa_002dget_002dexception_002dptr"></a> +<a name="index-fuse_002dcxa_002dget_002dexception_002dptr"></a> +</dd> +<dt><code>-fno-use-cxa-get-exception-ptr</code></dt> +<dd><p>Don’t use the <code>__cxa_get_exception_ptr</code> runtime routine. This +causes <code>std::uncaught_exception</code> to be incorrect, but is necessary +if the runtime routine is not available. +</p> +<a name="index-fvisibility_002dinlines_002dhidden"></a> +</dd> +<dt><code>-fvisibility-inlines-hidden</code></dt> +<dd><p>This switch declares that the user does not attempt to compare +pointers to inline functions or methods where the addresses of the two functions +are taken in different shared objects. +</p> +<p>The effect of this is that GCC may, effectively, mark inline methods with +<code>__attribute__ ((visibility ("hidden")))</code> so that they do not +appear in the export table of a DSO and do not require a PLT indirection +when used within the DSO. Enabling this option can have a dramatic effect +on load and link times of a DSO as it massively reduces the size of the +dynamic export table when the library makes heavy use of templates. +</p> +<p>The behavior of this switch is not quite the same as marking the +methods as hidden directly, because it does not affect static variables +local to the function or cause the compiler to deduce that +the function is defined in only one shared object. +</p> +<p>You may mark a method as having a visibility explicitly to negate the +effect of the switch for that method. For example, if you do want to +compare pointers to a particular inline method, you might mark it as +having default visibility. Marking the enclosing class with explicit +visibility has no effect. +</p> +<p>Explicitly instantiated inline methods are unaffected by this option +as their linkage might otherwise cross a shared library boundary. +See <a href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</a>. +</p> +<a name="index-fvisibility_002dms_002dcompat"></a> +</dd> +<dt><code>-fvisibility-ms-compat</code></dt> +<dd><p>This flag attempts to use visibility settings to make GCC’s C++ +linkage model compatible with that of Microsoft Visual Studio. +</p> +<p>The flag makes these changes to GCC’s linkage model: +</p> +<ol> +<li> It sets the default visibility to <code>hidden</code>, like +<samp>-fvisibility=hidden</samp>. + +</li><li> Types, but not their members, are not hidden by default. + +</li><li> The One Definition Rule is relaxed for types without explicit +visibility specifications that are defined in more than one +shared object: those declarations are permitted if they are +permitted when this option is not used. +</li></ol> + +<p>In new code it is better to use <samp>-fvisibility=hidden</samp> and +export those classes that are intended to be externally visible. +Unfortunately it is possible for code to rely, perhaps accidentally, +on the Visual Studio behavior. +</p> +<p>Among the consequences of these changes are that static data members +of the same type with the same name but defined in different shared +objects are different, so changing one does not change the other; +and that pointers to function members defined in different shared +objects may not compare equal. When this flag is given, it is a +violation of the ODR to define types with the same name differently. +</p> +<a name="index-fno_002dweak"></a> +<a name="index-fweak"></a> +</dd> +<dt><code>-fno-weak</code></dt> +<dd><p>Do not use weak symbol support, even if it is provided by the linker. +By default, G++ uses weak symbols if they are available. This +option exists only for testing, and should not be used by end-users; +it results in inferior code and has no benefits. This option may +be removed in a future release of G++. +</p> +<a name="index-fext_002dnumeric_002dliterals"></a> +<a name="index-fno_002dext_002dnumeric_002dliterals"></a> +</dd> +<dt><code>-fext-numeric-literals <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Accept imaginary, fixed-point, or machine-defined +literal number suffixes as GNU extensions. +When this option is turned off these suffixes are treated +as C++11 user-defined literal numeric suffixes. +This is on by default for all pre-C++11 dialects and all GNU dialects: +<samp>-std=c++98</samp>, <samp>-std=gnu++98</samp>, <samp>-std=gnu++11</samp>, +<samp>-std=gnu++14</samp>. +This option is off by default +for ISO C++11 onwards (<samp>-std=c++11</samp>, ...). +</p> +<a name="index-nostdinc_002b_002b"></a> +</dd> +<dt><code>-nostdinc++</code></dt> +<dd><p>Do not search for header files in the standard directories specific to +C++, but do still search the other standard directories. (This option +is used when building the C++ library.) +</p> +<a name="index-flang_002dinfo_002dinclude_002dtranslate"></a> +<a name="index-flang_002dinfo_002dinclude_002dtranslate_002dnot"></a> +</dd> +<dt><code>-flang-info-include-translate</code></dt> +<dt><code>-flang-info-include-translate-not</code></dt> +<dt><code>-flang-info-include-translate=<var>header</var></code></dt> +<dd><p>Inform of include translation events. The first will note accepted +include translations, the second will note declined include +translations. The <var>header</var> form will inform of include +translations relating to that specific header. If <var>header</var> is of +the form <code>"user"</code> or <code><system></code> it will be resolved to a +specific user or system header using the include path. +</p> +<a name="index-flang_002dinfo_002dmodule_002dcmi"></a> +</dd> +<dt><code>-flang-info-module-cmi</code></dt> +<dt><code>-flang-info-module-cmi=<var>module</var></code></dt> +<dd><p>Inform of Compiled Module Interface pathnames. The first will note +all read CMI pathnames. The <var>module</var> form will not reading a +specific module’s CMI. <var>module</var> may be a named module or a +header-unit (the latter indicated by either being a pathname containing +directory separators or enclosed in <code><></code> or <code>""</code>). +</p> +<a name="index-stdlib"></a> +</dd> +<dt><code>-stdlib=<var>libstdc++,libc++</var></code></dt> +<dd><p>When G++ is configured to support this option, it allows specification of +alternate C++ runtime libraries. Two options are available: <var>libstdc++</var> +(the default, native C++ runtime for G++) and <var>libc++</var> which is the +C++ runtime installed on some operating systems (e.g. Darwin versions from +Darwin11 onwards). The option switches G++ to use the headers from the +specified library and to emit <code>-lstdc++</code> or <code>-lc++</code> respectively, +when a C++ runtime is required for linking. +</p></dd> +</dl> + +<p>In addition, these warning options have meanings only for C++ programs: +</p> +<dl compact="compact"> +<dd><a name="index-Wabi_002dtag"></a> +</dd> +<dt><code>-Wabi-tag <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when a type with an ABI tag is used in a context that does not +have that ABI tag. See <a href="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</a> for more information +about ABI tags. +</p> +<a name="index-Wcomma_002dsubscript"></a> +<a name="index-Wno_002dcomma_002dsubscript"></a> +</dd> +<dt><code>-Wcomma-subscript <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn about uses of a comma expression within a subscripting expression. +This usage was deprecated in C++20 and is going to be removed in C++23. +However, a comma expression wrapped in <code>( )</code> is not deprecated. Example: +</p> +<div class="smallexample"> +<pre class="smallexample">void f(int *a, int b, int c) { + a[b,c]; // deprecated in C++20, invalid in C++23 + a[(b,c)]; // OK +} +</pre></div> + +<p>In C++23 it is valid to have comma separated expressions in a subscript +when an overloaded subscript operator is found and supports the right +number and types of arguments. G++ will accept the formerly valid syntax +for code that is not valid in C++23 but used to be valid but deprecated +in C++20 with a pedantic warning that can be disabled with +<samp>-Wno-comma-subscript</samp>. +</p> +<p>Enabled by default with <samp>-std=c++20</samp> unless <samp>-Wno-deprecated</samp>, +and with <samp>-std=c++23</samp> regardless of <samp>-Wno-deprecated</samp>. +</p> +<a name="index-Wctad_002dmaybe_002dunsupported"></a> +<a name="index-Wno_002dctad_002dmaybe_002dunsupported"></a> +</dd> +<dt><code>-Wctad-maybe-unsupported <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when performing class template argument deduction (CTAD) on a type with +no explicitly written deduction guides. This warning will point out cases +where CTAD succeeded only because the compiler synthesized the implicit +deduction guides, which might not be what the programmer intended. Certain +style guides allow CTAD only on types that specifically "opt-in"; i.e., on +types that are designed to support CTAD. This warning can be suppressed with +the following pattern: +</p> +<div class="smallexample"> +<pre class="smallexample">struct allow_ctad_t; // any name works +template <typename T> struct S { + S(T) { } +}; +// Guide with incomplete parameter type will never be considered. +S(allow_ctad_t) -> S<void>; +</pre></div> + +<a name="index-Wctor_002ddtor_002dprivacy"></a> +<a name="index-Wno_002dctor_002ddtor_002dprivacy"></a> +</dd> +<dt><code>-Wctor-dtor-privacy <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when a class seems unusable because all the constructors or +destructors in that class are private, and it has neither friends nor +public static member functions. Also warn if there are no non-private +methods, and there’s at least one private member function that isn’t +a constructor or destructor. +</p> +<a name="index-Wdangling_002dreference"></a> +<a name="index-Wno_002ddangling_002dreference"></a> +</dd> +<dt><code>-Wdangling-reference <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when a reference is bound to a temporary whose lifetime has ended. +For example: +</p> +<div class="smallexample"> +<pre class="smallexample">int n = 1; +const int& r = std::max(n - 1, n + 1); // r is dangling +</pre></div> + +<p>In the example above, two temporaries are created, one for each +argument, and a reference to one of the temporaries is returned. +However, both temporaries are destroyed at the end of the full +expression, so the reference <code>r</code> is dangling. This warning +also detects dangling references in member initializer lists: +</p> +<div class="smallexample"> +<pre class="smallexample">const int& f(const int& i) { return i; } +struct S { + const int &r; // r is dangling + S() : r(f(10)) { } +}; +</pre></div> + +<p>Member functions are checked as well, but only their object argument: +</p> +<div class="smallexample"> +<pre class="smallexample">struct S { + const S& self () { return *this; } +}; +const S& s = S().self(); // s is dangling +</pre></div> + +<p>Certain functions are safe in this respect, for example <code>std::use_facet</code>: +they take and return a reference, but they don’t return one of its arguments, +which can fool the warning. Such functions can be excluded from the warning +by wrapping them in a <code>#pragma</code>: +</p> +<div class="smallexample"> +<pre class="smallexample">#pragma GCC diagnostic push +#pragma GCC diagnostic ignored "-Wdangling-reference" +const T& foo (const T&) { … } +#pragma GCC diagnostic pop +</pre></div> + +<p><samp>-Wdangling-reference</samp> also warns about code like +</p> +<div class="smallexample"> +<pre class="smallexample">auto p = std::minmax(1, 2); +</pre></div> + +<p>where <code>std::minmax</code> returns <code>std::pair<const int&, const int&></code>, and +both references dangle after the end of the full expression that contains +the call to <code>std::minmax</code>. +</p> +<p>This warning is enabled by <samp>-Wextra</samp>. +</p> +<a name="index-Wdelete_002dnon_002dvirtual_002ddtor"></a> +<a name="index-Wno_002ddelete_002dnon_002dvirtual_002ddtor"></a> +</dd> +<dt><code>-Wdelete-non-virtual-dtor <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when <code>delete</code> is used to destroy an instance of a class that +has virtual functions and non-virtual destructor. It is unsafe to delete +an instance of a derived class through a pointer to a base class if the +base class does not have a virtual destructor. This warning is enabled +by <samp>-Wall</samp>. +</p> +<a name="index-Wdeprecated_002dcopy"></a> +<a name="index-Wno_002ddeprecated_002dcopy"></a> +</dd> +<dt><code>-Wdeprecated-copy <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn that the implicit declaration of a copy constructor or copy +assignment operator is deprecated if the class has a user-provided +copy constructor or copy assignment operator, in C++11 and up. This +warning is enabled by <samp>-Wextra</samp>. With +<samp>-Wdeprecated-copy-dtor</samp>, also deprecate if the class has a +user-provided destructor. +</p> +<a name="index-Wdeprecated_002denum_002denum_002dconversion"></a> +<a name="index-Wno_002ddeprecated_002denum_002denum_002dconversion"></a> +</dd> +<dt><code>-Wno-deprecated-enum-enum-conversion <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Disable the warning about the case when the usual arithmetic conversions +are applied on operands where one is of enumeration type and the other is +of a different enumeration type. This conversion was deprecated in C++20. +For example: +</p> +<div class="smallexample"> +<pre class="smallexample">enum E1 { e }; +enum E2 { f }; +int k = f - e; +</pre></div> + +<p><samp>-Wdeprecated-enum-enum-conversion</samp> is enabled by default with +<samp>-std=c++20</samp>. In pre-C++20 dialects, this warning can be enabled +by <samp>-Wenum-conversion</samp>. +</p> +<a name="index-Wdeprecated_002denum_002dfloat_002dconversion"></a> +<a name="index-Wno_002ddeprecated_002denum_002dfloat_002dconversion"></a> +</dd> +<dt><code>-Wno-deprecated-enum-float-conversion <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Disable the warning about the case when the usual arithmetic conversions +are applied on operands where one is of enumeration type and the other is +of a floating-point type. This conversion was deprecated in C++20. For +example: +</p> +<div class="smallexample"> +<pre class="smallexample">enum E1 { e }; +enum E2 { f }; +bool b = e <= 3.7; +</pre></div> + +<p><samp>-Wdeprecated-enum-float-conversion</samp> is enabled by default with +<samp>-std=c++20</samp>. In pre-C++20 dialects, this warning can be enabled +by <samp>-Wenum-conversion</samp>. +</p> +<a name="index-Winit_002dlist_002dlifetime"></a> +<a name="index-Wno_002dinit_002dlist_002dlifetime"></a> +</dd> +<dt><code>-Wno-init-list-lifetime <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Do not warn about uses of <code>std::initializer_list</code> that are likely +to result in dangling pointers. Since the underlying array for an +<code>initializer_list</code> is handled like a normal C++ temporary object, +it is easy to inadvertently keep a pointer to the array past the end +of the array’s lifetime. For example: +</p> +<ul> +<li> If a function returns a temporary <code>initializer_list</code>, or a local +<code>initializer_list</code> variable, the array’s lifetime ends at the end +of the return statement, so the value returned has a dangling pointer. + +</li><li> If a new-expression creates an <code>initializer_list</code>, the array only +lives until the end of the enclosing full-expression, so the +<code>initializer_list</code> in the heap has a dangling pointer. + +</li><li> When an <code>initializer_list</code> variable is assigned from a +brace-enclosed initializer list, the temporary array created for the +right side of the assignment only lives until the end of the +full-expression, so at the next statement the <code>initializer_list</code> +variable has a dangling pointer. + +<div class="smallexample"> +<pre class="smallexample">// li's initial underlying array lives as long as li +std::initializer_list<int> li = { 1,2,3 }; +// assignment changes li to point to a temporary array +li = { 4, 5 }; +// now the temporary is gone and li has a dangling pointer +int i = li.begin()[0] // undefined behavior +</pre></div> + +</li><li> When a list constructor stores the <code>begin</code> pointer from the +<code>initializer_list</code> argument, this doesn’t extend the lifetime of +the array, so if a class variable is constructed from a temporary +<code>initializer_list</code>, the pointer is left dangling by the end of +the variable declaration statement. + +</li></ul> + +<a name="index-Winvalid_002dconstexpr"></a> +<a name="index-Wno_002dinvalid_002dconstexpr"></a> +</dd> +<dt><code>-Winvalid-constexpr</code></dt> +<dd> +<p>Warn when a function never produces a constant expression. In C++20 +and earlier, for every <code>constexpr</code> function and function template, +there must be at least one set of function arguments in at least one +instantiation such that an invocation of the function or constructor +could be an evaluated subexpression of a core constant expression. +C++23 removed this restriction, so it’s possible to have a function +or a function template marked <code>constexpr</code> for which no invocation +satisfies the requirements of a core constant expression. +</p> +<p>This warning is enabled as a pedantic warning by default in C++20 and +earlier. In C++23, <samp>-Winvalid-constexpr</samp> can be turned on, in +which case it will be an ordinary warning. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">void f (int& i); +constexpr void +g (int& i) +{ + // Warns by default in C++20, in C++23 only with -Winvalid-constexpr. + f(i); +} +</pre></div> + +<a name="index-Winvalid_002dimported_002dmacros"></a> +<a name="index-Wno_002dinvalid_002dimported_002dmacros"></a> +</dd> +<dt><code>-Winvalid-imported-macros</code></dt> +<dd><p>Verify all imported macro definitions are valid at the end of +compilation. This is not enabled by default, as it requires +additional processing to determine. It may be useful when preparing +sets of header-units to ensure consistent macros. +</p> +<a name="index-Wliteral_002dsuffix"></a> +<a name="index-Wno_002dliteral_002dsuffix"></a> +</dd> +<dt><code>-Wno-literal-suffix <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Do not warn when a string or character literal is followed by a +ud-suffix which does not begin with an underscore. As a conforming +extension, GCC treats such suffixes as separate preprocessing tokens +in order to maintain backwards compatibility with code that uses +formatting macros from <code><inttypes.h></code>. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">#define __STDC_FORMAT_MACROS +#include <inttypes.h> +#include <stdio.h> + +int main() { + int64_t i64 = 123; + printf("My int64: %" PRId64"\n", i64); +} +</pre></div> + +<p>In this case, <code>PRId64</code> is treated as a separate preprocessing token. +</p> +<p>This option also controls warnings when a user-defined literal +operator is declared with a literal suffix identifier that doesn’t +begin with an underscore. Literal suffix identifiers that don’t begin +with an underscore are reserved for future standardization. +</p> +<p>These warnings are enabled by default. +</p> +<a name="index-Wnarrowing"></a> +<a name="index-Wno_002dnarrowing"></a> +</dd> +<dt><code>-Wno-narrowing <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>For C++11 and later standards, narrowing conversions are diagnosed by default, +as required by the standard. A narrowing conversion from a constant produces +an error, and a narrowing conversion from a non-constant produces a warning, +but <samp>-Wno-narrowing</samp> suppresses the diagnostic. +Note that this does not affect the meaning of well-formed code; +narrowing conversions are still considered ill-formed in SFINAE contexts. +</p> +<p>With <samp>-Wnarrowing</samp> in C++98, warn when a narrowing +conversion prohibited by C++11 occurs within +‘<samp>{ }</samp>’, e.g. +</p> +<div class="smallexample"> +<pre class="smallexample">int i = { 2.2 }; // error: narrowing from double to int +</pre></div> + +<p>This flag is included in <samp>-Wall</samp> and <samp>-Wc++11-compat</samp>. +</p> +<a name="index-Wnoexcept"></a> +<a name="index-Wno_002dnoexcept"></a> +</dd> +<dt><code>-Wnoexcept <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when a noexcept-expression evaluates to false because of a call +to a function that does not have a non-throwing exception +specification (i.e. <code>throw()</code> or <code>noexcept</code>) but is known by +the compiler to never throw an exception. +</p> +<a name="index-Wnoexcept_002dtype"></a> +<a name="index-Wno_002dnoexcept_002dtype"></a> +</dd> +<dt><code>-Wnoexcept-type <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn if the C++17 feature making <code>noexcept</code> part of a function +type changes the mangled name of a symbol relative to C++14. Enabled +by <samp>-Wabi</samp> and <samp>-Wc++17-compat</samp>. +</p> +<p>As an example: +</p> +<div class="smallexample"> +<pre class="smallexample">template <class T> void f(T t) { t(); }; +void g() noexcept; +void h() { f(g); } +</pre></div> + +<p>In C++14, <code>f</code> calls <code>f<void(*)()></code>, but in +C++17 it calls <code>f<void(*)()noexcept></code>. +</p> +<a name="index-Wclass_002dmemaccess"></a> +<a name="index-Wno_002dclass_002dmemaccess"></a> +</dd> +<dt><code>-Wclass-memaccess <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when the destination of a call to a raw memory function such as +<code>memset</code> or <code>memcpy</code> is an object of class type, and when writing +into such an object might bypass the class non-trivial or deleted constructor +or copy assignment, violate const-correctness or encapsulation, or corrupt +virtual table pointers. Modifying the representation of such objects may +violate invariants maintained by member functions of the class. For example, +the call to <code>memset</code> below is undefined because it modifies a non-trivial +class object and is, therefore, diagnosed. The safe way to either initialize +or clear the storage of objects of such types is by using the appropriate +constructor or assignment operator, if one is available. +</p><div class="smallexample"> +<pre class="smallexample">std::string str = "abc"; +memset (&str, 0, sizeof str); +</pre></div> +<p>The <samp>-Wclass-memaccess</samp> option is enabled by <samp>-Wall</samp>. +Explicitly casting the pointer to the class object to <code>void *</code> or +to a type that can be safely accessed by the raw memory function suppresses +the warning. +</p> +<a name="index-Wnon_002dvirtual_002ddtor"></a> +<a name="index-Wno_002dnon_002dvirtual_002ddtor"></a> +</dd> +<dt><code>-Wnon-virtual-dtor <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when a class has virtual functions and an accessible non-virtual +destructor itself or in an accessible polymorphic base class, in which +case it is possible but unsafe to delete an instance of a derived +class through a pointer to the class itself or base class. This +warning is automatically enabled if <samp>-Weffc++</samp> is specified. +The <samp>-Wdelete-non-virtual-dtor</samp> option (enabled by <samp>-Wall</samp>) +should be preferred because it warns about the unsafe cases without false +positives. +</p> +<a name="index-Wregister"></a> +<a name="index-Wno_002dregister"></a> +</dd> +<dt><code>-Wregister <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn on uses of the <code>register</code> storage class specifier, except +when it is part of the GNU <a href="Explicit-Register-Variables.html#Explicit-Register-Variables">Explicit Register Variables</a> extension. +The use of the <code>register</code> keyword as storage class specifier has +been deprecated in C++11 and removed in C++17. +Enabled by default with <samp>-std=c++17</samp>. +</p> +<a name="index-Wreorder"></a> +<a name="index-Wno_002dreorder"></a> +<a name="index-reordering_002c-warning"></a> +<a name="index-warning-for-reordering-of-member-initializers"></a> +</dd> +<dt><code>-Wreorder <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when the order of member initializers given in the code does not +match the order in which they must be executed. For instance: +</p> +<div class="smallexample"> +<pre class="smallexample">struct A { + int i; + int j; + A(): j (0), i (1) { } +}; +</pre></div> + +<p>The compiler rearranges the member initializers for <code>i</code> +and <code>j</code> to match the declaration order of the members, emitting +a warning to that effect. This warning is enabled by <samp>-Wall</samp>. +</p> +<a name="index-Wpessimizing_002dmove"></a> +<a name="index-Wno_002dpessimizing_002dmove"></a> +</dd> +<dt><code>-Wno-pessimizing-move <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>This warning warns when a call to <code>std::move</code> prevents copy +elision. A typical scenario when copy elision can occur is when returning in +a function with a class return type, when the expression being returned is the +name of a non-volatile automatic object, and is not a function parameter, and +has the same type as the function return type. +</p> +<div class="smallexample"> +<pre class="smallexample">struct T { +… +}; +T fn() +{ + T t; + … + return std::move (t); +} +</pre></div> + +<p>But in this example, the <code>std::move</code> call prevents copy elision. +</p> +<p>This warning is enabled by <samp>-Wall</samp>. +</p> +<a name="index-Wredundant_002dmove"></a> +<a name="index-Wno_002dredundant_002dmove"></a> +</dd> +<dt><code>-Wno-redundant-move <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>This warning warns about redundant calls to <code>std::move</code>; that is, when +a move operation would have been performed even without the <code>std::move</code> +call. This happens because the compiler is forced to treat the object as if +it were an rvalue in certain situations such as returning a local variable, +where copy elision isn’t applicable. Consider: +</p> +<div class="smallexample"> +<pre class="smallexample">struct T { +… +}; +T fn(T t) +{ + … + return std::move (t); +} +</pre></div> + +<p>Here, the <code>std::move</code> call is redundant. Because G++ implements Core +Issue 1579, another example is: +</p> +<div class="smallexample"> +<pre class="smallexample">struct T { // convertible to U +… +}; +struct U { +… +}; +U fn() +{ + T t; + … + return std::move (t); +} +</pre></div> +<p>In this example, copy elision isn’t applicable because the type of the +expression being returned and the function return type differ, yet G++ +treats the return value as if it were designated by an rvalue. +</p> +<p>This warning is enabled by <samp>-Wextra</samp>. +</p> +<a name="index-Wrange_002dloop_002dconstruct"></a> +<a name="index-Wno_002drange_002dloop_002dconstruct"></a> +</dd> +<dt><code>-Wrange-loop-construct <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>This warning warns when a C++ range-based for-loop is creating an unnecessary +copy. This can happen when the range declaration is not a reference, but +probably should be. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">struct S { char arr[128]; }; +void fn () { + S arr[5]; + for (const auto x : arr) { … } +} +</pre></div> + +<p>It does not warn when the type being copied is a trivially-copyable type whose +size is less than 64 bytes. +</p> +<p>This warning also warns when a loop variable in a range-based for-loop is +initialized with a value of a different type resulting in a copy. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">void fn() { + int arr[10]; + for (const double &x : arr) { … } +} +</pre></div> + +<p>In the example above, in every iteration of the loop a temporary value of +type <code>double</code> is created and destroyed, to which the reference +<code>const double &</code> is bound. +</p> +<p>This warning is enabled by <samp>-Wall</samp>. +</p> +<a name="index-Wredundant_002dtags"></a> +<a name="index-Wno_002dredundant_002dtags"></a> +</dd> +<dt><code>-Wredundant-tags <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn about redundant class-key and enum-key in references to class types +and enumerated types in contexts where the key can be eliminated without +causing an ambiguity. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">struct foo; +struct foo *p; // warn that keyword struct can be eliminated +</pre></div> + +<p>On the other hand, in this example there is no warning: +</p> +<div class="smallexample"> +<pre class="smallexample">struct foo; +void foo (); // "hides" struct foo +void bar (struct foo&); // no warning, keyword struct is necessary +</pre></div> + +<a name="index-Wsubobject_002dlinkage"></a> +<a name="index-Wno_002dsubobject_002dlinkage"></a> +</dd> +<dt><code>-Wno-subobject-linkage <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Do not warn +if a class type has a base or a field whose type uses the anonymous +namespace or depends on a type with no linkage. If a type A depends on +a type B with no or internal linkage, defining it in multiple +translation units would be an ODR violation because the meaning of B +is different in each translation unit. If A only appears in a single +translation unit, the best way to silence the warning is to give it +internal linkage by putting it in an anonymous namespace as well. The +compiler doesn’t give this warning for types defined in the main .C +file, as those are unlikely to have multiple definitions. +<samp>-Wsubobject-linkage</samp> is enabled by default. +</p> +<a name="index-Weffc_002b_002b"></a> +<a name="index-Wno_002deffc_002b_002b"></a> +</dd> +<dt><code>-Weffc++ <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn about violations of the following style guidelines from Scott Meyers’ +<cite>Effective C++</cite> series of books: +</p> +<ul> +<li> Define a copy constructor and an assignment operator for classes +with dynamically-allocated memory. + +</li><li> Prefer initialization to assignment in constructors. + +</li><li> Have <code>operator=</code> return a reference to <code>*this</code>. + +</li><li> Don’t try to return a reference when you must return an object. + +</li><li> Distinguish between prefix and postfix forms of increment and +decrement operators. + +</li><li> Never overload <code>&&</code>, <code>||</code>, or <code>,</code>. + +</li></ul> + +<p>This option also enables <samp>-Wnon-virtual-dtor</samp>, which is also +one of the effective C++ recommendations. However, the check is +extended to warn about the lack of virtual destructor in accessible +non-polymorphic bases classes too. +</p> +<p>When selecting this option, be aware that the standard library +headers do not obey all of these guidelines; use ‘<samp>grep -v</samp>’ +to filter out those warnings. +</p> +<a name="index-Wexceptions"></a> +<a name="index-Wno_002dexceptions"></a> +</dd> +<dt><code>-Wno-exceptions <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Disable the warning about the case when an exception handler is shadowed by +another handler, which can point out a wrong ordering of exception handlers. +</p> +<a name="index-Wstrict_002dnull_002dsentinel"></a> +<a name="index-Wno_002dstrict_002dnull_002dsentinel"></a> +</dd> +<dt><code>-Wstrict-null-sentinel <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn about the use of an uncasted <code>NULL</code> as sentinel. When +compiling only with GCC this is a valid sentinel, as <code>NULL</code> is defined +to <code>__null</code>. Although it is a null pointer constant rather than a +null pointer, it is guaranteed to be of the same size as a pointer. +But this use is not portable across different compilers. +</p> +<a name="index-Wno_002dnon_002dtemplate_002dfriend"></a> +<a name="index-Wnon_002dtemplate_002dfriend"></a> +</dd> +<dt><code>-Wno-non-template-friend <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Disable warnings when non-template friend functions are declared +within a template. In very old versions of GCC that predate implementation +of the ISO standard, declarations such as +‘<samp>friend int foo(int)</samp>’, where the name of the friend is an unqualified-id, +could be interpreted as a particular specialization of a template +function; the warning exists to diagnose compatibility problems, +and is enabled by default. +</p> +<a name="index-Wold_002dstyle_002dcast"></a> +<a name="index-Wno_002dold_002dstyle_002dcast"></a> +</dd> +<dt><code>-Wold-style-cast <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn if an old-style (C-style) cast to a non-void type is used within +a C++ program. The new-style casts (<code>dynamic_cast</code>, +<code>static_cast</code>, <code>reinterpret_cast</code>, and <code>const_cast</code>) are +less vulnerable to unintended effects and much easier to search for. +</p> +<a name="index-Woverloaded_002dvirtual"></a> +<a name="index-Wno_002doverloaded_002dvirtual"></a> +<a name="index-overloaded-virtual-function_002c-warning"></a> +<a name="index-warning-for-overloaded-virtual-function"></a> +</dd> +<dt><code>-Woverloaded-virtual <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dt><code>-Woverloaded-virtual=<var>n</var></code></dt> +<dd><p>Warn when a function declaration hides virtual functions from a +base class. For example, in: +</p> +<div class="smallexample"> +<pre class="smallexample">struct A { + virtual void f(); +}; + +struct B: public A { + void f(int); // does not override +}; +</pre></div> + +<p>the <code>A</code> class version of <code>f</code> is hidden in <code>B</code>, and code +like: +</p> +<div class="smallexample"> +<pre class="smallexample">B* b; +b->f(); +</pre></div> + +<p>fails to compile. +</p> +<p>In cases where the different signatures are not an accident, the +simplest solution is to add a using-declaration to the derived class +to un-hide the base function, e.g. add <code>using A::f;</code> to <code>B</code>. +</p> +<p>The optional level suffix controls the behavior when all the +declarations in the derived class override virtual functions in the +base class, even if not all of the base functions are overridden: +</p> +<div class="smallexample"> +<pre class="smallexample">struct C { + virtual void f(); + virtual void f(int); +}; + +struct D: public C { + void f(int); // does override +} +</pre></div> + +<p>This pattern is less likely to be a mistake; if D is only used +virtually, the user might have decided that the base class semantics +for some of the overloads are fine. +</p> +<p>At level 1, this case does not warn; at level 2, it does. +<samp>-Woverloaded-virtual</samp> by itself selects level 2. Level 1 is +included in <samp>-Wall</samp>. +</p> +<a name="index-Wno_002dpmf_002dconversions"></a> +<a name="index-Wpmf_002dconversions"></a> +</dd> +<dt><code>-Wno-pmf-conversions <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Disable the diagnostic for converting a bound pointer to member function +to a plain pointer. +</p> +<a name="index-Wsign_002dpromo"></a> +<a name="index-Wno_002dsign_002dpromo"></a> +</dd> +<dt><code>-Wsign-promo <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when overload resolution chooses a promotion from unsigned or +enumerated type to a signed type, over a conversion to an unsigned type of +the same size. Previous versions of G++ tried to preserve +unsignedness, but the standard mandates the current behavior. +</p> +<a name="index-Wtemplates"></a> +<a name="index-Wno_002dtemplates"></a> +</dd> +<dt><code>-Wtemplates <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when a primary template declaration is encountered. Some coding +rules disallow templates, and this may be used to enforce that rule. +The warning is inactive inside a system header file, such as the STL, so +one can still use the STL. One may also instantiate or specialize +templates. +</p> +<a name="index-Wmismatched_002dnew_002ddelete"></a> +<a name="index-Wno_002dmismatched_002dnew_002ddelete"></a> +</dd> +<dt><code>-Wmismatched-new-delete <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn for mismatches between calls to <code>operator new</code> or <code>operator +delete</code> and the corresponding call to the allocation or deallocation function. +This includes invocations of C++ <code>operator delete</code> with pointers +returned from either mismatched forms of <code>operator new</code>, or from other +functions that allocate objects for which the <code>operator delete</code> isn’t +a suitable deallocator, as well as calls to other deallocation functions +with pointers returned from <code>operator new</code> for which the deallocation +function isn’t suitable. +</p> +<p>For example, the <code>delete</code> expression in the function below is diagnosed +because it doesn’t match the array form of the <code>new</code> expression +the pointer argument was returned from. Similarly, the call to <code>free</code> +is also diagnosed. +</p> +<div class="smallexample"> +<pre class="smallexample">void f () +{ + int *a = new int[n]; + delete a; // warning: mismatch in array forms of expressions + + char *p = new char[n]; + free (p); // warning: mismatch between new and free +} +</pre></div> + +<p>The related option <samp>-Wmismatched-dealloc</samp> diagnoses mismatches +involving allocation and deallocation functions other than <code>operator +new</code> and <code>operator delete</code>. +</p> +<p><samp>-Wmismatched-new-delete</samp> is included in <samp>-Wall</samp>. +</p> +<a name="index-Wmismatched_002dtags"></a> +<a name="index-Wno_002dmismatched_002dtags"></a> +</dd> +<dt><code>-Wmismatched-tags <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn for declarations of structs, classes, and class templates and their +specializations with a class-key that does not match either the definition +or the first declaration if no definition is provided. +</p> +<p>For example, the declaration of <code>struct Object</code> in the argument list +of <code>draw</code> triggers the warning. To avoid it, either remove the redundant +class-key <code>struct</code> or replace it with <code>class</code> to match its definition. +</p><div class="smallexample"> +<pre class="smallexample">class Object { +public: + virtual ~Object () = 0; +}; +void draw (struct Object*); +</pre></div> + +<p>It is not wrong to declare a class with the class-key <code>struct</code> as +the example above shows. The <samp>-Wmismatched-tags</samp> option is intended +to help achieve a consistent style of class declarations. In code that is +intended to be portable to Windows-based compilers the warning helps prevent +unresolved references due to the difference in the mangling of symbols +declared with different class-keys. The option can be used either on its +own or in conjunction with <samp>-Wredundant-tags</samp>. +</p> +<a name="index-Wmultiple_002dinheritance"></a> +<a name="index-Wno_002dmultiple_002dinheritance"></a> +</dd> +<dt><code>-Wmultiple-inheritance <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when a class is defined with multiple direct base classes. Some +coding rules disallow multiple inheritance, and this may be used to +enforce that rule. The warning is inactive inside a system header file, +such as the STL, so one can still use the STL. One may also define +classes that indirectly use multiple inheritance. +</p> +<a name="index-Wvirtual_002dinheritance"></a> +<a name="index-Wno_002dvirtual_002dinheritance"></a> +</dd> +<dt><code>-Wvirtual-inheritance</code></dt> +<dd><p>Warn when a class is defined with a virtual direct base class. Some +coding rules disallow multiple inheritance, and this may be used to +enforce that rule. The warning is inactive inside a system header file, +such as the STL, so one can still use the STL. One may also define +classes that indirectly use virtual inheritance. +</p> +<a name="index-Wvirtual_002dmove_002dassign"></a> +<a name="index-Wno_002dvirtual_002dmove_002dassign"></a> +</dd> +<dt><code>-Wno-virtual-move-assign</code></dt> +<dd><p>Suppress warnings about inheriting from a virtual base with a +non-trivial C++11 move assignment operator. This is dangerous because +if the virtual base is reachable along more than one path, it is +moved multiple times, which can mean both objects end up in the +moved-from state. If the move assignment operator is written to avoid +moving from a moved-from object, this warning can be disabled. +</p> +<a name="index-Wnamespaces"></a> +<a name="index-Wno_002dnamespaces"></a> +</dd> +<dt><code>-Wnamespaces</code></dt> +<dd><p>Warn when a namespace definition is opened. Some coding rules disallow +namespaces, and this may be used to enforce that rule. The warning is +inactive inside a system header file, such as the STL, so one can still +use the STL. One may also use using directives and qualified names. +</p> +<a name="index-Wterminate"></a> +<a name="index-Wno_002dterminate"></a> +</dd> +<dt><code>-Wno-terminate <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Disable the warning about a throw-expression that will immediately +result in a call to <code>terminate</code>. +</p> +<a name="index-Wvexing_002dparse"></a> +<a name="index-Wno_002dvexing_002dparse"></a> +</dd> +<dt><code>-Wno-vexing-parse <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn about the most vexing parse syntactic ambiguity. This warns about +the cases when a declaration looks like a variable definition, but the +C++ language requires it to be interpreted as a function declaration. +For instance: +</p> +<div class="smallexample"> +<pre class="smallexample">void f(double a) { + int i(); // extern int i (void); + int n(int(a)); // extern int n (int); +} +</pre></div> + +<p>Another example: +</p> +<div class="smallexample"> +<pre class="smallexample">struct S { S(int); }; +void f(double a) { + S x(int(a)); // extern struct S x (int); + S y(int()); // extern struct S y (int (*) (void)); + S z(); // extern struct S z (void); +} +</pre></div> + +<p>The warning will suggest options how to deal with such an ambiguity; e.g., +it can suggest removing the parentheses or using braces instead. +</p> +<p>This warning is enabled by default. +</p> +<a name="index-Wno_002dclass_002dconversion"></a> +<a name="index-Wclass_002dconversion"></a> +</dd> +<dt><code>-Wno-class-conversion <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Do not warn when a conversion function converts an +object to the same type, to a base class of that type, or to void; such +a conversion function will never be called. +</p> +<a name="index-Wvolatile"></a> +<a name="index-Wno_002dvolatile"></a> +</dd> +<dt><code>-Wvolatile <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn about deprecated uses of the <code>volatile</code> qualifier. This includes +postfix and prefix <code>++</code> and <code>--</code> expressions of +<code>volatile</code>-qualified types, using simple assignments where the left +operand is a <code>volatile</code>-qualified non-class type for their value, +compound assignments where the left operand is a <code>volatile</code>-qualified +non-class type, <code>volatile</code>-qualified function return type, +<code>volatile</code>-qualified parameter type, and structured bindings of a +<code>volatile</code>-qualified type. This usage was deprecated in C++20. +</p> +<p>Enabled by default with <samp>-std=c++20</samp>. +</p> +<a name="index-Wzero_002das_002dnull_002dpointer_002dconstant"></a> +<a name="index-Wno_002dzero_002das_002dnull_002dpointer_002dconstant"></a> +</dd> +<dt><code>-Wzero-as-null-pointer-constant <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when a literal ‘<samp>0</samp>’ is used as null pointer constant. This can +be useful to facilitate the conversion to <code>nullptr</code> in C++11. +</p> +<a name="index-Waligned_002dnew"></a> +<a name="index-Wno_002daligned_002dnew"></a> +</dd> +<dt><code>-Waligned-new</code></dt> +<dd><p>Warn about a new-expression of a type that requires greater alignment +than the <code>alignof(std::max_align_t)</code> but uses an allocation +function without an explicit alignment parameter. This option is +enabled by <samp>-Wall</samp>. +</p> +<p>Normally this only warns about global allocation functions, but +<samp>-Waligned-new=all</samp> also warns about class member allocation +functions. +</p> +<a name="index-Wplacement_002dnew"></a> +<a name="index-Wno_002dplacement_002dnew"></a> +</dd> +<dt><code>-Wno-placement-new</code></dt> +<dt><code>-Wplacement-new=<var>n</var></code></dt> +<dd><p>Warn about placement new expressions with undefined behavior, such as +constructing an object in a buffer that is smaller than the type of +the object. For example, the placement new expression below is diagnosed +because it attempts to construct an array of 64 integers in a buffer only +64 bytes large. +</p><div class="smallexample"> +<pre class="smallexample">char buf [64]; +new (buf) int[64]; +</pre></div> +<p>This warning is enabled by default. +</p> +<dl compact="compact"> +<dt><code>-Wplacement-new=1</code></dt> +<dd><p>This is the default warning level of <samp>-Wplacement-new</samp>. At this +level the warning is not issued for some strictly undefined constructs that +GCC allows as extensions for compatibility with legacy code. For example, +the following <code>new</code> expression is not diagnosed at this level even +though it has undefined behavior according to the C++ standard because +it writes past the end of the one-element array. +</p><div class="smallexample"> +<pre class="smallexample">struct S { int n, a[1]; }; +S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]); +new (s->a)int [32](); +</pre></div> + +</dd> +<dt><code>-Wplacement-new=2</code></dt> +<dd><p>At this level, in addition to diagnosing all the same constructs as at level +1, a diagnostic is also issued for placement new expressions that construct +an object in the last member of structure whose type is an array of a single +element and whose size is less than the size of the object being constructed. +While the previous example would be diagnosed, the following construct makes +use of the flexible member array extension to avoid the warning at level 2. +</p><div class="smallexample"> +<pre class="smallexample">struct S { int n, a[]; }; +S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]); +new (s->a)int [32](); +</pre></div> + +</dd> +</dl> + +<a name="index-Wcatch_002dvalue"></a> +<a name="index-Wno_002dcatch_002dvalue"></a> +</dd> +<dt><code>-Wcatch-value</code></dt> +<dt><code>-Wcatch-value=<var>n</var> <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn about catch handlers that do not catch via reference. +With <samp>-Wcatch-value=1</samp> (or <samp>-Wcatch-value</samp> for short) +warn about polymorphic class types that are caught by value. +With <samp>-Wcatch-value=2</samp> warn about all class types that are caught +by value. With <samp>-Wcatch-value=3</samp> warn about all types that are +not caught by reference. <samp>-Wcatch-value</samp> is enabled by <samp>-Wall</samp>. +</p> +<a name="index-Wconditionally_002dsupported"></a> +<a name="index-Wno_002dconditionally_002dsupported"></a> +</dd> +<dt><code>-Wconditionally-supported <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn for conditionally-supported (C++11 [intro.defs]) constructs. +</p> +<a name="index-Wdelete_002dincomplete"></a> +<a name="index-Wno_002ddelete_002dincomplete"></a> +</dd> +<dt><code>-Wno-delete-incomplete <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Do not warn when deleting a pointer to incomplete type, which may cause +undefined behavior at runtime. This warning is enabled by default. +</p> +<a name="index-Wextra_002dsemi"></a> +<a name="index-Wno_002dextra_002dsemi"></a> +</dd> +<dt><code>-Wextra-semi <span class="roman">(C++, Objective-C++ only)</span></code></dt> +<dd><p>Warn about redundant semicolons after in-class function definitions. +</p> +<a name="index-Winaccessible_002dbase"></a> +<a name="index-Wno_002dinaccessible_002dbase"></a> +</dd> +<dt><code>-Wno-inaccessible-base <span class="roman">(C++, Objective-C++ only)</span></code></dt> +<dd><p>This option controls warnings +when a base class is inaccessible in a class derived from it due to +ambiguity. The warning is enabled by default. +Note that the warning for ambiguous virtual +bases is enabled by the <samp>-Wextra</samp> option. +</p><div class="smallexample"> +<pre class="smallexample">struct A { int a; }; + +struct B : A { }; + +struct C : B, A { }; +</pre></div> + +<a name="index-Winherited_002dvariadic_002dctor"></a> +<a name="index-Wno_002dinherited_002dvariadic_002dctor"></a> +</dd> +<dt><code>-Wno-inherited-variadic-ctor</code></dt> +<dd><p>Suppress warnings about use of C++11 inheriting constructors when the +base class inherited from has a C variadic constructor; the warning is +on by default because the ellipsis is not inherited. +</p> +<a name="index-Wno_002dinvalid_002doffsetof"></a> +<a name="index-Winvalid_002doffsetof"></a> +</dd> +<dt><code>-Wno-invalid-offsetof <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Suppress warnings from applying the <code>offsetof</code> macro to a non-POD +type. According to the 2014 ISO C++ standard, applying <code>offsetof</code> +to a non-standard-layout type is undefined. In existing C++ implementations, +however, <code>offsetof</code> typically gives meaningful results. +This flag is for users who are aware that they are +writing nonportable code and who have deliberately chosen to ignore the +warning about it. +</p> +<p>The restrictions on <code>offsetof</code> may be relaxed in a future version +of the C++ standard. +</p> +<a name="index-Wsized_002ddeallocation"></a> +<a name="index-Wno_002dsized_002ddeallocation"></a> +</dd> +<dt><code>-Wsized-deallocation <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn about a definition of an unsized deallocation function +</p><div class="smallexample"> +<pre class="smallexample">void operator delete (void *) noexcept; +void operator delete[] (void *) noexcept; +</pre></div> +<p>without a definition of the corresponding sized deallocation function +</p><div class="smallexample"> +<pre class="smallexample">void operator delete (void *, std::size_t) noexcept; +void operator delete[] (void *, std::size_t) noexcept; +</pre></div> +<p>or vice versa. Enabled by <samp>-Wextra</samp> along with +<samp>-fsized-deallocation</samp>. +</p> +<a name="index-Wno_002dsuggest_002dfinal_002dtypes"></a> +<a name="index-Wsuggest_002dfinal_002dtypes"></a> +</dd> +<dt><code>-Wsuggest-final-types</code></dt> +<dd><p>Warn about types with virtual methods where code quality would be improved +if the type were declared with the C++11 <code>final</code> specifier, +or, if possible, +declared in an anonymous namespace. This allows GCC to more aggressively +devirtualize the polymorphic calls. This warning is more effective with +link-time optimization, +where the information about the class hierarchy graph is +more complete. +</p> +<a name="index-Wno_002dsuggest_002dfinal_002dmethods"></a> +<a name="index-Wsuggest_002dfinal_002dmethods"></a> +</dd> +<dt><code>-Wsuggest-final-methods</code></dt> +<dd><p>Warn about virtual methods where code quality would be improved if the method +were declared with the C++11 <code>final</code> specifier, +or, if possible, its type were +declared in an anonymous namespace or with the <code>final</code> specifier. +This warning is +more effective with link-time optimization, where the information about the +class hierarchy graph is more complete. It is recommended to first consider +suggestions of <samp>-Wsuggest-final-types</samp> and then rebuild with new +annotations. +</p> +<a name="index-Wsuggest_002doverride"></a> +<a name="index-Wno_002dsuggest_002doverride"></a> +</dd> +<dt><code>-Wsuggest-override</code></dt> +<dd><p>Warn about overriding virtual functions that are not marked with the +<code>override</code> keyword. +</p> +<a name="index-Wuse_002dafter_002dfree"></a> +<a name="index-Wno_002duse_002dafter_002dfree"></a> +</dd> +<dt><code>-Wuse-after-free</code></dt> +<dt><code>-Wuse-after-free=<var>n</var></code></dt> +<dd><p>Warn about uses of pointers to dynamically allocated objects that have +been rendered indeterminate by a call to a deallocation function. +The warning is enabled at all optimization levels but may yield different +results with optimization than without. +</p> +<dl compact="compact"> +<dt><code>-Wuse-after-free=1</code></dt> +<dd><p>At level 1 the warning attempts to diagnose only unconditional uses +of pointers made indeterminate by a deallocation call or a successful +call to <code>realloc</code>, regardless of whether or not the call resulted +in an actual reallocatio of memory. This includes double-<code>free</code> +calls as well as uses in arithmetic and relational expressions. Although +undefined, uses of indeterminate pointers in equality (or inequality) +expressions are not diagnosed at this level. +</p></dd> +<dt><code>-Wuse-after-free=2</code></dt> +<dd><p>At level 2, in addition to unconditional uses, the warning also diagnoses +conditional uses of pointers made indeterminate by a deallocation call. +As at level 2, uses in equality (or inequality) expressions are not +diagnosed. For example, the second call to <code>free</code> in the following +function is diagnosed at this level: +</p><div class="smallexample"> +<pre class="smallexample">struct A { int refcount; void *data; }; + +void release (struct A *p) +{ + int refcount = --p->refcount; + free (p); + if (refcount == 0) + free (p->data); // warning: p may be used after free +} +</pre></div> +</dd> +<dt><code>-Wuse-after-free=3</code></dt> +<dd><p>At level 3, the warning also diagnoses uses of indeterminate pointers in +equality expressions. All uses of indeterminate pointers are undefined +but equality tests sometimes appear after calls to <code>realloc</code> as +an attempt to determine whether the call resulted in relocating the object +to a different address. They are diagnosed at a separate level to aid +legacy code gradually transition to safe alternatives. For example, +the equality test in the function below is diagnosed at this level: +</p><div class="smallexample"> +<pre class="smallexample">void adjust_pointers (int**, int); + +void grow (int **p, int n) +{ + int **q = (int**)realloc (p, n *= 2); + if (q == p) + return; + adjust_pointers ((int**)q, n); +} +</pre></div> +<p>To avoid the warning at this level, store offsets into allocated memory +instead of pointers. This approach obviates needing to adjust the stored +pointers after reallocation. +</p></dd> +</dl> + +<p><samp>-Wuse-after-free=2</samp> is included in <samp>-Wall</samp>. +</p> +<a name="index-Wuseless_002dcast"></a> +<a name="index-Wno_002duseless_002dcast"></a> +</dd> +<dt><code>-Wuseless-cast <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Warn when an expression is cast to its own type. This warning does not +occur when a class object is converted to a non-reference type as that +is a way to create a temporary: +</p> +<div class="smallexample"> +<pre class="smallexample">struct S { }; +void g (S&&); +void f (S&& arg) +{ + g (S(arg)); // make arg prvalue so that it can bind to S&& +} +</pre></div> + +<a name="index-Wconversion_002dnull"></a> +<a name="index-Wno_002dconversion_002dnull"></a> +</dd> +<dt><code>-Wno-conversion-null <span class="roman">(C++ and Objective-C++ only)</span></code></dt> +<dd><p>Do not warn for conversions between <code>NULL</code> and non-pointer +types. <samp>-Wconversion-null</samp> is enabled by default. +</p> +</dd> +</dl> + +<hr> +<div class="header"> +<p> +Next: <a href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options" accesskey="n" rel="next">Objective-C and Objective-C++ Dialect Options</a>, Previous: <a href="C-Dialect-Options.html#C-Dialect-Options" accesskey="p" rel="previous">C Dialect Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |