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diff --git a/share/doc/gcc/Statement-Exprs.html b/share/doc/gcc/Statement-Exprs.html new file mode 100644 index 0000000..7e9b609 --- /dev/null +++ b/share/doc/gcc/Statement-Exprs.html @@ -0,0 +1,241 @@ +<!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): Statement Exprs</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Statement Exprs"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Statement Exprs"> +<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="C-Extensions.html#C-Extensions" rel="up" title="C Extensions"> +<link href="Local-Labels.html#Local-Labels" rel="next" title="Local Labels"> +<link href="C-Extensions.html#C-Extensions" rel="previous" title="C Extensions"> +<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="Statement-Exprs"></a> +<div class="header"> +<p> +Next: <a href="Local-Labels.html#Local-Labels" accesskey="n" rel="next">Local Labels</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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="Statements-and-Declarations-in-Expressions"></a> +<h3 class="section">6.1 Statements and Declarations in Expressions</h3> +<a name="index-statements-inside-expressions"></a> +<a name="index-declarations-inside-expressions"></a> +<a name="index-expressions-containing-statements"></a> +<a name="index-macros_002c-statements-in-expressions"></a> + +<p>A compound statement enclosed in parentheses may appear as an expression +in GNU C. This allows you to use loops, switches, and local variables +within an expression. +</p> +<p>Recall that a compound statement is a sequence of statements surrounded +by braces; in this construct, parentheses go around the braces. For +example: +</p> +<div class="smallexample"> +<pre class="smallexample">({ int y = foo (); int z; + if (y > 0) z = y; + else z = - y; + z; }) +</pre></div> + +<p>is a valid (though slightly more complex than necessary) expression +for the absolute value of <code>foo ()</code>. +</p> +<p>The last thing in the compound statement should be an expression +followed by a semicolon; the value of this subexpression serves as the +value of the entire construct. (If you use some other kind of statement +last within the braces, the construct has type <code>void</code>, and thus +effectively no value.) +</p> +<p>This feature is especially useful in making macro definitions “safe” (so +that they evaluate each operand exactly once). For example, the +“maximum” function is commonly defined as a macro in standard C as +follows: +</p> +<div class="smallexample"> +<pre class="smallexample">#define max(a,b) ((a) > (b) ? (a) : (b)) +</pre></div> + +<a name="index-side-effects_002c-macro-argument"></a> +<p>But this definition computes either <var>a</var> or <var>b</var> twice, with bad +results if the operand has side effects. In GNU C, if you know the +type of the operands (here taken as <code>int</code>), you can avoid this +problem by defining the macro as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">#define maxint(a,b) \ + ({int _a = (a), _b = (b); _a > _b ? _a : _b; }) +</pre></div> + +<p>Note that introducing variable declarations (as we do in <code>maxint</code>) can +cause variable shadowing, so while this example using the <code>max</code> macro +produces correct results: +</p><div class="smallexample"> +<pre class="smallexample">int _a = 1, _b = 2, c; +c = max (_a, _b); +</pre></div> +<p>this example using maxint will not: +</p><div class="smallexample"> +<pre class="smallexample">int _a = 1, _b = 2, c; +c = maxint (_a, _b); +</pre></div> + +<p>This problem may for instance occur when we use this pattern recursively, like +so: +</p> +<div class="smallexample"> +<pre class="smallexample">#define maxint3(a, b, c) \ + ({int _a = (a), _b = (b), _c = (c); maxint (maxint (_a, _b), _c); }) +</pre></div> + +<p>Embedded statements are not allowed in constant expressions, such as +the value of an enumeration constant, the width of a bit-field, or +the initial value of a static variable. +</p> +<p>If you don’t know the type of the operand, you can still do this, but you +must use <code>typeof</code> or <code>__auto_type</code> (see <a href="Typeof.html#Typeof">Typeof</a>). +</p> +<p>In G++, the result value of a statement expression undergoes array and +function pointer decay, and is returned by value to the enclosing +expression. For instance, if <code>A</code> is a class, then +</p> +<div class="smallexample"> +<pre class="smallexample"> A a; + + ({a;}).Foo () +</pre></div> + +<p>constructs a temporary <code>A</code> object to hold the result of the +statement expression, and that is used to invoke <code>Foo</code>. +Therefore the <code>this</code> pointer observed by <code>Foo</code> is not the +address of <code>a</code>. +</p> +<p>In a statement expression, any temporaries created within a statement +are destroyed at that statement’s end. This makes statement +expressions inside macros slightly different from function calls. In +the latter case temporaries introduced during argument evaluation are +destroyed at the end of the statement that includes the function +call. In the statement expression case they are destroyed during +the statement expression. For instance, +</p> +<div class="smallexample"> +<pre class="smallexample">#define macro(a) ({__typeof__(a) b = (a); b + 3; }) +template<typename T> T function(T a) { T b = a; return b + 3; } + +void foo () +{ + macro (X ()); + function (X ()); +} +</pre></div> + +<p>has different places where temporaries are destroyed. For the +<code>macro</code> case, the temporary <code>X</code> is destroyed just after +the initialization of <code>b</code>. In the <code>function</code> case that +temporary is destroyed when the function returns. +</p> +<p>These considerations mean that it is probably a bad idea to use +statement expressions of this form in header files that are designed to +work with C++. (Note that some versions of the GNU C Library contained +header files using statement expressions that lead to precisely this +bug.) +</p> +<p>Jumping into a statement expression with <code>goto</code> or using a +<code>switch</code> statement outside the statement expression with a +<code>case</code> or <code>default</code> label inside the statement expression is +not permitted. Jumping into a statement expression with a computed +<code>goto</code> (see <a href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>) has undefined behavior. +Jumping out of a statement expression is permitted, but if the +statement expression is part of a larger expression then it is +unspecified which other subexpressions of that expression have been +evaluated except where the language definition requires certain +subexpressions to be evaluated before or after the statement +expression. A <code>break</code> or <code>continue</code> statement inside of +a statement expression used in <code>while</code>, <code>do</code> or <code>for</code> +loop or <code>switch</code> statement condition +or <code>for</code> statement init or increment expressions jumps to an +outer loop or <code>switch</code> statement if any (otherwise it is an error), +rather than to the loop or <code>switch</code> statement in whose condition +or init or increment expression it appears. +In any case, as with a function call, the evaluation of a +statement expression is not interleaved with the evaluation of other +parts of the containing expression. For example, +</p> +<div class="smallexample"> +<pre class="smallexample"> foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz(); +</pre></div> + +<p>calls <code>foo</code> and <code>bar1</code> and does not call <code>baz</code> but +may or may not call <code>bar2</code>. If <code>bar2</code> is called, it is +called after <code>foo</code> and before <code>bar1</code>. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Local-Labels.html#Local-Labels" accesskey="n" rel="next">Local Labels</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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> |