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diff --git a/share/doc/cpp/Object_002dlike-Macros.html b/share/doc/cpp/Object_002dlike-Macros.html new file mode 100644 index 0000000..70bc407 --- /dev/null +++ b/share/doc/cpp/Object_002dlike-Macros.html @@ -0,0 +1,207 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 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. A copy of +the license is included in the +section entitled "GNU Free Documentation License". + +This manual contains no Invariant Sections. 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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>The C Preprocessor: Object-like Macros</title> + +<meta name="description" content="The C Preprocessor: Object-like Macros"> +<meta name="keywords" content="The C Preprocessor: Object-like Macros"> +<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="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Macros.html#Macros" rel="up" title="Macros"> +<link href="Function_002dlike-Macros.html#Function_002dlike-Macros" rel="next" title="Function-like Macros"> +<link href="Macros.html#Macros" rel="previous" title="Macros"> +<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" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Object_002dlike-Macros"></a> +<div class="header"> +<p> +Next: <a href="Function_002dlike-Macros.html#Function_002dlike-Macros" accesskey="n" rel="next">Function-like Macros</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Object_002dlike-Macros-1"></a> +<h3 class="section">3.1 Object-like Macros</h3> +<a name="index-object_002dlike-macro"></a> +<a name="index-symbolic-constants"></a> +<a name="index-manifest-constants"></a> + +<p>An <em>object-like macro</em> is a simple identifier which will be replaced +by a code fragment. It is called object-like because it looks like a +data object in code that uses it. They are most commonly used to give +symbolic names to numeric constants. +</p> +<a name="index-_0023define"></a> +<p>You create macros with the ‘<samp>#define</samp>’ directive. ‘<samp>#define</samp>’ is +followed by the name of the macro and then the token sequence it should +be an abbreviation for, which is variously referred to as the macro’s +<em>body</em>, <em>expansion</em> or <em>replacement list</em>. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">#define BUFFER_SIZE 1024 +</pre></div> + +<p>defines a macro named <code>BUFFER_SIZE</code> as an abbreviation for the +token <code>1024</code>. If somewhere after this ‘<samp>#define</samp>’ directive +there comes a C statement of the form +</p> +<div class="smallexample"> +<pre class="smallexample">foo = (char *) malloc (BUFFER_SIZE); +</pre></div> + +<p>then the C preprocessor will recognize and <em>expand</em> the macro +<code>BUFFER_SIZE</code>. The C compiler will see the same tokens as it would +if you had written +</p> +<div class="smallexample"> +<pre class="smallexample">foo = (char *) malloc (1024); +</pre></div> + +<p>By convention, macro names are written in uppercase. Programs are +easier to read when it is possible to tell at a glance which names are +macros. +</p> +<p>The macro’s body ends at the end of the ‘<samp>#define</samp>’ line. You may +continue the definition onto multiple lines, if necessary, using +backslash-newline. When the macro is expanded, however, it will all +come out on one line. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">#define NUMBERS 1, \ + 2, \ + 3 +int x[] = { NUMBERS }; + → int x[] = { 1, 2, 3 }; +</pre></div> + +<p>The most common visible consequence of this is surprising line numbers +in error messages. +</p> +<p>There is no restriction on what can go in a macro body provided it +decomposes into valid preprocessing tokens. Parentheses need not +balance, and the body need not resemble valid C code. (If it does not, +you may get error messages from the C compiler when you use the macro.) +</p> +<p>The C preprocessor scans your program sequentially. Macro definitions +take effect at the place you write them. Therefore, the following input +to the C preprocessor +</p> +<div class="smallexample"> +<pre class="smallexample">foo = X; +#define X 4 +bar = X; +</pre></div> + +<p>produces +</p> +<div class="smallexample"> +<pre class="smallexample">foo = X; +bar = 4; +</pre></div> + +<p>When the preprocessor expands a macro name, the macro’s expansion +replaces the macro invocation, then the expansion is examined for more +macros to expand. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">#define TABLESIZE BUFSIZE +#define BUFSIZE 1024 +TABLESIZE + → BUFSIZE + → 1024 +</pre></div> + +<p><code>TABLESIZE</code> is expanded first to produce <code>BUFSIZE</code>, then that +macro is expanded to produce the final result, <code>1024</code>. +</p> +<p>Notice that <code>BUFSIZE</code> was not defined when <code>TABLESIZE</code> was +defined. The ‘<samp>#define</samp>’ for <code>TABLESIZE</code> uses exactly the +expansion you specify—in this case, <code>BUFSIZE</code>—and does not +check to see whether it too contains macro names. Only when you +<em>use</em> <code>TABLESIZE</code> is the result of its expansion scanned for +more macro names. +</p> +<p>This makes a difference if you change the definition of <code>BUFSIZE</code> +at some point in the source file. <code>TABLESIZE</code>, defined as shown, +will always expand using the definition of <code>BUFSIZE</code> that is +currently in effect: +</p> +<div class="smallexample"> +<pre class="smallexample">#define BUFSIZE 1020 +#define TABLESIZE BUFSIZE +#undef BUFSIZE +#define BUFSIZE 37 +</pre></div> + +<p>Now <code>TABLESIZE</code> expands (in two stages) to <code>37</code>. +</p> +<p>If the expansion of a macro contains its own name, either directly or +via intermediate macros, it is not expanded again when the expansion is +examined for more macros. This prevents infinite recursion. +See <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">Self-Referential Macros</a>, for the precise details. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Function_002dlike-Macros.html#Function_002dlike-Macros" accesskey="n" rel="next">Function-like Macros</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |