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diff --git a/share/doc/cpp/Concatenation.html b/share/doc/cpp/Concatenation.html new file mode 100644 index 0000000..625aff7 --- /dev/null +++ b/share/doc/cpp/Concatenation.html @@ -0,0 +1,168 @@ +<!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. The Front-Cover Texts are +(a) (see below), and the Back-Cover Texts are (b) (see below). + +(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>The C Preprocessor: Concatenation</title> + +<meta name="description" content="The C Preprocessor: Concatenation"> +<meta name="keywords" content="The C Preprocessor: Concatenation"> +<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="Variadic-Macros.html#Variadic-Macros" rel="next" title="Variadic Macros"> +<link href="Stringizing.html#Stringizing" rel="previous" title="Stringizing"> +<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="Concatenation"></a> +<div class="header"> +<p> +Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringizing.html#Stringizing" accesskey="p" rel="previous">Stringizing</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="Concatenation-1"></a> +<h3 class="section">3.5 Concatenation</h3> +<a name="index-concatenation"></a> +<a name="index-token-pasting"></a> +<a name="index-token-concatenation"></a> +<a name="index-_0023_0023-operator"></a> + +<p>It is often useful to merge two tokens into one while expanding macros. +This is called <em>token pasting</em> or <em>token concatenation</em>. The +‘<samp>##</samp>’ preprocessing operator performs token pasting. When a macro +is expanded, the two tokens on either side of each ‘<samp>##</samp>’ operator +are combined into a single token, which then replaces the ‘<samp>##</samp>’ and +the two original tokens in the macro expansion. Usually both will be +identifiers, or one will be an identifier and the other a preprocessing +number. When pasted, they make a longer identifier. This isn’t the +only valid case. It is also possible to concatenate two numbers (or a +number and a name, such as <code>1.5</code> and <code>e3</code>) into a number. +Also, multi-character operators such as <code>+=</code> can be formed by +token pasting. +</p> +<p>However, two tokens that don’t together form a valid token cannot be +pasted together. For example, you cannot concatenate <code>x</code> with +<code>+</code> in either order. If you try, the preprocessor issues a warning +and emits the two tokens. Whether it puts white space between the +tokens is undefined. It is common to find unnecessary uses of ‘<samp>##</samp>’ +in complex macros. If you get this warning, it is likely that you can +simply remove the ‘<samp>##</samp>’. +</p> +<p>Both the tokens combined by ‘<samp>##</samp>’ could come from the macro body, +but you could just as well write them as one token in the first place. +Token pasting is most useful when one or both of the tokens comes from a +macro argument. If either of the tokens next to an ‘<samp>##</samp>’ is a +parameter name, it is replaced by its actual argument before ‘<samp>##</samp>’ +executes. As with stringizing, the actual argument is not +macro-expanded first. If the argument is empty, that ‘<samp>##</samp>’ has no +effect. +</p> +<p>Keep in mind that the C preprocessor converts comments to whitespace +before macros are even considered. Therefore, you cannot create a +comment by concatenating ‘<samp>/</samp>’ and ‘<samp>*</samp>’. You can put as much +whitespace between ‘<samp>##</samp>’ and its operands as you like, including +comments, and you can put comments in arguments that will be +concatenated. However, it is an error if ‘<samp>##</samp>’ appears at either +end of a macro body. +</p> +<p>Consider a C program that interprets named commands. There probably +needs to be a table of commands, perhaps an array of structures declared +as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">struct command +{ + char *name; + void (*function) (void); +}; +</pre><pre class="smallexample"> +</pre><pre class="smallexample">struct command commands[] = +{ + { "quit", quit_command }, + { "help", help_command }, + … +}; +</pre></div> + +<p>It would be cleaner not to have to give each command name twice, once in +the string constant and once in the function name. A macro which takes the +name of a command as an argument can make this unnecessary. The string +constant can be created with stringizing, and the function name by +concatenating the argument with ‘<samp>_command</samp>’. Here is how it is done: +</p> +<div class="smallexample"> +<pre class="smallexample">#define COMMAND(NAME) { #NAME, NAME ## _command } + +struct command commands[] = +{ + COMMAND (quit), + COMMAND (help), + … +}; +</pre></div> + +<hr> +<div class="header"> +<p> +Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringizing.html#Stringizing" accesskey="p" rel="previous">Stringizing</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> |