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169 lines
7.4 KiB
HTML
169 lines
7.4 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled "GNU Free Documentation License".
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Concatenation (The C Preprocessor)</title>
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<meta name="description" content="Concatenation (The C Preprocessor)">
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<meta name="keywords" content="Concatenation (The C Preprocessor)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Macros.html#Macros" rel="up" title="Macros">
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<link href="Variadic-Macros.html#Variadic-Macros" rel="next" title="Variadic Macros">
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<link href="Stringizing.html#Stringizing" rel="prev" title="Stringizing">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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div.example {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Concatenation"></a>
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<div class="header">
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<p>
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Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringizing.html#Stringizing" accesskey="p" rel="prev">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>
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</div>
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<hr>
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<a name="Concatenation-1"></a>
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<h3 class="section">3.5 Concatenation</h3>
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<a name="index-concatenation"></a>
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<a name="index-token-pasting"></a>
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<a name="index-token-concatenation"></a>
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<a name="index-_0023_0023-operator"></a>
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<p>It is often useful to merge two tokens into one while expanding macros.
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This is called <em>token pasting</em> or <em>token concatenation</em>. The
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‘<samp>##</samp>’ preprocessing operator performs token pasting. When a macro
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is expanded, the two tokens on either side of each ‘<samp>##</samp>’ operator
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are combined into a single token, which then replaces the ‘<samp>##</samp>’ and
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the two original tokens in the macro expansion. Usually both will be
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identifiers, or one will be an identifier and the other a preprocessing
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number. When pasted, they make a longer identifier. This isn’t the
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only valid case. It is also possible to concatenate two numbers (or a
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number and a name, such as <code>1.5</code> and <code>e3</code>) into a number.
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Also, multi-character operators such as <code>+=</code> can be formed by
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token pasting.
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</p>
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<p>However, two tokens that don’t together form a valid token cannot be
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pasted together. For example, you cannot concatenate <code>x</code> with
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<code>+</code> in either order. If you try, the preprocessor issues a warning
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and emits the two tokens. Whether it puts white space between the
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tokens is undefined. It is common to find unnecessary uses of ‘<samp>##</samp>’
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in complex macros. If you get this warning, it is likely that you can
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simply remove the ‘<samp>##</samp>’.
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</p>
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<p>Both the tokens combined by ‘<samp>##</samp>’ could come from the macro body,
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but you could just as well write them as one token in the first place.
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Token pasting is most useful when one or both of the tokens comes from a
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macro argument. If either of the tokens next to an ‘<samp>##</samp>’ is a
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parameter name, it is replaced by its actual argument before ‘<samp>##</samp>’
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executes. As with stringizing, the actual argument is not
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macro-expanded first. If the argument is empty, that ‘<samp>##</samp>’ has no
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effect.
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</p>
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<p>Keep in mind that the C preprocessor converts comments to whitespace
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before macros are even considered. Therefore, you cannot create a
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comment by concatenating ‘<samp>/</samp>’ and ‘<samp>*</samp>’. You can put as much
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whitespace between ‘<samp>##</samp>’ and its operands as you like, including
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comments, and you can put comments in arguments that will be
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concatenated. However, it is an error if ‘<samp>##</samp>’ appears at either
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end of a macro body.
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</p>
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<p>Consider a C program that interprets named commands. There probably
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needs to be a table of commands, perhaps an array of structures declared
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as follows:
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct command
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{
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char *name;
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void (*function) (void);
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};
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</pre><pre class="smallexample">
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</pre><pre class="smallexample">struct command commands[] =
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{
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{ "quit", quit_command },
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{ "help", help_command },
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…
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};
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</pre></div>
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<p>It would be cleaner not to have to give each command name twice, once in
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the string constant and once in the function name. A macro which takes the
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name of a command as an argument can make this unnecessary. The string
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constant can be created with stringizing, and the function name by
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concatenating the argument with ‘<samp>_command</samp>’. Here is how it is done:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define COMMAND(NAME) { #NAME, NAME ## _command }
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struct command commands[] =
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{
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COMMAND (quit),
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COMMAND (help),
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…
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};
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringizing.html#Stringizing" accesskey="p" rel="prev">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>
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</div>
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</body>
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</html>
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