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132 lines
6.2 KiB
HTML
132 lines
6.2 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Concatenation - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Macros.html#Macros" title="Macros">
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<link rel="prev" href="Stringification.html#Stringification" title="Stringification">
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<link rel="next" href="Variadic-Macros.html#Variadic-Macros" title="Variadic Macros">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1987-2015 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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span.sansserif { font-family:sans-serif; font-weight:normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Concatenation"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Variadic-Macros.html#Variadic-Macros">Variadic Macros</a>,
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Previous: <a rel="previous" accesskey="p" href="Stringification.html#Stringification">Stringification</a>,
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Up: <a rel="up" accesskey="u" href="Macros.html#Macros">Macros</a>
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<hr>
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</div>
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<h3 class="section">3.5 Concatenation</h3>
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<p><a name="index-concatenation-52"></a><a name="index-token-pasting-53"></a><a name="index-token-concatenation-54"></a><a name="index-g_t_0040samp_007b_0023_0023_007d-operator-55"></a>
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It is often useful to merge two tokens into one while expanding macros.
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This is called <dfn>token pasting</dfn> or <dfn>token concatenation</dfn>. The
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‘<samp><span class="samp">##</span></samp>’ preprocessing operator performs token pasting. When a macro
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is expanded, the two tokens on either side of each ‘<samp><span class="samp">##</span></samp>’ operator
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are combined into a single token, which then replaces the ‘<samp><span class="samp">##</span></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>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><span class="samp">##</span></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><span class="samp">##</span></samp>’.
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<p>Both the tokens combined by ‘<samp><span class="samp">##</span></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><span class="samp">##</span></samp>’ is a
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parameter name, it is replaced by its actual argument before ‘<samp><span class="samp">##</span></samp>’
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executes. As with stringification, the actual argument is not
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macro-expanded first. If the argument is empty, that ‘<samp><span class="samp">##</span></samp>’ has no
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effect.
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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><span class="samp">/</span></samp>’ and ‘<samp><span class="samp">*</span></samp>’. You can put as much
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whitespace between ‘<samp><span class="samp">##</span></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><span class="samp">##</span></samp>’ appears at either
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end of a macro body.
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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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<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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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>
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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 stringification, and the function name by
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concatenating the argument with ‘<samp><span class="samp">_command</span></samp>’. Here is how it is done:
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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>
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</body></html>
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