You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
169 lines
7.4 KiB
HTML
169 lines
7.4 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1987-2018 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 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Concatenation (The C Preprocessor)</title>
|
|
|
|
<meta name="description" content="Concatenation (The C Preprocessor)">
|
|
<meta name="keywords" content="Concatenation (The C Preprocessor)">
|
|
<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="prev" title="Stringizing">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
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.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<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="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>
|
|
</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="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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|