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| 22 | <title>Concatenation (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Concatenation (GNU C Language Manual)">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Macros.html" rel="up" title="Macros">
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| 33 | <link href="Variadic-Macros.html" rel="next" title="Variadic Macros">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Concatenation"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Variadic-Macros.html" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringification.html" accesskey="p" rel="prev">Stringification</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Concatenation-1"></span><h4 class="subsection">26.5.5 Concatenation</h4>
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| 64 | <span id="index-concatenation"></span>
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| 65 | <span id="index-token-pasting"></span>
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| 66 | <span id="index-token-concatenation"></span>
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| 67 | <span id="index-_0023_0023-operator"></span>
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| 68 |
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| 69 | <p>It is often useful to merge two tokens into one while expanding macros.
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| 70 | This is called <em>token pasting</em> or <em>token concatenation</em>. The
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| 71 | <code>##</code> preprocessing operator performs token pasting. When a macro
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| 72 | is expanded, the two tokens on either side of each <code>##</code> operator
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| 73 | are combined into a single token, which then replaces the <code>##</code> and
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| 74 | the two original tokens in the macro expansion. Usually both will be
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| 75 | identifiers, or one will be an identifier and the other a preprocessing
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| 76 | number. When pasted, they make a longer identifier.
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| 77 | </p>
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| 78 | <p>Concatenation into an identifier isn’t the only valid case. It is
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| 79 | also possible to concatenate two numbers (or a number and a name, such
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| 80 | as <code>1.5</code> and <code>e3</code>) into a number. Also, multi-character
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| 81 | operators such as <code>+=</code> can be formed by token pasting.
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| 82 | </p>
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| 83 | <p>However, two tokens that don’t together form a valid token cannot be
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| 84 | pasted together. For example, you cannot concatenate <code>x</code> with
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| 85 | <code>+</code>, not in either order. Trying this issues a warning and keeps
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| 86 | the two tokens separate. Whether it puts white space between the
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| 87 | tokens is undefined. It is common to find unnecessary uses of
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| 88 | <code>##</code> in complex macros. If you get this warning, it is likely
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| 89 | that you can simply remove the <code>##</code>.
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| 90 | </p>
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| 91 | <p>The tokens combined by <code>##</code> could both come from the macro body,
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| 92 | but then you could just as well write them as one token in the first place.
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| 93 | Token pasting is useful when one or both of the tokens comes from a
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| 94 | macro argument. If either of the tokens next to an <code>##</code> is a
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| 95 | parameter name, it is replaced by its actual argument before <code>##</code>
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| 96 | executes. As with stringification, the actual argument is not
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| 97 | macro-expanded first. If the argument is empty, that <code>##</code> has no
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| 98 | effect.
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| 99 | </p>
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| 100 | <p>Keep in mind that preprocessing converts comments to whitespace before
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| 101 | it looks for uses of macros. Therefore, you cannot create a comment
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| 102 | by concatenating ‘<samp>/</samp>’ and ‘<samp>*</samp>’. You can put as much
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| 103 | whitespace between <code>##</code> and its operands as you like, including
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| 104 | comments, and you can put comments in arguments that will be
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| 105 | concatenated.
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| 106 | </p>
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| 107 | <p>It is an error to use <code>##</code> at the beginning or end of a macro
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| 108 | body.
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| 109 | </p>
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| 110 | <p>Multiple <code>##</code> operators are handled left-to-right, so that
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| 111 | ‘<samp>1 ## e ## -2</samp>’ pastes into ‘<samp>1e-2</samp>’. (Right-to-left
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| 112 | processing would first generate ‘<samp>e-2</samp>’, which is an invalid token.)
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| 113 | When <code>#</code> and <code>##</code> are used together, they are all handled
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| 114 | left-to-right.
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| 115 | </p>
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| 116 | <p>Consider a C program that interprets named commands. There probably
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| 117 | needs to be a table of commands, perhaps an array of structures declared
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| 118 | as follows:
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| 119 | </p>
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| 120 | <div class="example">
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| 121 | <pre class="example">struct command
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| 122 | {
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| 123 | char *name;
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| 124 | void (*function) (void);
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| 125 | };
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| 126 | </pre><pre class="example">
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| 127 |
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| 128 | </pre><pre class="example">struct command commands[] =
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| 129 | {
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| 130 | { "quit", quit_command },
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| 131 | { "help", help_command },
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| 132 | /* <span class="roman">…</span> */
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| 133 | };
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| 134 | </pre></div>
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| 135 |
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| 136 | <p>It would be cleaner not to have to write each command name twice, once
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| 137 | in the string constant and once in the function name. A macro that
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| 138 | takes the name of a command as an argument can make this unnecessary.
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| 139 | It can create the string constant with stringification, and the
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| 140 | function name by concatenating the argument with ‘<samp>_command</samp>’.
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| 141 | Here is how it is done:
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| 142 | </p>
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| 143 | <div class="example">
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| 144 | <pre class="example">#define COMMAND(NAME) { #NAME, NAME ## _command }
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| 145 |
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| 146 | struct command commands[] =
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| 147 | {
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| 148 | COMMAND (quit),
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| 149 | COMMAND (help),
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| 150 | /* <span class="roman">…</span> */
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| 151 | };
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| 152 | </pre></div>
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| 153 |
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| 154 | <hr>
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| 155 | <div class="header">
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| 156 | <p>
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| 157 | Next: <a href="Variadic-Macros.html" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringification.html" accesskey="p" rel="prev">Stringification</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 158 | </div>
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