1<html lang="en"> 2<head> 3<title>Concatenation - The C Preprocessor</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="The C Preprocessor"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Macros.html#Macros" title="Macros"> 9<link rel="prev" href="Stringification.html#Stringification" title="Stringification"> 10<link rel="next" href="Variadic-Macros.html#Variadic-Macros" title="Variadic Macros"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 141997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 152008, 2009, 2010, 2011 16Free Software Foundation, Inc. 17 18Permission is granted to copy, distribute and/or modify this document 19under the terms of the GNU Free Documentation License, Version 1.3 or 20any later version published by the Free Software Foundation. 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The 66‘<samp><span class="samp">##</span></samp>’ preprocessing operator performs token pasting. When a macro 67is expanded, the two tokens on either side of each ‘<samp><span class="samp">##</span></samp>’ operator 68are combined into a single token, which then replaces the ‘<samp><span class="samp">##</span></samp>’ and 69the two original tokens in the macro expansion. Usually both will be 70identifiers, or one will be an identifier and the other a preprocessing 71number. When pasted, they make a longer identifier. This isn't the 72only valid case. It is also possible to concatenate two numbers (or a 73number and a name, such as <code>1.5</code> and <code>e3</code>) into a number. 74Also, multi-character operators such as <code>+=</code> can be formed by 75token pasting. 76 77 <p>However, two tokens that don't together form a valid token cannot be 78pasted together. For example, you cannot concatenate <code>x</code> with 79<code>+</code> in either order. If you try, the preprocessor issues a warning 80and emits the two tokens. Whether it puts white space between the 81tokens is undefined. It is common to find unnecessary uses of ‘<samp><span class="samp">##</span></samp>’ 82in complex macros. If you get this warning, it is likely that you can 83simply remove the ‘<samp><span class="samp">##</span></samp>’. 84 85 <p>Both the tokens combined by ‘<samp><span class="samp">##</span></samp>’ could come from the macro body, 86but you could just as well write them as one token in the first place. 87Token pasting is most useful when one or both of the tokens comes from a 88macro argument. If either of the tokens next to an ‘<samp><span class="samp">##</span></samp>’ is a 89parameter name, it is replaced by its actual argument before ‘<samp><span class="samp">##</span></samp>’ 90executes. As with stringification, the actual argument is not 91macro-expanded first. If the argument is empty, that ‘<samp><span class="samp">##</span></samp>’ has no 92effect. 93 94 <p>Keep in mind that the C preprocessor converts comments to whitespace 95before macros are even considered. Therefore, you cannot create a 96comment by concatenating ‘<samp><span class="samp">/</span></samp>’ and ‘<samp><span class="samp">*</span></samp>’. You can put as much 97whitespace between ‘<samp><span class="samp">##</span></samp>’ and its operands as you like, including 98comments, and you can put comments in arguments that will be 99concatenated. However, it is an error if ‘<samp><span class="samp">##</span></samp>’ appears at either 100end of a macro body. 101 102 <p>Consider a C program that interprets named commands. There probably 103needs to be a table of commands, perhaps an array of structures declared 104as follows: 105 106<pre class="smallexample"> struct command 107 { 108 char *name; 109 void (*function) (void); 110 }; 111 112 struct command commands[] = 113 { 114 { "quit", quit_command }, 115 { "help", help_command }, 116 ... 117 }; 118</pre> 119 <p>It would be cleaner not to have to give each command name twice, once in 120the string constant and once in the function name. A macro which takes the 121name of a command as an argument can make this unnecessary. The string 122constant can be created with stringification, and the function name by 123concatenating the argument with ‘<samp><span class="samp">_command</span></samp>’. Here is how it is done: 124 125<pre class="smallexample"> #define COMMAND(NAME) { #NAME, NAME ## _command } 126 127 struct command commands[] = 128 { 129 COMMAND (quit), 130 COMMAND (help), 131 ... 132 }; 133</pre> 134 </body></html> 135 136