| <html lang="en"> |
| <head> |
| <title>Typeof - Using the GNU Compiler Collection (GCC)</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Using the GNU Compiler Collection (GCC)"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> |
| <link rel="prev" href="Constructing-Calls.html#Constructing-Calls" title="Constructing Calls"> |
| <link rel="next" href="Conditionals.html#Conditionals" title="Conditionals"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
| 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008 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 1.2 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Funding Free Software'', the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| ``GNU Free Documentation License''. |
| |
| (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.--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Typeof"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Conditionals.html#Conditionals">Conditionals</a>, |
| Previous: <a rel="previous" accesskey="p" href="Constructing-Calls.html#Constructing-Calls">Constructing Calls</a>, |
| Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">6.6 Referring to a Type with <code>typeof</code></h3> |
| |
| <p><a name="index-typeof-2201"></a><a name="index-sizeof-2202"></a><a name="index-macros_002c-types-of-arguments-2203"></a> |
| Another way to refer to the type of an expression is with <code>typeof</code>. |
| The syntax of using of this keyword looks like <code>sizeof</code>, but the |
| construct acts semantically like a type name defined with <code>typedef</code>. |
| |
| <p>There are two ways of writing the argument to <code>typeof</code>: with an |
| expression or with a type. Here is an example with an expression: |
| |
| <pre class="smallexample"> typeof (x[0](1)) |
| </pre> |
| <p class="noindent">This assumes that <code>x</code> is an array of pointers to functions; |
| the type described is that of the values of the functions. |
| |
| <p>Here is an example with a typename as the argument: |
| |
| <pre class="smallexample"> typeof (int *) |
| </pre> |
| <p class="noindent">Here the type described is that of pointers to <code>int</code>. |
| |
| <p>If you are writing a header file that must work when included in ISO C |
| programs, write <code>__typeof__</code> instead of <code>typeof</code>. |
| See <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>. |
| |
| <p>A <code>typeof</code>-construct can be used anywhere a typedef name could be |
| used. For example, you can use it in a declaration, in a cast, or inside |
| of <code>sizeof</code> or <code>typeof</code>. |
| |
| <p>The operand of <code>typeof</code> is evaluated for its side effects if and |
| only if it is an expression of variably modified type or the name of |
| such a type. |
| |
| <p><code>typeof</code> is often useful in conjunction with the |
| statements-within-expressions feature. Here is how the two together can |
| be used to define a safe “maximum” macro that operates on any |
| arithmetic type and evaluates each of its arguments exactly once: |
| |
| <pre class="smallexample"> #define max(a,b) \ |
| ({ typeof (a) _a = (a); \ |
| typeof (b) _b = (b); \ |
| _a > _b ? _a : _b; }) |
| </pre> |
| <p><a name="index-underscores-in-variables-in-macros-2204"></a><a name="index-g_t_0040samp_007b_005f_007d-in-variables-in-macros-2205"></a><a name="index-local-variables-in-macros-2206"></a><a name="index-variables_002c-local_002c-in-macros-2207"></a><a name="index-macros_002c-local-variables-in-2208"></a> |
| The reason for using names that start with underscores for the local |
| variables is to avoid conflicts with variable names that occur within the |
| expressions that are substituted for <code>a</code> and <code>b</code>. Eventually we |
| hope to design a new form of declaration syntax that allows you to declare |
| variables whose scopes start only after their initializers; this will be a |
| more reliable way to prevent such conflicts. |
| |
| <p class="noindent">Some more examples of the use of <code>typeof</code>: |
| |
| <ul> |
| <li>This declares <code>y</code> with the type of what <code>x</code> points to. |
| |
| <pre class="smallexample"> typeof (*x) y; |
| </pre> |
| <li>This declares <code>y</code> as an array of such values. |
| |
| <pre class="smallexample"> typeof (*x) y[4]; |
| </pre> |
| <li>This declares <code>y</code> as an array of pointers to characters: |
| |
| <pre class="smallexample"> typeof (typeof (char *)[4]) y; |
| </pre> |
| <p class="noindent">It is equivalent to the following traditional C declaration: |
| |
| <pre class="smallexample"> char *y[4]; |
| </pre> |
| <p>To see the meaning of the declaration using <code>typeof</code>, and why it |
| might be a useful way to write, rewrite it with these macros: |
| |
| <pre class="smallexample"> #define pointer(T) typeof(T *) |
| #define array(T, N) typeof(T [N]) |
| </pre> |
| <p class="noindent">Now the declaration can be rewritten this way: |
| |
| <pre class="smallexample"> array (pointer (char), 4) y; |
| </pre> |
| <p class="noindent">Thus, <code>array (pointer (char), 4)</code> is the type of arrays of 4 |
| pointers to <code>char</code>. |
| </ul> |
| |
| <p><em>Compatibility Note:</em> In addition to <code>typeof</code>, GCC 2 supported |
| a more limited extension which permitted one to write |
| |
| <pre class="smallexample"> typedef <var>T</var> = <var>expr</var>; |
| </pre> |
| <p class="noindent">with the effect of declaring <var>T</var> to have the type of the expression |
| <var>expr</var>. This extension does not work with GCC 3 (versions between |
| 3.0 and 3.2 will crash; 3.2.1 and later give an error). Code which |
| relies on it should be rewritten to use <code>typeof</code>: |
| |
| <pre class="smallexample"> typedef typeof(<var>expr</var>) <var>T</var>; |
| </pre> |
| <p class="noindent">This will work with all versions of GCC. |
| |
| </body></html> |
| |
