📄 typeof.html

📁 自己收集的linux入门到学懂高级编程书集包括linux程序设计第三版
💻 HTML
字号:
<html lang="en"><head><title>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 4.6"><!--Copyright &copy; 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.   <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below).  A copy of the license isincluded in the section entitled "GNU Free Documentation License".   <p>(a) The FSF's Front-Cover Text is:   <p>A GNU Manual   <p>(b) The FSF's Back-Cover Text is:   <p>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 }--></style></head><body><div class="node"><p>Node:&nbsp;<a name="Typeof">Typeof</a>,Next:&nbsp;<a rel="next" accesskey="n" href="Conditionals.html#Conditionals">Conditionals</a>,Previous:&nbsp;<a rel="previous" accesskey="p" href="Constructing-Calls.html#Constructing%20Calls">Constructing Calls</a>,Up:&nbsp;<a rel="up" accesskey="u" href="C-Extensions.html#C%20Extensions">C Extensions</a><hr><br></div><h3 class="section">Referring to a Type with <code>typeof</code></h3><p>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 theconstruct 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 anexpression or with a type.  Here is an example with an expression:<pre class="smallexample">     typeof (x[0](1))     </pre><p>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>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 Cprograms, write <code>__typeof__</code> instead of <code>typeof</code>. See <a href="Alternate-Keywords.html#Alternate%20Keywords">Alternate Keywords</a>.   <p>A <code>typeof</code>-construct can be used anywhere a typedef name could beused.  For example, you can use it in a declaration, in a cast, or insideof <code>sizeof</code> or <code>typeof</code>.   <p><code>typeof</code> is often useful in conjunction with thestatements-within-expressions feature.  Here is how the two together canbe used to define a safe "maximum" macro that operates on anyarithmetic 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 &gt; _b ? _a : _b; })     </pre>   <p>The reason for using names that start with underscores for the localvariables is to avoid conflicts with variable names that occur within theexpressions that are substituted for <code>a</code> and <code>b</code>.  Eventually wehope to design a new form of declaration syntax that allows you to declarevariables whose scopes start only after their initializers; this will be amore reliable way to prevent such conflicts.<p>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>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 itmight be a useful way to write, let's rewrite it with these macros:     <pre class="smallexample">          #define pointer(T)  typeof(T *)          #define array(T, N) typeof(T [N])          </pre>     <p>Now the declaration can be rewritten this way:     <pre class="smallexample">          array (pointer (char), 4) y;          </pre>     <p>Thus, <code>array (pointer (char), 4)</code> is the type of arrays of 4pointers to <code>char</code>. </ul>   <p><em>Compatibility Note:</em> In addition to <code>typeof</code>, GCC 2 supporteda more limited extension which permitted one to write<pre class="smallexample">     typedef <var>T</var> = <var>expr</var>;     </pre><p>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 between3.0 and 3.2 will crash; 3.2.1 and later give an error).  Code whichrelies on it should be rewritten to use <code>typeof</code>:<pre class="smallexample">     typedef typeof(<var>expr</var>) <var>T</var>;     </pre><p>This will work with all versions of GCC.   </body></html>
⌨️ 快捷键说明

复制代码 Ctrl + C
搜索代码 Ctrl + F
全屏模式 F11
切换主题 Ctrl + Shift + D
显示快捷键 ?
增大字号 Ctrl + =
减小字号 Ctrl + -