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<title>Using the GNU Compiler Collection (GCC): Typeof</title>
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<a name="Typeof"></a>
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<div class="header">
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<p>
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Next: <a href="Conditionals.html#Conditionals" accesskey="n" rel="next">Conditionals</a>, Previous: <a href="Constructing-Calls.html#Constructing-Calls" accesskey="p" rel="prev">Constructing Calls</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Referring-to-a-Type-with-typeof"></a>
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<h3 class="section">6.6 Referring to a Type with <code>typeof</code></h3>
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<a name="index-typeof"></a>
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<a name="index-sizeof"></a>
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<a name="index-macros_002c-types-of-arguments"></a>
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<p>Another way to refer to the type of an expression is with <code>typeof</code>.
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The syntax of using of this keyword looks like <code>sizeof</code>, but the
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construct acts semantically like a type name defined with <code>typedef</code>.
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</p>
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<p>There are two ways of writing the argument to <code>typeof</code>: with an
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expression or with a type. Here is an example with an expression:
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</p>
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<div class="smallexample">
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<pre class="smallexample">typeof (x[0](1))
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</pre></div>
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<p>This assumes that <code>x</code> is an array of pointers to functions;
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the type described is that of the values of the functions.
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</p>
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<p>Here is an example with a typename as the argument:
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</p>
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<div class="smallexample">
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<pre class="smallexample">typeof (int *)
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</pre></div>
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<p>Here the type described is that of pointers to <code>int</code>.
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</p>
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<p>If you are writing a header file that must work when included in ISO C
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programs, write <code>__typeof__</code> instead of <code>typeof</code>.
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See <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>.
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</p>
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<p>A <code>typeof</code> construct can be used anywhere a typedef name can be
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used. For example, you can use it in a declaration, in a cast, or inside
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of <code>sizeof</code> or <code>typeof</code>.
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</p>
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<p>The operand of <code>typeof</code> is evaluated for its side effects if and
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only if it is an expression of variably modified type or the name of
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such a type.
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</p>
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<p><code>typeof</code> is often useful in conjunction with
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statement expressions (see <a href="Statement-Exprs.html#Statement-Exprs">Statement Exprs</a>).
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Here is how the two together can
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be used to define a safe “maximum” macro which operates on any
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arithmetic type and evaluates each of its arguments exactly once:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define max(a,b) \
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({ typeof (a) _a = (a); \
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typeof (b) _b = (b); \
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_a > _b ? _a : _b; })
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</pre></div>
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<a name="index-underscores-in-variables-in-macros"></a>
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<a name="index-_005f-in-variables-in-macros"></a>
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<a name="index-local-variables-in-macros"></a>
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<a name="index-variables_002c-local_002c-in-macros"></a>
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<a name="index-macros_002c-local-variables-in"></a>
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<p>The reason for using names that start with underscores for the local
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variables is to avoid conflicts with variable names that occur within the
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expressions that are substituted for <code>a</code> and <code>b</code>. Eventually we
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hope to design a new form of declaration syntax that allows you to declare
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variables whose scopes start only after their initializers; this will be a
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more reliable way to prevent such conflicts.
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</p>
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<p>Some more examples of the use of <code>typeof</code>:
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</p>
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<ul>
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<li> This declares <code>y</code> with the type of what <code>x</code> points to.
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<div class="smallexample">
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<pre class="smallexample">typeof (*x) y;
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</pre></div>
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</li><li> This declares <code>y</code> as an array of such values.
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<div class="smallexample">
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<pre class="smallexample">typeof (*x) y[4];
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</pre></div>
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</li><li> This declares <code>y</code> as an array of pointers to characters:
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<div class="smallexample">
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<pre class="smallexample">typeof (typeof (char *)[4]) y;
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</pre></div>
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<p>It is equivalent to the following traditional C declaration:
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</p>
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<div class="smallexample">
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<pre class="smallexample">char *y[4];
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</pre></div>
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<p>To see the meaning of the declaration using <code>typeof</code>, and why it
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might be a useful way to write, rewrite it with these macros:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define pointer(T) typeof(T *)
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#define array(T, N) typeof(T [N])
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</pre></div>
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<p>Now the declaration can be rewritten this way:
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</p>
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<div class="smallexample">
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<pre class="smallexample">array (pointer (char), 4) y;
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</pre></div>
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<p>Thus, <code>array (pointer (char), 4)</code> is the type of arrays of 4
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pointers to <code>char</code>.
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</p></li></ul>
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<p>In GNU C, but not GNU C++, you may also declare the type of a variable
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as <code>__auto_type</code>. In that case, the declaration must declare
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only one variable, whose declarator must just be an identifier, the
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declaration must be initialized, and the type of the variable is
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determined by the initializer; the name of the variable is not in
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scope until after the initializer. (In C++, you should use C++11
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<code>auto</code> for this purpose.) Using <code>__auto_type</code>, the
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“maximum” macro above could be written as:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define max(a,b) \
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({ __auto_type _a = (a); \
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__auto_type _b = (b); \
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_a > _b ? _a : _b; })
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</pre></div>
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<p>Using <code>__auto_type</code> instead of <code>typeof</code> has two advantages:
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</p>
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<ul>
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<li> Each argument to the macro appears only once in the expansion of
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the macro. This prevents the size of the macro expansion growing
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exponentially when calls to such macros are nested inside arguments of
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such macros.
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</li><li> If the argument to the macro has variably modified type, it is
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evaluated only once when using <code>__auto_type</code>, but twice if
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<code>typeof</code> is used.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Conditionals.html#Conditionals" accesskey="n" rel="next">Conditionals</a>, Previous: <a href="Constructing-Calls.html#Constructing-Calls" accesskey="p" rel="prev">Constructing Calls</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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