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134 lines
6.5 KiB
HTML
134 lines
6.5 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Compound Literals - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Initializers.html#Initializers" title="Initializers">
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<link rel="next" href="Designated-Inits.html#Designated-Inits" title="Designated Inits">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Compound-Literals"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Designated-Inits.html#Designated-Inits">Designated Inits</a>,
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Previous: <a rel="previous" accesskey="p" href="Initializers.html#Initializers">Initializers</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.26 Compound Literals</h3>
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<p><a name="index-constructor-expressions-3016"></a><a name="index-initializations-in-expressions-3017"></a><a name="index-structures_002c-constructor-expression-3018"></a><a name="index-expressions_002c-constructor-3019"></a><a name="index-compound-literals-3020"></a><!-- The GNU C name for what C99 calls compound literals was "constructor expressions". -->
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<p>ISO C99 supports compound literals. A compound literal looks like
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a cast containing an initializer. Its value is an object of the
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type specified in the cast, containing the elements specified in
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the initializer; it is an lvalue. As an extension, GCC supports
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compound literals in C90 mode and in C++, though the semantics are
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somewhat different in C++.
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<p>Usually, the specified type is a structure. Assume that
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<code>struct foo</code> and <code>structure</code> are declared as shown:
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<pre class="smallexample"> struct foo {int a; char b[2];} structure;
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</pre>
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<p class="noindent">Here is an example of constructing a <code>struct foo</code> with a compound literal:
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<pre class="smallexample"> structure = ((struct foo) {x + y, 'a', 0});
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</pre>
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<p class="noindent">This is equivalent to writing the following:
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<pre class="smallexample"> {
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struct foo temp = {x + y, 'a', 0};
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structure = temp;
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}
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</pre>
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<p>You can also construct an array, though this is dangerous in C++, as
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explained below. If all the elements of the compound literal are
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(made up of) simple constant expressions, suitable for use in
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initializers of objects of static storage duration, then the compound
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literal can be coerced to a pointer to its first element and used in
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such an initializer, as shown here:
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<pre class="smallexample"> char **foo = (char *[]) { "x", "y", "z" };
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</pre>
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<p>Compound literals for scalar types and union types are
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also allowed, but then the compound literal is equivalent
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to a cast.
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<p>As a GNU extension, GCC allows initialization of objects with static storage
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duration by compound literals (which is not possible in ISO C99, because
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the initializer is not a constant).
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It is handled as if the object is initialized only with the bracket
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enclosed list if the types of the compound literal and the object match.
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The initializer list of the compound literal must be constant.
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If the object being initialized has array type of unknown size, the size is
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determined by compound literal size.
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<pre class="smallexample"> static struct foo x = (struct foo) {1, 'a', 'b'};
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static int y[] = (int []) {1, 2, 3};
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static int z[] = (int [3]) {1};
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</pre>
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<p class="noindent">The above lines are equivalent to the following:
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<pre class="smallexample"> static struct foo x = {1, 'a', 'b'};
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static int y[] = {1, 2, 3};
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static int z[] = {1, 0, 0};
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</pre>
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<p>In C, a compound literal designates an unnamed object with static or
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automatic storage duration. In C++, a compound literal designates a
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temporary object, which only lives until the end of its
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full-expression. As a result, well-defined C code that takes the
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address of a subobject of a compound literal can be undefined in C++,
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so the C++ compiler rejects the conversion of a temporary array to a pointer.
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For instance, if the array compound literal example above appeared
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inside a function, any subsequent use of ‘<samp><span class="samp">foo</span></samp>’ in C++ has
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undefined behavior because the lifetime of the array ends after the
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declaration of ‘<samp><span class="samp">foo</span></samp>’.
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<p>As an optimization, the C++ compiler sometimes gives array compound
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literals longer lifetimes: when the array either appears outside a
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function or has const-qualified type. If ‘<samp><span class="samp">foo</span></samp>’ and its
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initializer had elements of ‘<samp><span class="samp">char *const</span></samp>’ type rather than
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‘<samp><span class="samp">char *</span></samp>’, or if ‘<samp><span class="samp">foo</span></samp>’ were a global variable, the array
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would have static storage duration. But it is probably safest just to
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avoid the use of array compound literals in code compiled as C++.
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</body></html>
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