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| <title>Using the GNU Compiler Collection (GCC): Compound Literals</title> |
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| <a name="Compound-Literals"></a> |
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| <a name="Compound-Literals-1"></a> |
| <h3 class="section">6.25 Compound Literals</h3> |
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| <a name="index-compound-literals"></a> |
| |
| <p>ISO C99 supports compound literals. A compound literal looks like |
| a cast containing an initializer. Its value is an object of the |
| type specified in the cast, containing the elements specified in |
| the initializer; it is an lvalue. As an extension, GCC supports |
| compound literals in C90 mode and in C++, though the semantics are |
| somewhat different in C++. |
| </p> |
| <p>Usually, the specified type is a structure. Assume that |
| <code>struct foo</code> and <code>structure</code> are declared as shown: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">struct foo {int a; char b[2];} structure; |
| </pre></div> |
| |
| <p>Here is an example of constructing a <code>struct foo</code> with a compound literal: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">structure = ((struct foo) {x + y, 'a', 0}); |
| </pre></div> |
| |
| <p>This is equivalent to writing the following: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">{ |
| struct foo temp = {x + y, 'a', 0}; |
| structure = temp; |
| } |
| </pre></div> |
| |
| <p>You can also construct an array, though this is dangerous in C++, as |
| explained below. If all the elements of the compound literal are |
| (made up of) simple constant expressions, suitable for use in |
| initializers of objects of static storage duration, then the compound |
| literal can be coerced to a pointer to its first element and used in |
| such an initializer, as shown here: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">char **foo = (char *[]) { "x", "y", "z" }; |
| </pre></div> |
| |
| <p>Compound literals for scalar types and union types are |
| also allowed, but then the compound literal is equivalent |
| to a cast. |
| </p> |
| <p>As a GNU extension, GCC allows initialization of objects with static storage |
| duration by compound literals (which is not possible in ISO C99, because |
| the initializer is not a constant). |
| It is handled as if the object is initialized only with the bracket |
| enclosed list if the types of the compound literal and the object match. |
| The initializer list of the compound literal must be constant. |
| If the object being initialized has array type of unknown size, the size is |
| determined by compound literal size. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">static struct foo x = (struct foo) {1, 'a', 'b'}; |
| static int y[] = (int []) {1, 2, 3}; |
| static int z[] = (int [3]) {1}; |
| </pre></div> |
| |
| <p>The above lines are equivalent to the following: |
| </p><div class="smallexample"> |
| <pre class="smallexample">static struct foo x = {1, 'a', 'b'}; |
| static int y[] = {1, 2, 3}; |
| static int z[] = {1, 0, 0}; |
| </pre></div> |
| |
| <p>In C, a compound literal designates an unnamed object with static or |
| automatic storage duration. In C++, a compound literal designates a |
| temporary object, which only lives until the end of its |
| full-expression. As a result, well-defined C code that takes the |
| address of a subobject of a compound literal can be undefined in C++. |
| For instance, if the array compound literal example above appeared |
| inside a function, any subsequent use of ‘<samp>foo</samp>’ in C++ has |
| undefined behavior because the lifetime of the array ends after the |
| declaration of ‘<samp>foo</samp>’. As a result, the C++ compiler now rejects |
| the conversion of a temporary array to a pointer. |
| </p> |
| <p>As an optimization, the C++ compiler sometimes gives array compound |
| literals longer lifetimes: when the array either appears outside a |
| function or has const-qualified type. If ‘<samp>foo</samp>’ and its |
| initializer had elements of ‘<samp>char *const</samp>’ type rather than |
| ‘<samp>char *</samp>’, or if ‘<samp>foo</samp>’ were a global variable, the array |
| would have static storage duration. But it is probably safest just to |
| avoid the use of array compound literals in code compiled as C++. |
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