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| <h3 class="section">6.24 Compound Literals</h3> |
| |
| <p><a name="index-constructor-expressions-2314"></a><a name="index-initializations-in-expressions-2315"></a><a name="index-structures_002c-constructor-expression-2316"></a><a name="index-expressions_002c-constructor-2317"></a><a name="index-compound-literals-2318"></a><!-- The GNU C name for what C99 calls compound literals was "constructor expressions". --> |
| |
| <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++. |
| |
| <p>Usually, the specified type is a structure. Assume that |
| <code>struct foo</code> and <code>structure</code> are declared as shown: |
| |
| <pre class="smallexample"> struct foo {int a; char b[2];} structure; |
| </pre> |
| <p class="noindent">Here is an example of constructing a <code>struct foo</code> with a compound literal: |
| |
| <pre class="smallexample"> structure = ((struct foo) {x + y, 'a', 0}); |
| </pre> |
| <p class="noindent">This is equivalent to writing the following: |
| |
| <pre class="smallexample"> { |
| struct foo temp = {x + y, 'a', 0}; |
| structure = temp; |
| } |
| </pre> |
| <p>You can also construct an array. 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: |
| |
| <pre class="smallexample"> char **foo = (char *[]) { "x", "y", "z" }; |
| </pre> |
| <p>Compound literals for scalar types and union types are is |
| also allowed, but then the compound literal is equivalent |
| to a cast. |
| |
| <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 was 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. |
| |
| <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> |
| <p class="noindent">The above lines are equivalent to the following: |
| <pre class="smallexample"> static struct foo x = {1, 'a', 'b'}; |
| static int y[] = {1, 2, 3}; |
| static int z[] = {1, 0, 0}; |
| </pre> |
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