Compound Literals - Red Hat ENTERPRISE LINUX 4 Manual

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6.20. Compound Literals

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 C89 mode and in C++.
Usually, the specified type is a structure. Assume that
shown:
struct foo {int a; char b[2];} structure;
Here is an example of constructing a
structure = ((struct foo) {x + y, 'a', 0});
This is equivalent to writing the following:
{
struct foo temp = {x + y, 'a', 0};
structure = temp;
}
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:
char **foo = (char *[]) { "x", "y", "z" };
Compound literals for scalar types and union types are is also allowed, but then the compound literal
is equivalent to a cast.
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 compound literal's and object types
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.
static struct foo x = (struct foo) {1, 'a', 'b'};
static int y[] = (int []) {1, 2, 3};
static int z[] = (int [3]) {1};
The above lines are equivalent to the following:
static struct foo x = {1, 'a', 'b'};
static int y[] = {1, 2, 3};
static int z[] = {1, 0, 0};
Chapter 6. Extensions to the C Language Family
struct foo
with a compound literal:
struct foo
and are declared as
structure