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4.19 Constructor Expressions

GNU C supports constructor expressions. A constructor 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.

Usually, the specified type is a structure. Assume that struct foo and structure are declared as shown:

struct foo {int a; char b[2];} structure;

Here is an example of constructing a struct foo with a constructor:

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 constructor are (made up of) simple constant expressions, suitable for use in initializers, then the constructor is an lvalue and can be coerced to a pointer to its first element, as shown here:

char **foo = (char *[]) { "x", "y", "z" };

Array constructors whose elements are not simple constants are not very useful, because the constructor is not an lvalue. There are only two valid ways to use it: to subscript it, or initialize an array variable with it. The former is probably slower than a switch statement, while the latter does the same thing an ordinary C initializer would do. Here is an example of subscripting an array constructor:

output = ((int[]) { 2, x, 28 }) [input];

Constructor expressions for scalar types and union types are is also allowed, but then the constructor expression is equivalent to a cast.