Using and Porting GNU CC

5.7: Referring to a Type with typeof

Another way to refer to the type of an expression is with typeof. The syntax of using of this keyword looks like sizeof, but the construct acts semantically like a type name defined with typedef.

There are two ways of writing the argument to typeof: with an expression or with a type. Here is an example with an expression:

typeof (x[0](1))

This assumes that x is an array of functions; the type described is that of the values of the functions.

Here is an example with a typename as the argument:

typeof (int *)

Here the type described is that of pointers to int.

If you are writing a header file that must work when included in ANSI C programs, write __typeof__ instead of typeof. See Alternate Keywords.

A typeof-construct can be used anywhere a typedef name could be used. For example, you can use it in a declaration, in a cast, or inside of sizeof or typeof.

• This declares y with the type of what x points to.

  typeof (*x) y;
  

• This declares y as an array of such values.

  typeof (*x) y[4];
  

• This declares y as an array of pointers to characters:

  typeof (typeof (char *)[4]) y;
  

  It is equivalent to the following traditional C declaration:

  char *y[4];
  

  To see the meaning of the declaration using typeof, and why it might be a useful way to write, let's rewrite it with these macros:

  #define pointer(T)  typeof(T *)
  #define array(T, N) typeof(T [N])
  

  Now the declaration can be rewritten this way:

  array (pointer (char), 4) y;
  

  Thus, array (pointer (char), 4) is the type of arrays of 4 pointers to char.