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:
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 Section 2.35.
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;
This declares y as an array of pointers to characters:
typeof (typeof (char *)) y;
It is equivalent to the following traditional C declaration:
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.