13.2 Type References

Another way to refer to the type of an expression is with the typeof keyword. This is a non-standard extension to the language. Using this feature reduces your code portability.

The syntax for using 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 a pointer to int.

If you are writing a header file that must work when included in ANSI C programs, write _ _typeof_ _ instead of typeof.

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 four pointers to char.