20.4 Referring to a Type with __auto_type

You can declare a variable copying the type from the initializer by using __auto_type instead of a particular type. Here’s an example:

#define max(a,b) \
  ({ __auto_type _a = (a); \
      __auto_type _b = (b); \
    _a > _b ? _a : _b })

This defines _a to be of the same type as a, and _b to be of the same type as b. This is a useful thing to do in a macro that ought to be able to handle any type of data (see Using __auto_type for Local Variables).

The original GNU C method for obtaining the type of a value is to use typeof, which takes as an argument either a value or the name of a type. The previous example could also be written as:

#define max(a,b) \
  ({ typeof(a) _a = (a); \
      typeof(b) _b = (b); \
    _a > _b ? _a : _b })

typeof is more flexible than __auto_type; however, the principal use case for typeof is in variable declarations with initialization, which is exactly what __auto_type handles.