26.5.10.5 Using __auto_type for Local Variables

The operator __auto_type makes it possible to define macros that can work on any data type even though they need to generate local variable declarations. See Auto Type.

For instance, here’s how to define a safe “maximum” macro that operates on any arithmetic type and computes each of its arguments exactly once:

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

The ‘({ … })’ notation produces statement expression—a statement that can be used as an expression (see Statement Exprs). Its value is the value of its last statement. This permits us to define local variables and store each argument value into one.

The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a and b. Underscore followed by a lower case letter won’t be predefined by the system in any way.