The mechanism of typedef in c/c++ is substitution of symbol.In detail, the declaration of typedef consist with undeclared symbol and declared symbol, the undeclared symbol is just the new type.Using new type to declare identifier means substituting the new type symbol by identifier.Such as bellow:

typedef void (*func)(void);
func f = NULL;  //!< means `void (*f)(void) = NULL;`

typedef unsigned long size_t;
size_t s = 0;  //!< means `unsigend long s = 0;`