In my years of C++ (MFC) programming in I never felt the need to use typedef, so I don’t really know what is it used for. Where should I use it? Are there any real situations where the use of typedef is preferred? Or is this really more a C-specific keyword?
Answer
Template Metaprogramming
typedef is necessary for many template metaprogramming tasks — whenever a class is treated as a “compile-time type function”, a typedef is used as a “compile-time type value” to obtain the resulting type. E.g. consider a simple metafunction for converting a pointer type to its base type:
template<typename T>
struct strip_pointer_from;
template<typename T>
struct strip_pointer_from<T*> { // Partial specialisation for pointer types
typedef T type;
};
Example: the type expression strip_pointer_from<double*>::type evaluates to double. Note that template metaprogramming is not commonly used outside of library development.
Simplifying Function Pointer Types
typedef is helpful for giving a short, sharp alias to complicated function pointer types:
typedef int (*my_callback_function_type)(int, double, std::string);
void RegisterCallback(my_callback_function_type fn) {
...
}