When should I use typedef in C++?

Question

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 1

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) {     ... } 

Answer 2

In Bjarne's book he states that you can use typedef to deal with portability problems between systems that have different integer sizes. (this is a paraphrase)

On a machine where sizeof(int) is 4 you can

typedef int int32; 

Then use int32 everywhere in your code. When you move to an implementation of C++ where sizeof(int) is 2, then you can just change the typdef

typedef long int32; 

and your program will still work on the new implementation.