When should I use typedef in C++?

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) {
    ...
}

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.