When should I use typedef in C++?

c++ typedef
djeidot picture djeidot · Feb 5, 2009 · Viewed 56.9k times · Source

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

j_random_hacker picture j_random_hacker · Feb 5, 2009

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) {
    ...
}

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