Metafunction

Metafunction is a more general idiom than type generator. Metafunctions that produce type(s) as a result is used as type generators.

Appearance:

• a class template in which all the parameters are types
• a class with publicly accessible type type

Intent

• To encapsulate a complex type computation algorithm
• To generate a type using compile-time type selection tech

Metafunction idiom is the principal way of writing compile-time algorithms in C++.

Implementation of metafunctions is often based on template specializations.

Example

template<bool, class L, class R>
struct IF{
    typedef R type;
};
template<class L, class R>
struct IF<true, L, R>{
    typedef L type;
};
IF<false, int, long>::type i; // long i
IF<true,  int, long>::type i; // int i

Reference

More C++ Idioms/Metafunction