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