metafunction.qbk
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[#metafunction]
[section Template metafunction]
A ['template metafunction] represents a function over types that is evaluated at
compile-time. It is implemented by a template class.
The template arguments of that class are expected to be types (`class` or
`typename` template arguments). They represent the arguments of the
metafunction.
Instances of the template class are expected to have a public nested type called
`type`. This type is the type the metafunction returns.
Template metafunction are expected to be called with
[link metaprogramming_value template metaprogramming values] as arguments only.
Template metafunctions are expected to return template metaprogramming values.
For example this is the identity template metafunction:
template <class T>
struct identity
{
using type = T;
};
This metafunction is called `identity`. It takes one argument, `T`. The result
of calling this metafunction is its argument, `T`.
To call this metafunction, one has to instantiate the template class. The
template arguments it is instantiated with are the arguments the metafunction is
called with. For example:
identity<std::integral_constant<int, 13>>::type
The above example calls the metafunction `identity` with
`std::integral_constant<int, 13>` as its argument.
[endsect]