ceph/src/boost/libs/metaparse/doc/metafunction.qbk

   1 [#metafunction]
   2 [section Template metafunction]
   3
   4 A ['template metafunction] represents a function over types that is evaluated at
   5 compile-time. It is implemented by a template class.
   6
   7 The template arguments of that class are expected to be types (`class` or
   8 `typename` template arguments). They represent the arguments of the
   9 metafunction.
  10
  11 Instances of the template class are expected to have a public nested type called
  12 `type`. This type is the type the metafunction returns.
  13
  14 Template metafunction are expected to be called with
  15 [link metaprogramming_value template metaprogramming values] as arguments only.
  16
  17 Template metafunctions are expected to return template metaprogramming values.
  18
  19 For example this is the identity template metafunction:
  20
  21   template <class T>
  22   struct identity
  23   {
  24     using type = T;
  25   };
  26
  27 This metafunction is called `identity`. It takes one argument, `T`. The result
  28 of calling this metafunction is its argument, `T`.
  29
  30 To call this metafunction, one has to instantiate the template class. The
  31 template arguments it is instantiated with are the arguments the metafunction is
  32 called with. For example:
  33
  34   identity<std::integral_constant<int, 13>>::type
  35
  36 The above example calls the metafunction `identity` with
  37 `std::integral_constant<int, 13>` as its argument.
  38
  39 [endsect]
  40