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

   1 [#metafunction_class]
   2 [section Template metafunction class]
   3
   4 A ['template metafunction class] is a type with a public nested
   5 [link metafunction template metafunction] called `apply`. Since it is a type, it can be
   6 passed to template metafunctions as arguments and metafunctions can return it as
   7 their result. This makes it possible to implement
   8 ['higher-order template metafunctions], which are template metafunctions taking
   9 template metafunctions as arguments or returning template metafunctions as their
  10 result.
  11
  12 For example this is the identity template metafunction class:
  13
  14   struct identity
  15   {
  16     template <class T>
  17     struct apply
  18     {
  19       using type = T;
  20     };
  21     using type = identity;
  22   };
  23
  24 This metafunction class is called `identity`. It takes one argument, `T`. The
  25 result of calling this metafunction class is its argument, `T`. Note that the
  26 `identity` metafunction class is also a
  27 [link metaprogramming_value template metaprogramming value], so it can be an
  28 argument or the result of a template metafunction.
  29
  30 To call this metafunction, one has to call the nested template metafunction.
  31 For example:
  32
  33   identity::apply<std::integral_constant<int, 13>>::type
  34
  35 The above example calls the metafunction class `identity` with
  36 `std::integral_constant<int, 13>` as its argument.
  37
  38 [endsect]
  39