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

   1 [#predicate]
   2 [section Predicate]
   3
   4 A ['predicate] (or unary predicate) is a
   5 [link metafunction_class template metafunction class] taking one argument and
   6 returning a [link boxed_value boxed value] of type `bool`.
   7
   8 For example the following predicate checks if its argument is the boxed `char`
   9 value `x`:
  10
  11   struct is_x
  12   {
  13     template <class C>
  14     struct apply
  15     {
  16       static constexpr bool value = (C::value == 'x');
  17       using type = apply;
  18     };
  19
  20     using type = is_x;
  21   };
  22
  23 [endsect]
  24