A predicate (or unary predicate) is a template metafunction class taking one argument and returning a boxed value of type bool.

For example the following predicate checks if its argument is the boxed char value x:

struct is_x
{
  template <class C>
  struct apply
  {
    static constexpr bool value = (C::value == 'x');
    using type = apply;
  };

  using type = is_x;
};