[#nullary_metafunction]
[section Nullary template metafunction]

A ['nullary template metafuncion] is a [link metafunction template metafunction]
taking 0 arguments. It is a type with a nested type called `type`, which is the
return value of the nullary metafunction. For example:

  struct always13
  {
    using type = std::integral_constant<int, 13>;
  };

This metafunction is called `always13`. It is a nullary metafunction, because it
takes no arguments. It always returns `std::integral_constant<int, 13>`.

To call this metafunction, one has to access its `::type`. For example:

  always13::type

The above example calls the metafunction to get
`std::integral_constant<int, 13>`.

[endsect]