16 Library introduction [library]

A customization point object is a function object ([function.objects]) with a literal class type that interacts with program-defined types while enforcing semantic requirements on that interaction.

The type of a customization point object, ignoring cv-qualifiers, shall model semiregular ([concepts.object]).

All instances of a specific customization point object type shall be equal ([concepts.equality]).

The effects of invoking different instances of a specific customization point object type on the same arguments are equivalent.

The type T of a customization point object, ignoring cv-qualifiers, shall model invocable<T&, Args...>, invocable<const T&, Args...>, invocable<T, Args...>, and invocable<const T, Args...> ([concept.invocable]) when the types in Args... meet the requirements specified in that customization point object's definition.

When the types of Args... do not meet the customization point object's requirements, T shall not have a function call operator that participates in overload resolution.

For a given customization point object o, let p be a variable initialized as if by auto p = o;.

Then for any sequence of arguments args..., the following expressions have effects equivalent to o(args...):

• (5.1)
  p(args...)
• (5.2)
  as_const(p)(args...)
• (5.3)
  std​::​move(p)(args...)
• (5.4)
  std​::​move(as_const(p))(args...)

Each customization point object type constrains its return type to model a particular concept.