16 Library introduction [library]

16.4 Method of description [description]

16.4.2 Other conventions [conventions] Type descriptions [type.descriptions] Customization Point Object types [customization.point.object]

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 shall satisfy Semiregular ([concepts.object]).
All instances of a specific customization point object type shall be equal ([concepts.equality]).
The type T of a customization point object shall satisfy 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.
Each customization point object type constrains its return type to satisfy a particular concept.
Many of the customization point objects in the library evaluate function call expressions with an unqualified name which results in a call to a program-defined function found by argument dependent name lookup ([basic.lookup.argdep]).
To preclude such an expression resulting in a call to unconstrained functions with the same name in namespace std, customization point objects specify that lookup for these expressions is performed in a context that includes deleted overloads matching the signatures of overloads defined in namespace std.
When the deleted overloads are viable, program-defined overloads need be more specialized ([temp.func.order]) or more constrained ([temp.constr.order]) to be used by a customization point object.
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