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C++ named requirements: RangeAdaptorObject (since C++20)

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Range adaptor objects are customization point objects that accept viewable_range as their first arguments and return a view. Some range adaptor objects are unary, i.e. they take one viewable_range as their only argument. Other range adaptor objects take a viewable_range and other trailing arguments.

If a range adaptor object takes only one argument, it is also a RangeAdaptorClosureObject.

If a range adaptor object takes more than one argument, it also supports partial application: let

  • a be such a range adaptor object, and
  • args... be arguments (generally suitable for trailing arguments),

expression a(args...) has following properties:

  • it is valid if and only if for every argument e in args... such that E is decltype((e)), std::is_constructible_v<std::decay_t<E>, E> is true,
  • when the call is valid, its result object stores a subobject of type std::decay_t<E> direct-non-list-initialized with std::forward<E>(e), for every argument e in args... (in other words, range adaptor objects bind arguments by value), and
  • the result object is a RangeAdaptorClosureObject.

Like other customization point objects, let

  • a be an object of the cv-unqualified version of the type of any range adaptor objects,
  • args... be any group of arguments that satisfies the constraints of the operator() of the type of a,

calls to

are all equivalent.

The result object of each of these expressions is either a view object or a RangeAdaptorClosureObject.

[edit] Notes

operator() is unsupported for volatile-qualified or const-volatile-qualified version of range adaptor object types. Arrays and functions are converted to pointers while binding.