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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/RangeAdaptorObject.html below:

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

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),
• the result object is a RangeAdaptorClosureObject,
• calling the RangeAdaptorClosureObject forwards the bound arguments (if any) to the associated range adaptor object. The bound arguments (if any) are considered to have the value category and cv-qualification of the RangeAdaptorClosureObject. In other words, a(args...)(r) is equivalent to std::bind_back(a, args...)(r) (but the former also supports the pipe syntax).(since C++23)

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

• a(args...),
• std::as_const(a)(args...),
• std::move(a)(args...), and
• std::move(std::as_const(a))(args...)

are all equivalent.

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

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.

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