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 .

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.