26 Ranges library [ranges]

26.7 Range adaptors [range.adaptors]

26.7.2 Range adaptor objects [range.adaptor.object]

A range adaptor closure object is a unary function object that accepts a range argument.
For a range adaptor closure object C and an expression R such that decltype((R)) models range, the following expressions are equivalent: C(R) R | C
Given an additional range adaptor closure object D, the expression C | D produces another range adaptor closure object E.
E is a perfect forwarding call wrapper ([func.require]) with the following properties:
  • Its target object is an object d of type decay_­t<decltype((D))> direct-non-list-initialized with D.
  • It has one bound argument entity, an object c of type decay_­t<decltype((C))> direct-non-list-initialized with C.
  • Its call pattern is d(c(arg)), where arg is the argument used in a function call expression of E.
The expression C | D is well-formed if and only if the initializations of the state entities of E are all well-formed.
Given an object t of type T, where
  • t is a unary function object that accepts a range argument,
  • T models derived_­from<range_­adaptor_­closure<T>>,
  • T has no other base classes of type range_­adaptor_­closure<U> for any other type U, and
  • T does not model range
then the implementation ensures that t is a range adaptor closure object.
The template parameter D for range_­adaptor_­closure may be an incomplete type.
If an expression of type cv D is used as an operand to the | operator, D shall be complete and model derived_­from<range_­adaptor_­closure<D>>.
The behavior of an expression involving an object of type cv D as an operand to the | operator is undefined if overload resolution selects a program-defined operator| function.
If an expression of type cv U is used as an operand to the | operator, where U has a base class of type range_­adaptor_­closure<T> for some type T other than U, the behavior is undefined.
The behavior of a program that adds a specialization for range_­adaptor_­closure is undefined.
A range adaptor object is a customization point object ([customization.point.object]) that accepts a viewable_­range as its first argument and returns a view.
If a range adaptor object accepts only one argument, then it is a range adaptor closure object.
If a range adaptor object adaptor accepts more than one argument, then let range be an expression such that decltype((range)) models viewable_­range, let args... be arguments such that adaptor(range, args...) is a well-formed expression as specified in the rest of subclause [range.adaptors], and let BoundArgs be a pack that denotes decay_­t<decltype((args))>....
The expression adaptor(args...) produces a range adaptor closure object f that is a perfect forwarding call wrapper ([func.require]) with the following properties:
  • Its target object is a copy of adaptor.
  • Its bound argument entities bound_­args consist of objects of types BoundArgs... direct-non-list-initialized with std​::​forward<decltype((args))>(args)..., respectively.
  • Its call pattern is adaptor(r, bound_­args...), where r is the argument used in a function call expression of f.
The expression adaptor(args...) is well-formed if and only if the initialization of the bound argument entities of the result, as specified above, are all well-formed.