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

Range adaptor closure objects are FunctionObjects that are callable via the pipe operator: if C is a range adaptor closure object and R is a range, these two expressions are equivalent

C(R)
R | C

Two range adaptor closure objects can be chained by operator| to produce another range adaptor closure object: if C and D are range adaptor closure objects, then C | D produces a range adaptor closure object E

• E stores a copy of C and D, direct-non-list-initialized from std::forward <decltype( (C) ) > (C) and std::forward <decltype( (D) ) > (D) respectively. If such initialization is invalid, C | D
• Let c and d be the stored copies (with the same constness and value category as E), and R be a range object, the following expressions are equivalent

d(c(R))
R | c | d
E(R)
R | E // R | (C | D)

Notes: operator()

Objects whose type is the same as one of the following objects (ignoring cv-qualification) are range adaptor closure objects:

• unary range adaptor objects,

• the results of binding trailing arguments by range adaptor objects, and
• the results of chaining two range adaptor closure objects by operator|.

See also