std::indirect_unary_predicate

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Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
indirect_unary_predicate
(C++20)
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)
Iterator adaptors
Range access
(C++11)(C++14)
(C++14)(C++14)  
(C++11)(C++14)
(C++14)(C++14)  
(C++17)(C++20)
(C++17)
(C++17)
 
Defined in header <iterator>
template< class F, class I >

concept indirect_unary_predicate =
    std::indirectly_readable<I> &&
    std::copy_constructible<F> &&
    std::predicate<F&, /*indirect-value-t*/<I>> &&

    std::predicate<F&, std::iter_reference_t<I>>;
(since C++20)

The concept indirect_unary_predicate specifies requirements for algorithms that call unary predicates as their arguments. The key difference between this concept and std::predicate is that it is applied to the type that I references, rather than I itself.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2609R3 C++20 one of the requirements was defined in terms of std::iter_value_t<I>&
which mishandled projections resulting in incompatibility with predicate F&
defined in terms of /*indirect-value-t*/<I>
to correctly handle such projections
P2997R1 C++20 indirect_unary_predicate required F& to satisfy predicate with
std::iter_common_reference_t<I>
does not require