std::indirect_unary_predicate

Defined in header <iterator>

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)

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)

For the definition of /*indirect-value-t*/, see this page.

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

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