From cppreference.com
template<classF,classI>conceptindirect_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
.
The concept indirect_unary_predicate specifies requirements for algorithms that call unary predicates as their arguments. The key difference between this concept and
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
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
C++20 indirect_unary_predicate required F& to satisfy
with std::iter_common_reference_t<I>does not require