# std::indirect_equivalence_relation

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
template<class F, class I1, class I2 = I1> concept indirect_equivalence_relation = std::indirectly_readable<I1> && std::indirectly_readable<I2> && std::copy_constructible<F> && std::equivalence_relation<F&, std::iter_value_t<I1>&, std::iter_value_t<I2>&> && std::equivalence_relation<F&, std::iter_value_t<I1>&, std::iter_reference_t<I2>> && std::equivalence_relation<F&, std::iter_reference_t<I1>, std::iter_value_t<I2>&> && std::equivalence_relation<F&, std::iter_reference_t<I1>, std::iter_reference_t<I2>> && std::equivalence_relation<F&, std::iter_common_reference_t<I1>, std::iter_common_reference_t<I2>>; |  | (since C++20)

The concept `indirect_equivalence_relation` specifies requirements for algorithms that call equivalence relations as their arguments. The key difference between this concept and std::equivalence_relation is that it is applied to the types that `I1` and `I2` references, rather than `I1` and `I2` themselves.

### Semantic requirements

`F`, `I1`, and `I2` model `indirect_equivalence_relation` only if all concepts it subsumes are modeled.

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