# std::equivalence_relation

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Defined in header `<concepts>` |  |
template< class R, class T, class U > concept equivalence_relation = std::relation <R, T, U>; |  | (since C++20)

The concept `equivalence_relation<R, T, U>` specifies that the `relation` `R` imposes an equivalence relation on its arguments.

### Semantic requirements

A relation `r` is an equivalence relation if

- it is reflexive: for all `x`, `r(x, x)` is true;
- it is symmetric: for all `a` and `b`, `r(a, b)` is true if and only if `r(b, a)` is true;
- it is transitive: `r(a, b) && r(b, c)` implies `r(a, c)`.

### Notes

The distinction between `relation` and `equivalence_relation` is purely semantic.

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 18.7.6 Concept `equivalence_relation` [concept.equiv]

- C++20 standard (ISO/IEC 14882:2020):

  - 18.7.6 Concept `equivalence_relation` [concept.equiv]

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