# std::relation

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< cpp‎ | concepts

Defined in header `<concepts>` |  |
template< class R, class T, class U > concept relation = std::predicate <R, T, T> && std::predicate <R, U, U> && std::predicate <R, T, U> && std::predicate <R, U, T>; | (1) | (since C++20)

The concept `relation<R, T, U>` specifies that `R` defines a binary relation over the set of expressions whose type and value category are those encoded by either `T` or `U`.

### References

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

  - 18.7.5 Concept `relation` [concept.relation]

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

  - 18.7.5 Concept `relation` [concept.relation]

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