< cpp‎ | experimental‎ | ranges
Technical specifications
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals 2 TS)
Extensions for parallelism (parallelism TS)
Extensions for parallelism 2 (parallelism TS v2)
Extensions for concurrency (concurrency TS)
Concepts (concepts TS)
Ranges (ranges TS)
Special mathematical functions (special math TR)
Concepts library
Core language concepts
Object concepts
Comparison concepts
Callable concepts
URNG concept
template <class R, class T, class U>

concept bool Relation =
  Predicate<R, T, T> &&
  Predicate<R, U, U> &&
    const std::remove_reference_t<T>&,
    const std::remove_reference_t<U>&> &&
      const std::remove_reference_t<T>&,
      const std::remove_reference_t<U>&>,
      const std::remove_reference_t<T>&,
      const std::remove_reference_t<U>&>> &&
  Predicate<R, T, U> &&

  Predicate<R, U, T>;
(1) (ranges TS)

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.


  • r, an expression such that decltype((r)) is R,
  • t, an expression such that decltype((t)) is T,
  • u, an expression such that decltype((u)) is U,

and let C be ranges::common_reference_t<const std::remove_reference_t<T>&, const std::remove_reference_t<U>&>,

then Relation<R, T, U> is satisfied only if

  • bool(r(t, u)) == bool(r(C(t), C(u))) and
  • bool(r(u, t)) == bool(r(C(u), C(t))).