Standard library header <concepts>

This header is part of the concepts library.

Concepts

Core language concepts
(C++20)
 specifies that a type is the same as another type
(concept)
(C++20)
 specifies that a type is derived from another type
(concept)
(C++20)
 specifies that a type is implicitly convertible to another type
(concept)
(C++20)
 specifies that two types share a common reference type
(concept)
(C++20)
 specifies that two types share a common type
(concept)
(C++20)
 specifies that a type is an integral type
(concept)
(C++20)
 specifies that a type is an integral type that is signed
(concept)
(C++20)
 specifies that a type is an integral type that is unsigned
(concept)
(C++20)
 specifies that a type is assignable from another type
(concept)
(C++20)
 specifies that a type can be swapped or that two types can be swapped with each other
(concept)
(C++20)
 specifies that an object of the type can be destroyed
(concept)
(C++20)
 specifies that a variable of the type can be constructed from or bound to a set of argument types
(concept)
(C++20)
 specifies that an object of a type can be default constructed
(concept)
(C++20)
 specifies that an object of a type can be move constructed
(concept)
(C++20)
 specifies that an object of a type can be copy constructed and move constructed
(concept)
Comparison concepts
(C++20)
 specifies that a type can be used in Boolean contexts
(concept)
(C++20)
 specifies that operator == is an equivalence relation
(concept)
(C++20)
 specifies that the comparison operators on the type yield a total order
(concept)
Object concepts
(C++20)
 specifies that an object of a type can be moved and swapped
(concept)
(C++20)
 specifies that an object of a type can be copied, moved, and swapped
(concept)
(C++20)
 specifies that an object of a type can be copied, moved, swapped, and default constructed
(concept)
(C++20)
 specifies that a type is regular, that is, it is both Semiregular and EqualityComparable
(concept)
Callable concepts
(C++20)
 specifies that a callable type can be invoked with a given set of argument types
(concept)
(C++20)
 specifies that a callable type is a Boolean predicate
(concept)
(C++20)
 specifies that a callable type is a binary relation
(concept)
(C++20)
 specifies that a Relation imposes a strict weak ordering
(concept)

Customization point objects
(C++20)
 swaps the values of two objects
(customization point object)

Synopsis

namespace std { 
 
    // Core language concepts
 
    template <class T, class U>
    concept Same = /* see description */;
 
    template <class Derived, class Base>
    concept DerivedFrom = /* see description */;
 
    template <class From, class To>
    concept ConvertibleTo = /* see description */;
 
    template <class T, class U>
    concept CommonReference = /* see description */;
 
    template <class T, class U>
    concept Common = /* see description */;
 
    template <class T>
    concept Integral = /* see description */;
 
    template <class T>
    concept SignedIntegral = /* see description */;
 
    template <class T>
    concept UnsignedIntegral = /* see description */;
 
    template <class LHS, class RHS>
    concept Assignable = /* see description */;
 
    namespace ranges {
        inline namespace /* unspecified */ {
            inline constexpr /* unspecified */ swap = /* unspecified */;
        }
    }
 
    template <class T>
    concept Swappable = /* see description */;
 
    template <class T, class U>
    concept SwappableWith = /* see description */;
 
    template <class T>
    concept Destructible = /* see description */;
 
    template <class T, class... Args>
    concept Constructible = /* see description */;
 
    template <class T>
    concept DefaultConstructible = /* see description */;
 
    template <class T>
    concept MoveConstructible = /* see description */;
 
    template <class T>
    concept CopyConstructible = /* see description */;
 
    // Comparison concepts
 
    template <class B>
    concept Boolean = /* see description */;
 
    template <class T>
    concept EqualityComparable = /* see description */;
    template <class T, class U>
    concept EqualityComparableWith = /* see description */;
 
    template <class T>
    concept StrictTotallyOrdered = /* see description */;
    template <class T, class U>
    concept StrictTotallyOrderedWith = /* see description */;
 
    // Object concepts
 
    template <class T>
    concept Movable = /* see description */;
 
    template <class T>
    concept Copyable = /* see description */;
 
    template <class T>
    concept Semiregular = /* see description */;
 
    template <class T>
    concept Regular = /* see description */;
 
    // Callable concepts
 
    template <class F, class... Args>
    concept Invocable = /* see description */;
 
    template <class F, class... Args>
    concept RegularInvocable = /* see description */;
 
    template <class F, class... Args>
    concept Predicate = /* see description */;
 
    template <class R, class T, class U>
    concept Relation = /* see description */;
 
    template <class R, class T, class U>
    concept StrictWeakOrder = /* see description */;
}

Concept Same 

template<class T, class U>
    concept __SameImpl = is_same_v<T, U>;  // exposition only
 
template<class T, class U>
    concept Same = __SameImpl<T, U> && __SameImpl<U, T>;

Concept DerivedFrom 

template<class Derived, class Base>
    concept DerivedFrom =
            is_base_of_v<Base, Derived> &&
            is_convertible_v<const volatile Derived*, const volatile Base*>;

Concept ConvertibleTo 

template<class From, class To>
    concept ConvertibleTo =
        is_convertible_v<From, To> &&
        requires(From (&f)()) {
            static_cast<To>(f());
        };

Concept CommonReference 

template<class T, class U>
    concept CommonReference =
        Same<common_reference_t<T, U>, common_reference_t<U, T>> &&
        ConvertibleTo<T, common_reference_t<T, U>> &&
        ConvertibleTo<U, common_reference_t<T, U>>;

Concept Common 

template<class T, class U>
    concept Common =
        Same<common_type_t<T, U>, common_type_t<U, T>> &&
        requires {
            static_cast<common_type_t<T, U>>(declval<T>());
            static_cast<common_type_t<T, U>>(declval<U>());
        } &&
        CommonReference<
            add_lvalue_reference_t<const T>,
            add_lvalue_reference_t<const U>> &&
        CommonReference<
            add_lvalue_reference_t<common_type_t<T, U>>,
            common_reference_t<
                add_lvalue_reference_t<const T>,
                add_lvalue_reference_t<const U>>>;

Concept Integral 

template<class T>
    concept Integral = is_integral_v<T>;

Concept SignedIntegral 

template<class T>
    concept SignedIntegral = Integral<T> && is_signed_v<T>;

Concept UnsignedIntegral 

template<class T>
    concept UnsignedIntegral = Integral<T> && !SignedIntegral<T>;

Concept Assignable 

template<class LHS, class RHS>
    concept Assignable =
        is_lvalue_reference_v<LHS> &&
        CommonReference<const remove_reference_t<LHS>&, const remove_reference_t<RHS>&> &&
        requires(LHS lhs, RHS&& rhs) {
            { lhs = std::forward<RHS>(rhs) } -> Same<LHS>;
        };

Concept Swappable 

template<class T>
    concept Swappable = requires(T& a, T& b) { ranges::swap(a, b); };

Concept SwappableWith 

template<class T, class U>
    concept SwappableWith =
        CommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
        requires(T&& t, U&& u) {
            ranges::swap(std::forward<T>(t), std::forward<T>(t));
            ranges::swap(std::forward<U>(u), std::forward<U>(u));
            ranges::swap(std::forward<T>(t), std::forward<U>(u));
            ranges::swap(std::forward<U>(u), std::forward<T>(t));
        };

Concept Destructible 

template<class T>
    concept Destructible = is_nothrow_destructible_v<T>;

Concept Constructible 

template<class T, class... Args>
    concept Constructible = Destructible<T> && is_constructible_v<T, Args>;

Concept DefaultConstructible 

template<class T>
    concept DefaultConstructible = Constructible<T>;

Concept MoveConstructible 

template<class T>
    concept MoveConstructible = Constructible<T, T> && ConvertibleTo<T, T>;

Concept CopyConstructible 

template<class T>
    concept CopyConstructible =
        MoveConstructible<T> &&
        Constructible<T, T&> && ConvertibleTo<T&, T> &&
        Constructible<T, const T&> && ConvertibleTo<const T&, T> &&
        Constructible<T, const T> && ConvertibleTo<const T, T>;

Concept Boolean 

template<class B>
    concept Boolean =
        Movable<remove_cvref_t<B>> &&
        requires(const remove_reference_t<B>& b1,
                 const remove_reference_t<B>& b2, const bool a) {
            requires ConvertibleTo<const remove_reference_t<B>&, bool>;
            { !b1 } -> ConvertibleTo<bool>;
            { b1 &&  a } -> Same<bool>;
            { b1 ||  a } -> Same<bool>;
            { b1 && b2 } -> Same<bool>;
            {  a && b2 } -> Same<bool>;
            { b1 || b2 } -> Same<bool>;
            {  a || b2 } -> Same<bool>;
            { b1 == b2 } -> ConvertibleTo<bool>;
            { b1 ==  a } -> ConvertibleTo<bool>;
            {  a == b2 } -> ConvertibleTo<bool>;
            { b1 != b2 } -> ConvertibleTo<bool>;
            { b1 !=  a } -> ConvertibleTo<bool>;
            {  a != b2 } -> ConvertibleTo<bool>;
        };

Concept EqualityComparable 

template<class T, class U>
    concept __WeaklyEqualityComparableWith = // exposition only
        requires(const remove_reference_t<T>& t,
                 const remove_reference_t<U>& u) {
            { t == u } -> Boolean;
            { t != u } -> Boolean;
            { u == t } -> Boolean;
            { u != t } -> Boolean;
        };
 
template<class T>
    concept EqualityComparable = __WeaklyEqualityComparableWith<T, T>;

Concept EqualityComparableWith 

template<class T, class U>
    concept EqualityComparableWith =
        EqualityComparable<T> && EqualityComparable<U> &&
        CommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
        EqualityComparable<
            common_reference_t<
                const remove_reference_t<T>&,
                const remove_reference_t<U>&>> &&
        __WeaklyEqualityComparableWith<T, U>;

Concept StrictTotallyOrdered 

template<class T>
    concept StrictTotallyOrdered =
        EqualityComparable<T> &&
        requires(const remove_reference_t<T>& a,
                 const remove_reference_t<T>& b) {
            { a <  b } -> Boolean;
            { a >  b } -> Boolean;
            { a <= b } -> Boolean;
            { a >= b } -> Boolean;
        };

Concept StrictTotallyOrderedWith 

template<class T, class U>
    concept StrictTotallyOrderedWith =
        StrictTotallyOrdered<T> && StrictTotallyOrdered<U> &&
        CommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
        StrictTotallyOrdered<
            common_reference_t<
                const remove_reference_t<T>&,
                const remove_reference_t<U>&>> &&
        EqualityComparableWith<T, U> &&
        requires(const remove_reference_t<T>& t,
                 const remove_reference_t<U>& u) {
            { t <  u } -> Boolean;
            { t >  u } -> Boolean;
            { t <= u } -> Boolean;
            { t >= u } -> Boolean;
            { u <  t } -> Boolean;
            { u >  t } -> Boolean;
            { u <= t } -> Boolean;
            { u >= t } -> Boolean;
        };

Concept Movable 

template<class T>
    concept Movable =
        is_object_v<T> && MoveConstructible<T> && Assignable<T&, T> && Swappable<T>;

Concept Copyable 

template<class T>
    concept Copyable = CopyConstructible<T> && Movable<T> && Assignable<T&, const T&>;

Concept Semiregular 

template<class T>
    concept Semiregular = Copyable<T> && DefaultConstructible<T>;

Concept Regular 

template<class T>
    concept Regular = Semiregular<T> && EqualityComparable<T>;

Concept Invocable 

template<class F, class... Args>
    concept Invocable = requires(F&& f, Args&&... args) {
        invoke(std::forward<F>(f), std::forward<Args>(args)...);
        // not required to be equality-preserving
    };

Concept RegularInvocable 

template<class F, class... Args>
    concept RegularInvocable = Invocable<F, Args...>;

Concept Predicate 

template<class F, class... Args>
    concept Predicate =
        RegularInvocable<F, Args...> && Boolean<invoke_result_t<F, Args...>>;

Concept Relation 

template<class R, class T, class U>
    concept Relation =
        Predicate<R, T, T> && Predicate<R, U, U> &&
        Predicate<R, T, U> && Predicate<R, U, T>;

Concept StrictWeakOrder 

template<class R, class T, class U>
    concept StrictWeakOrder = Relation<R, T, U>;

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