Standard library header <concepts>

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Standard Library headers

Language Support
<cstddef> <limits> <climits> <contract>	<cfloat> <cstdint> <new> <coroutine>	<typeinfo> <exception> <initializer_list> <version>	<csignal> <csetjmp> <cstdarg>
Concepts
<concepts>
Diagnostics
<stdexcept>	<cassert>	<cerrno>	<system_error>
General utilities
<utility> <memory> <memory_resource> <scoped_allocator>     <compare>	<bitset> <tuple> <optional> <any> <variant>	<type_traits> <ratio> <chrono> <typeindex> <functional>	<ctime> <cstdlib>
Strings
<string> <string_view>	<cctype> <cwctype>	<cwchar> <cuchar>	<cstring> <charconv>
Localization
<locale>	<codecvt>	<clocale>
Containers
<span> <array> <vector>	<deque> <forward_list> <list>	<map> <set> <queue>	<unordered_map> <unordered_set> <stack>
Iterators
<iterator>
Ranges
<ranges>
Algorithms
<algorithm>	<execution>
Numerics
<complex> <random>	<valarray> <numeric>	<cfenv> <cmath>	<bit>
Input/Output
<iosfwd> <ios> <iomanip> <streambuf>	<istream> <ostream> <iostream>	<sstream> <fstream> <syncstream>	<cstdio> <cinttypes> <strstream>
Regular expressions
<regex>
Filesystem support
<filesystem>
Thread support
<thread> <atomic>	<mutex> <shared_mutex>	<condition_variable> <future>
C compatibility
<ciso646> <cstdalign>	<cstdbool> <ccomplex>	<ctgmath>

 

This header is part of the concepts library.

Concepts
Core language concepts
Same (C++20)	specifies that a type is the same as another type (concept)
DerivedFrom (C++20)	specifies that a type is derived from another type (concept)
ConvertibleTo (C++20)	specifies that a type is implicitly convertible to another type (concept)
CommonReference (C++20)	specifies that two types share a common reference type (concept)
Common (C++20)	specifies that two types share a common type (concept)
Integral (C++20)	specifies that a type is an integral type (concept)
SignedIntegral (C++20)	specifies that a type is an integral type that is signed (concept)
UnsignedIntegral (C++20)	specifies that a type is an integral type that is unsigned (concept)
Assignable (C++20)	specifies that a type is assignable from another type (concept)
SwappableSwappableWith (C++20)	specifies that a type can be swapped or that two types can be swapped with each other (concept)
Destructible (C++20)	specifies that an object of the type can be destroyed (concept)
Constructible (C++20)	specifies that a variable of the type can be constructed from or bound to a set of argument types (concept)
DefaultConstructible (C++20)	specifies that an object of a type can be default constructed (concept)
MoveConstructible (C++20)	specifies that an object of a type can be move constructed (concept)
CopyConstructible (C++20)	specifies that an object of a type can be copy constructed and move constructed (concept)
Comparison concepts
Boolean (C++20)	specifies that a type can be used in Boolean contexts (concept)
EqualityComparableEqualityComparableWith (C++20)	specifies that operator == is an equivalence relation (concept)
StrictTotallyOrderedStrictTotallyOrderedWith (C++20)	specifies that the comparison operators on the type yield a total order (concept)
Object concepts
Movable (C++20)	specifies that an object of a type can be moved and swapped (concept)
Copyable (C++20)	specifies that an object of a type can be copied, moved, and swapped (concept)
Semiregular (C++20)	specifies that an object of a type can be copied, moved, swapped, and default constructed (concept)
Regular (C++20)	specifies that a type is regular, that is, it is both Semiregular and EqualityComparable (concept)
Callable concepts
InvocableRegularInvocable (C++20)	specifies that a callable type can be invoked with a given set of argument types (concept)
Predicate (C++20)	specifies that a callable type is a Boolean predicate (concept)
Relation (C++20)	specifies that a callable type is a binary relation (concept)
StrictWeakOrder (C++20)	specifies that a Relation imposes a strict weak ordering (concept)
Customization point objects
ranges::swap (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>;