[concept.swappable] (original) (raw)
18 Concepts library [concepts]
18.4 Language-related concepts [concepts.lang]
18.4.9 Concept swappable [concept.swappable]
Let t1 and t2 be equality-preserving expressions that denote distinct equal objects of type T, and let u1 and u2similarly denote distinct equal objects of type U.
[ Note
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t1 and u1 can denote distinct objects, or the same object.
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An operationexchanges the values denoted by t1 and u1 if and only if the operation modifies neither t2 nor u2 and:
- If T and U are the same type, the result of the operation is that t1 equals u2 and u1 equals t2.
- If T and U are different types andcommon_reference_with<decltype((t1)), decltype((u1))> is modeled, the result of the operation is thatC(t1) equals C(u2) andC(u1) equals C(t2) where C is common_reference_t<decltype((t1)), decltype((u1))>.
The expressionranges::swap(E1, E2) for subexpressions E1and E2 is expression-equivalent to an expressionS determined as follows:
- S is (void)swap(E1, E2)218 if E1 or E2 has class or enumeration type ([basic.compound]) and that expression is valid, with overload resolution performed in a context that includes the declaration
template
void swap(T&, T&) = delete;
and does not include a declaration of ranges::swap.
If the function selected by overload resolution does not exchange the values denoted byE1 and E2, the program is ill-formed, no diagnostic required. - Otherwise, if E1 and E2 are lvalues of array types ([basic.compound]) with equal extent and ranges::swap(*E1, *E2) is a valid expression,S is (void)ranges::swap_ranges(E1, E2), except thatnoexcept(S) is equal tonoexcept(ranges::swap(*E1, *E2)).
- Otherwise, if E1 and E2 are lvalues of the same type T that models move_constructible<T> andassignable_from<T&, T>,S is an expression that exchanges the denoted values.
S is a constant expression if- T is a literal type ([basic.types]),
- both E1 = std::move(E2) and E2 = std::move(E1) are constant subexpressions ([defns.const.subexpr]), and
- the full-expressions of the initializers in the declarations
T t1(std::move(E1));
T t2(std::move(E2));
are constant subexpressions.
noexcept(S) is equal tois_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>.
- Otherwise, ranges::swap(E1, E2) is ill-formed.
[ Note
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This case can result in substitution failure when ranges::swap(E1, E2) appears in the immediate context of a template instantiation.
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[ Note
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Whenever ranges::swap(E1, E2) is a valid expression, it exchanges the values denoted byE1 and E2 and has type void.
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template<class T> concept swappable = requires(T& a, T& b) { ranges::swap(a, b); };
template<class T, class U> concept swappable_with = common_reference_with<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));};
[ Note
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The semantics of the swappable and swappable_withconcepts are fully defined by the ranges::swap customization point.
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[ Example
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User code can ensure that the evaluation of swap calls is performed in an appropriate context under the various conditions as follows:
#include #include #include
namespace ranges = std::ranges;
template<class T, std::swappable_with U> void value_swap(T&& t, U&& u) { ranges::swap(std::forward(t), std::forward(u)); }
template<std::swappable T> void lv_swap(T& t1, T& t2) { ranges::swap(t1, t2); }
namespace N { struct A { int m; }; struct Proxy { A* a; Proxy(A& a) : a{&a} {} friend void swap(Proxy x, Proxy y) { ranges::swap(*x.a, *y.a); } }; Proxy proxy(A& a) { return Proxy{ a }; } }
int main() { int i = 1, j = 2; lv_swap(i, j); assert(i == 2 && j == 1);
N::A a1 = { 5 }, a2 = { -5 }; value_swap(a1, proxy(a2)); assert(a1.m == -5 && a2.m == 5); }
— end example
]