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Section: 18.4.9 [concept.swappable] Status: C++20 Submitter: Kostas Kyrimis Opened: 2018-12-14 Last modified: 2021-02-25
Priority: 1
View all other issues in [concept.swappable].
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Discussion:
The defect stems from the example found in sub-clause 18.4.9 [concept.swappable] p5:
[…]
template<class T, std::SwappableWith<T> U>
void value_swap(T&& t, U&& u) {
ranges::swap(std::forward<T>(t), std::forward<U>(u));
}
[…]
namespace N {
struct A { int m; };
struct Proxy { A* a; };
Proxy proxy(A& a) { return Proxy{ &a }; }
void swap(A& x, Proxy p) {
ranges::swap(x.m, p.a->m);
}
void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement
}
int main() {
[…]
N::A a1 = { 5 }, a2 = { -5 };
value_swap(a1, proxy(a2)); // diagnostic manifests here(#1)
assert(a1.m == -5 && a2.m == 5);
}
The call to value_swap(a1, proxy(a2)) resolves to [T = N::A&, U = N::Proxy] The compiler will issue a diagnostic for #1 because:
rvalue proxy(a2) is not swappable
concept SwappableWith<T, U> requires N::A and Proxy to model CommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&> It follows from the example that there is no common reference for [T = N::A&, U = N::Proxy]
[2019-06-20; Casey Carter comments and provides improved wording]
The purpose of the CommonReference requirements in the cross-type concepts is to provide a sanity check. The fact that two types satisfy a single-type concept, have a common reference type that satisfies that concept, and implement cross-type operations required by the cross-type flavor of that concept very strongly suggests the programmer intends them to model the cross-type concept. It's an opt-in that requires some actual work, so it's unlikely to be inadvertent.
The CommonReference<const T&, const U&> pattern makes sense for the comparison concepts which require that all variations of const and value category be comparable: we use const lvalues to trigger the "implicit expression variation" wording in 18.2 [concepts.equality]. SwappableWith, however, doesn't care about implicit expression variations: it only needs to witness that we can exchange the values denoted by two reference-y expressions E1 and E2. This suggests that CommonReference<decltype((E1)), decltype((E2))> is a more appropriate requirement than the current CommonReference<const remove_reference_t<…> mess that was blindly copied from the comparison concepts. We must change the definition of "exchange the values" in 18.4.9 [concept.swappable] — which refers to the common reference type — consistently. Previous resolution [SUPERSEDED]:This wording is relative to N4791.
Change 18.4.9 [concept.swappable] as indicated:
-3- […]
template<class T> concept Swappable = requires(T& a, T& b) { ranges::swap(a, b); }; template<class T, class U> concept SwappableWith = CommonReference<T, Uconst 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)); };-4- […]
-5- [Example: User code can ensure that the evaluation of swap calls is performed in an appropriate context under the various conditions as follows:#include <cassert> #include <concepts> #include <utility> namespace ranges = std::ranges; template<class T, std::SwappableWith<T> U> void value_swap(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<U>(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 }; } void swap(A& x, Proxy p) { ranges::swap(x.m, p.a->m); } void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement } 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); }
[2020-01-16 Priority set to 1 after discussion on the reflector.]
[2020-02-10 Move to Immediate Monday afternoon in Prague]
Proposed resolution:
This wording is relative to N4820.
Change 18.4.9 [concept.swappable] as indicated:
-1- Let t1 and t2 be equality-preserving expressions that denote distinct equal objects of type T, and let u1 and u2 similarly denote distinct equal objects of type U. [Note: t1 and u1 can denote distinct objects, or the same object. — end note] An operation exchanges the values denoted by t1 and u1 if and only if the operation modifies neither t2 nor u2 and:
(1.1) — If T and U are the same type, the result of the operation is that t1 equals u2 and u1 equals t2.
(1.2) — If T and U are different types
that model CommonReference<const T&, const U&>and CommonReference<decltype((t1)), decltype((u1))> is modeled, the result of the operation is that C(t1) equals C(u2) and C(u1) equals C(t2) where C is common_reference_t<const T&, const U&decltype((t1)), decltype((u1))>.-2- […]
-3- […]template<class T> concept Swappable = requires(T& a, T& b) { ranges::swap(a, b); }; template<class T, class U> concept SwappableWith = CommonReference<T, Uconst 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)); };-4- […]
-5- [Example: User code can ensure that the evaluation of swap calls is performed in an appropriate context under the various conditions as follows:#include <cassert> #include <concepts> #include <utility> namespace ranges = std::ranges; template<class T, std::SwappableWith<T> U> void value_swap(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<U>(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 }; }void swap(A& x, Proxy p) { ranges::swap(x.m, p.a->m); } void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement} 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); }