[swappable.requirements] (original) (raw)

:

User code can ensure that the evaluation of swap calls is performed in an appropriate context under the various conditions as follows:

#include

template<class T, class U> void value_swap(T&& t, U&& u) { using std::swap; swap(std::forward(t), std::forward(u));

}

template void lv_swap(T& t1, T& t2) { using std::swap; swap(t1, t2);
}

namespace N { struct A { int m; }; struct Proxy { A* a; }; Proxy proxy(A& a) { return Proxy{ &a }; }

void swap(A& x, Proxy p) { std::swap(x.m, p.a->m);

} void swap(Proxy p, A& x) { swap(x, p); }
}

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