An object t is *swappable with* an object u if and only if

- the expressions swap(t, u) and swap(u, t) are valid when evaluated in the context described below, and
- these expressions have the following effects:
- the object referred to by t has the value originally held by u and
- the object referred to by u has the value originally held by t.

The context in which swap(t, u) and swap(u, t) are evaluated shall
ensure that a binary non-member function named “swap” is selected via overload
resolution on a candidate set that includes:

- the two swap function templates defined in <utility> and
- the lookup set produced by argument-dependent lookup.

[*Note 1*: *end note*]

If T and U are both fundamental types or arrays of
fundamental types and the declarations from the header <utility> are in
scope, the overall lookup set described above is equivalent to that of the
qualified name lookup applied to the expression std::swap(t, u) or
std::swap(u, t) as appropriate.

— A type X meeting any of the iterator requirements ([iterator.requirements])
meets the *Cpp17ValueSwappable* requirements if,
for any dereferenceable object
x of type X,
*x is swappable.

[*Example 1*: *end 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 <utility>
// Preconditions: std::forward<T>(t) is swappable with std::forward<U>(u).
template<class T, class U>
void value_swap(T&& t, U&& u) {
using std::swap;
swap(std::forward<T>(t), std::forward<U>(u)); // OK, uses “swappable with'' conditions
// for rvalues and lvalues
}
// Preconditions: T meets the *Cpp17Swappable* requirements.
template<class T>
void lv_swap(T& t1, T& t2) {
using std::swap;
swap(t1, t2); // OK, uses swappable conditions for lvalues of type T
}
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); // OK, uses context equivalent to swappable
// conditions for fundamental types
}
void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry constraint
}
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);
}

—