C++ named requirements: Swappable

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Any lvalue or rvalue of this type can be swapped with any lvalue or rvalue of some other type, using unqualified function call swap() in the context where both std::swap and the user-defined swap()s are visible.

Requirements

Type U is swappable with type T if, for any object u of type U and any object t of type T,

Expression Requirements Semantics
#include <utility>

using std::swap;
swap(u, t);

After the call, the value of t is the value held by u before the call, and the value of u is the value held by t before the call. Calls the function named swap() found by overload resolution among all functions with that name that are found by argument-dependent lookup and the two std::swap templates defined in the header <utility>.
#include <utility>

using std::swap;
swap(t, u);

same same

Many standard library functions (for example, many algorithms) expect their arguments to satisfy Swappable, which means that any time the standard library performs a swap, it uses the equivalent of using std::swap; swap(t, u);.

Typical implementations either

1) Define a non-member swap in the enclosing namespace, which may forward to a member swap if access to non-public data members is required
2) Define a friend function in-class (this approach hides the class-specific swap from name lookup other than ADL)

Notes

It is unspecified whether <utility> is actually included when the standard library functions perform the swap, so the user-provided swap() should not expect it to be included.

Example

#include <iostream>
#include <vector>
 
struct IntVector {
    std::vector<int> v;
    IntVector& operator=(IntVector) = delete; // not assignable
    void swap(IntVector& other) {
        v.swap(other.v);
    }
    void operator()(auto rem, char term = ' ') {
        for(std::cout << rem << " { "; int e: v) { std::cout << e << ' '; }
        std::cout << "}" << term;
    }
};
void swap(IntVector& v1, IntVector& v2) {
    v1.swap(v2);
}
 
int main()
{
    IntVector v1{ {1,1,1,1,1} }, v2{ {2222,2222} };
    auto prn = [&]{ v1("v1"), v2("v2", '\n'); };
 
//  std::swap(v1, v2); // compiler error! std::swap requires MoveAssignable
    prn();
    std::iter_swap(&v1, &v2); // OK: library calls unqualified swap()
    prn();
    std::ranges::swap(v1, v2); // OK: library calls unqualified swap()
    prn();
}

Output:

v1 { 1 1 1 1 1 } v2 { 2222 2222 }
v1 { 2222 2222 } v2 { 1 1 1 1 1 }
v1 { 1 1 1 1 1 } v2 { 2222 2222 }

See also

checks if objects of a type can be swapped with objects of same or different type
(class template)
specifies that a type can be swapped or that two types can be swapped with each other
(concept)