C++ named requirements: Swappable

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C++
 
C++ named requirements
 

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 <algorithm> // until C++11

#include <utility> // since C++11
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 <algorithm>(until C++11)<utility>(since C++11).
#include <algorithm> // until C++11

#include <utility> // since C++11
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 <algorithm>(until C++11)<utility>(since C++11) is actually included when the standard library functions perform the swap, so the user-provided swap() should not expect it to be included.

Example

Run this code
#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, auto term = " ")
    {
        std::cout << rem << "{{";
        for (int n{}; int e : v)
            std::cout << (n++ ? ", " : "") << e;
        std::cout << "}}" << term;
    }
};
 
void swap(IntVector& v1, IntVector& v2)
{
    v1.swap(v2);
}
 
int main()
{
    IntVector v1{{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}}, v2{{2222, 2222}};
v1{{2222, 2222}}, v2{{1, 1, 1, 1}};
v1{{1, 1, 1, 1}}, v2{{2222, 2222}};

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 226 C++98 it was unclear how the standard library uses swap clarified to use both std:: and ADL-found swap

See also

(C++17)(C++17)(C++17)(C++17)
 checks if objects of a type can be swapped with objects of same or different type
(class template)
(C++20)
 specifies that a type can be swapped or that two types can be swapped with each other
(concept)
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