C++ named requirements: Swappable

 

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C++ named requirements

Basic DefaultConstructible MoveConstructible (C++11) CopyConstructible CopyAssignable MoveAssignable (C++11) Destructible Type properties ScalarType PODType TriviallyCopyable (C++11) TrivialType (C++11) StandardLayoutType (C++11)   ImplicitLifetimeType Library-wide BooleanTestable EqualityComparable LessThanComparable Swappable ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11) Allocator FunctionObject Callable Predicate BinaryPredicate Compare

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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 <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

is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (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)
swappableswappable_with (C++20)	specifies that a type can be swapped or that two types can be swapped with each other (concept)