C++ concepts: Swappable

 

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Technical Specifications

 

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TrivialType (C++11) TriviallyCopyable (C++11)	StandardLayoutType (C++11) PODType
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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

Run this code

#include <iostream>
#include <vector>
 
class IntVector {
    std::vector<int> v;
    IntVector& operator=(IntVector); // not assignable
 public:
    void swap(IntVector& other) {
        v.swap(other.v);
    }
};
void swap(IntVector& v1, IntVector& v2) {
    v1.swap(v2);
}
 
int main()
{
    IntVector v1, v2;
//  std::swap(v1, v2); // compiler error! std::swap requires MoveAssignable
    std::iter_swap(&v1, &v2); // OK: library calls unqualified 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)