C++ named requirements: ValueSwappable

From cppreference.com
< cpp‎ | named req
C++ named requirements

Two objects of this type can be dereferenced and the resulting values can be swapped using unqualified function call swap() in the context where both std::swap and the user-defined swap()s are visible.


A type T is ValueSwappable if

1) T satisfies the LegacyIterator requirements
2) For any dereferencable object x of type T (that is, any value other than the end iterator), *x satisfies the Swappable requirements.

Many standard library functions expect their arguments to satisfy ValueSwappable, which means that any time the standard library performs a swap, it uses the equivalent of using std::swap; swap(*iter1, *iter2);.


#include <iostream>
#include <vector>
class IntVector {
    std::vector<int> v;
    IntVector& operator=(IntVector); // not assignable
    void swap(IntVector& other) {
void swap(IntVector& v1, IntVector& v2) {
int main()
    IntVector v1, v2;    // IntVector is Swappable, but not MoveAssignable
    IntVector* p1 = &v1;
    IntVector* p2 = &v2; // IntVector* is ValueSwappable
    std::iter_swap(p1, p2); // OK: iter_swap requires ValueSwappable
//  std::swap(v1, v2); // compiler error! std::swap requires MoveAssignable