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.
Type T is ValueSwappable if.
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
public:
void swap(IntVector& other) {
v.swap(other.v);
}
};
void swap(IntVector& v1, IntVector& v2) {
v1.swap(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
}
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http://en.cppreference.com/w/cpp/concept/ValueSwappable