C++ named requirements: ValueSwappable (od C++11)
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.
Requirements
A type T is ValueSwappable if
- T satisfies the LegacyIterator requirements
- For any dereferenceable 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);
.
Example
#include <iostream>
#include <vector>
class IntVector {
std::vector<int> v;
// IntVector& operator=(IntVector); // not assignable (C++98 way)
public:
IntVector& operator=(IntVector) = delete; // not assignable
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
}