# C++ named requirements: ValueSwappable

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< cpp‎ | named req

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

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);.

### 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;    // 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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