ValueSwappable

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

Type T is ValueSwappable if.

1) Type T satisfies the Iterator 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

#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
}

Links:

http://en.cppreference.com/w/cpp/concept/ValueSwappable

doc_CPP

2016-10-11 10:09:04

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