Difference between revisions of "cpp/named req/ValueSwappable"
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Revision as of 13:50, 15 June 2018
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 Template:concept if
2) For any dereferencable object
x of type T (that is, any value other than the end iterator), *x satisfies the Template:concept requirements.Many standard library functions expect their arguments to satisfy Template:concept, 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 }
