Przejdź do głównej zawartości

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
}

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
}