How to swap two objects without copy assignment operator? - c++

Question

I have a class A where, the copy assignment operator is deleted. How should I swap two instances of A ?
I tried using std::swap but that did not work.
class A {
private:
int a;
public:
A& operator=(const A& other) = delete;
A(int _a = 0):a(_a){}
void showA() { std::cout << a << std::endl; }
};
int main()
{
A obj1(10);
A obj2(20);
obj1.showA();
obj2.showA();
//A temp;
//temp = obj1;
//obj1 = obj2;
//obj2 = temp;
obj1.showA();
obj2.showA();
}
I expect obj1 and obj2 to be swapped. Initially obj1.a is 10 and obj2.a is 20, I expect obj1.a to be 20 and obj2.ato be 10 when done.

Answer 1

As #Yksisarvinen indicated you need to have move constructor and move assignment defined in order to get std::move to work:
#include <iostream>
#include <utility>
class A {
private:
int a;
public:
A(int a_) : a(a_) {}
A(const A& other) = delete;
A& operator=(const A&) = delete;
A(A&& other) {
a = other.a;
}
A& operator=(A&& other) {
a = other.a;
return *this;
}
void showA() { std::cout << a << std::endl; }
};
int main(int argc, char* argv[]) {
A obj1(10);
A obj2(20);
obj1.showA();
obj2.showA();
std::swap(obj1, obj2);
std::cout << "swapped:" << std::endl;
obj1.showA();
obj2.showA();
return 0;
}