How is this object's destructor being called two times?-CodePudding

Can someone please explain how test1 is getting destroyed 2 times? Compiled with Visual Studio C 17

class test {
public:
    test(int val):val_(val) {
        cout << "test" << val_ << "constructor called\n";
    }
    ~test() {
        cout << "test" << val_ << " destructor called\n";
    }
    int val_;
};

int main() {
    auto t2 = test(2);
    {
        auto t1 = test(1);
         swap(t1, t2);
    }
    return 0;
}

Output:

test2constructor called

test1constructor called

test1 destructor called

test2 destructor called

test1 destructor called

CodePudding user response：

The general implementation of swap is as follows:

template<typename T> 
void swap(T& x, T& y) {
  T tmp = std::move(x);
  x = std::move(y);
  y = std::move(tmp);
}

There is a temporary variable tmp, which is destroyed when it leaves the scope of swap.

Since it is initialized by x at the beginning, its value is 1, and when it is assigned to y again, std::move will not change its value, so its value is still 1.