为什么析构函数和复制构造函数使用对象数组显示此行为？

Question

When array was initialized with anonymus objects, destructor is displaying valid values, but I create an array with objects, override copy constructor doesn't call and also destructor is displaying garbage values.

I have tried understanding this by displaying values but still confused.

class Check{
    private:
        int a;
    public:
        Check()
        {
            this->a = 9999;
            cout << "\n Default Constructor Called \n";
        }
        Check(int i)
        {
            this->a = i;
        }
        Check(const Check & obj)
        {
            cout << "COPY CONSTRUCTOR\n";
        }
        ~Check()
        {
            cout << this->a<<" DESTRUCTOR  \n";
        }
};

Check b[2] = {Check(5),Check(4)};
Check obj1(2);
Check obj2(3);
Check a[2] = {obj1,obj2};

I was expecting the "COPY CONSTRUCTOR" output 4 times but it was only two times and no garbage values. Actual output is shown below:

 COPY CONSTRUCTOR COPY CONSTRUCTOR 32649 DESTRUCTOR -1330935392 DESTRUCTOR 3 DESTRUCTOR 2 DESTRUCTOR 4 DESTRUCTOR 5 DESTRUCTOR 

Answer 1

The reason is because of copy elision.

In this case:

Check b[2] = {Check(5),Check(4)};

Since both values are temporaries, the compiler is allowed (or, in C++17, forced) to optimize the copy away and instead construct the objects in place.

However, in the second case:

Check a[2] = {obj1,obj2};

obj1 and obj2 are not temporaries, so real copies must occur in this case.

Answer 2

In:

Check b[2] = {Check(5),Check(4)};

this syntax means that the constructor arguments for the two array elements are 5 and 4 . It does not mean that the arguments are temporaries.

Check(5) is a prvalue expression , this does not necessarily mean a temporary is materialized. One of the ways a prvalue can be "consumed" is as the initializer for an object of the same type.

This behaviour changed in C++17; prior to that, there was conceptually always a temporary but the temporary could be omitted at the compiler's discretion.