struct AnimationData
{
    struct Frame
    {
        int x1, y1, x2, y2;
        float dt; // seconds
        Frame() : x1(0), y1(0), x2(1), y2(1), dt(1) {}
    } * frames;

    int frame_count;

    std::string name;

    AnimationData(int fc, std::string n) : frame_count(fc), name(n)
    {
        frames = new Frame[fc];
        for (int i = 0; i < fc; i  )
        {
            std::cout << frames[i].x2 << '\n';
        }
    }
};

int main()
{

    AnimationData data{10, "test"};
    data.frames[1].x1 = 666;

    std::cout << "Start" << '\n';
    for (int i = 0; i < data.frame_count; i  )
    {
        std::cout << data.frames[i].x1 << '\n';
    }

    return 0;
}

This outputs:

1
1
1
1
1
1
1
1
1
1
Start
0
666
0
0
0
0
0
0
0
0

Why Does it turn everything to zero except for the one I set? (Note I know nothing about copy and move constructors idk if they are related to this)

CodePudding user response：

Why Does it turn everything to zero except for the one I set?

Because the default constructor Frame::Frame() will initialize x1, y1 to 0 and x2, y2, dt to 1 when you wrote:

frames = new Frame[fc]; //default ctor will initialize x1, y1 to 0 and x2, y2, dt to 1

That is, the 10 Frame objects that will be allocated on the heap will be created using the default constructor of Frame which will initialize x1, y1 to 0 and x2, y2 and dt to 1. This is why when you wrote:

std::cout << frames[i].x2 << '\n'; //prints 1 becasue default ctor initialized `x2` to `1`

the above statement will print 1 as default ctor initialized x2 to 1

For the same reason when you wrote:

std::cout << data.frames[i].x1 << '\n'; //will print 0 as default ctro initialized x1 to 0 except for `data.frames[1].x1` which you set to `666`.

the above statement will print 0 since the default ctor initialized x1 to 0 except for data.frames[1].x1 which you specifically set to 666.

You can confirm that the default ctor is used by adding a cout statement inside it as done below:

Frame() : x1(0), y1(0), x2(1), y2(1), dt(1) {
        std::cout<<"default ctor called"<<std::endl;
    }

The output of the modified program is:

default ctor called
default ctor called
default ctor called
default ctor called
default ctor called
default ctor called
default ctor called
default ctor called
default ctor called
default ctor called
1
1
1
1
1
1
1
1
1
1
Start
0
666
0
0
0
0
0
0
0
0