Why I am getting a segmentation fault while inserting elements to the vector of data inside the class?

Question

I am receiving segmentation fault while inserting data inside the vector. I think the vector is not allocated.

I do not want to reserve size. How to do it?

class A
{
private:
    struct data
    {
        int x;
        int y;
    };
    std::vector<data>Set;

public:
    void insert()
    {
        Set[0].x = 5; Set[0].y = 6;
    }
};

A a;
a.insert(); // Segmentation Fault

Answer 1

Use std::vector::push_back().

Accessing the first element ( Set[0] ) is undefined behavior.

A default constructed vector is empty.