A(){} is the default constructor which initialize the variable x and y with default value 0 as evident from the output.

If user does not define any default constructor then compiler itself perform the same working, by automatically initializing the variable of the class on object creation.
Example:

#include<iostream>
using namespace std;
class A
{
    int x;
    float y;
public:
    void disp()
    {
        cout<<"\nx = "<<x;
        cout<<"\ny = "<<y;
    }
};

int main()
{
A a;
a.disp();
}

Above piece of code does not have default constructor, but when object is created for class A, its data members are automatically initialized to value 0. Thus its quite evident that even if we do not create any default constructor, complier itself will initialize every data member.