Why primitive to class type conversion destroys object values?

Why in the below code when I set c1 = 10 destroys all other values of variables (a, b, c) of the object. The statement should call the constructor and as the constructor is defined it sets the values of a to 10, but when I try to access values of b and c; it is giving me garbage values.

#include<iostream>
using namespace std;
class abc{
    private:
    // properties
    int a,b, c;
    
    public:
    
    void setdata(int x,int y)
    {
        a = x;
        b = y;
    }
    void showdata(){
        cout << "a = " << a << " b = " << b << "\n";
    }
    
    // constructors
    abc(){}
    
    abc(int k)
    {
        a=k;
    }
};
int main()
{
    
abc c1; // object intialization
c1.setdata(6,7); // setting values of properties
c1.showdata(); // printing values

c1 = 10; // primitive to class type conversion, constructor is being called
c1.showdata(); // why value of b and other variables is getting changed ?

return 0;

}