Operator= in class objects. (C++)

Question

What happens when we assign an object of the inherited class to a base class object?
class A{};
class B: public A{};
A a;
B b;
a=b;

Answer 1

You may find your answer here :

https://stackoverflow.com/questions/45857883/why-cant-a-base-class-object-assigned-to-a-derived-class-object

Answer 2

When you use the default assignment operator provided to assign a base class object with a derived class object, all the data present in the inherited members of the derived class is copied to the base class.

Eg - :

#include<iostream>
using namespace std;
class A
{
public:
int a;
A(int v):a(v){}
};
class B : public A
{
public:
int b;
B(int v):b(v),A(v){}
};
int main()
{
A o1(2);
B o2(4);
o1 = o2;
cout<<o1.a<<endl;
}

Note that o1 = o2 is possible, but o2 = o1 is not.
You need to define a new assignment operator in the derived class to use the object of the base class and copy just the useful data.

Answer 3

I know that it will work like this, but I can not understand why to equate an object in which only the variable a to the object in which both a and b is possible, and vice versa is impossible.