Object creation

Question

class A{};
class B : public A{};
int main() {

//Why can we write this:
A* obj = new B;

//and can't do this:
B*obj = new A; //error

return 0;
}

Answer 1

Class A is like a model for class B. B has just new features.
For example, you have a mother class Vehicle (A) and a derived class Motorbike (B) : all Motorbike are Vehicle but all Vehicle aren't Motorbike.