Why derived class pointer cannot be assigned a base classes' pointer?

Question

I m learning in c++ from the textbook but still confusing

C++ Programme:

using namespace std;
#include<iostream>

class A
{
public:
    A()
    {
        std::cout << "A\n";
    }
};

class B: public A
{
public:
    B()
    {
        std::cout << "B is derived class\n";
    }
};


int main()
{

    A * a=new A();  //valid 
    B * b=new B();  //valid

    A * aa=new B(); //valid    why?
    B * bb=new A(); //invalid  why?                                  run time error: invalid conversion from ‘A*’ to ‘B*’ [-fpermissive]

    return 0;
}

this line

A * a=new A();  //valid

use because I want to access class a

and

this line

B * b=new B();  //valid

use because I want to access class b

and

I m writing this line learning purpose I don't know but this is valid why??

A * aa=new B();  //valid

and

I m writing this line learning purpose I don't know but this is not valid why??

B * bb=new A(); //not valid

please explain the last two lines still confusion?

programme link: https://onlinegdb.com/Sy8xkt38L

Sounds like you need a [good C++ book](https://stackoverflow.com/q/388242/9716597). The "is-a" relationship is a very fundamental OOP concept. — L. F., Mar 28 '20 at 07:40
Replace `A` with `Shape` and `B` with `Rectangle`. Rectangle is a Shape, but a Shape is not necessarily a Rectangle. — selbie, Mar 28 '20 at 07:40
To add up on the comment from @L.F. : as a beginner, you should **not** use `new` to allocate object. With modern C++, this should be used only in very specific situations. Just instanciate with `A a;` — kebs, Mar 28 '20 at 07:43
@L.F. According to the first sentence they already have a book. (Whether good or not, I don't know. Probably not if it starts with `new`.) — walnut, Mar 28 '20 at 07:43

score 3 · Answer 1 · answered Mar 28 '20 at 07:42

This

class B: public A

effectively says all B's are A's, Its like

class Lion : public Animal

says all lions are animals.

So this is OK

Animal* a = new Lion();

because all lions are animals. It's safe to assume that a lion is an animal. But this is not OK

Lion* l = new Animal();

because not all animals are lions. You can't take any old animal and pretend it's a lion.

Why derived class pointer cannot be assigned a base classes' pointer?

1 Answers1