Why can't a Base class object be assigned to a Derived class object?

Question

A derived class object can be assigned to a base class object in C++.

Derived d;
Base b = d; // It's Ok

But why can't a base class object be assigned to a derived class object?

Base b;
Derived d = b; //Not Ok. Compiler give an error

Edit:

Sorry, but this question was actually asked durring an interview.

Answer 1

Inheritance is an "is-a" relationship, but it's one-way only.

If you have e.g.

struct Base { /* ... */ };
struct Derived : Base { /* ... */ };

Then Derived is a Base, but Base is not a Derived.

That's why you can assign or initialize a base-class instance with a derived object (but beware of object slicing), but not the other way around.

Answer 2

A derived object is a base object, with additional information.

You can initialize a complete base object from the base part of a derived object, no problem.

But if you want to construct a derived object from just a base object, what should the additional information be initialized with?

If you want to provide defaults for that additional information, you can do so by declaring a Derived(Base const &) constructor. But since it does not work in the general case, it isn't done for you.