Why is an assignment to a base class valid, but an assignment to a derived class a compilation error?

Question

This was an interview question. Consider the following:

struct A {};  struct B : A {};  A a;  B b;  a = b; b = a;  

Why does b = a; throw an error, while a = b; is perfectly fine?

Answer 1

Because the implicitly declared copy assignment operator of B hides the implicitly declared copy assignment operator of A.

So for the line b = a, only the the operator= of B is a candidate. But its parameter has type B const&, which cannot be initialized by an A argument (you would need a downcast). So you get an error.

Answer 2

Because every B is an A, but not every A is a B.

Edited following comments to make things a bit clearer (I modified your example):

struct A {int someInt;};  struct B : A {int anotherInt};  A a;  B b;   /* Compiler thinks: B inherits from A, so I'm going to create    a new A from b, stripping B-specific fields. Then, I assign it to a.    Let's do this!  */ a = b;  /* Compiler thinks: I'm missing some information here! If I create a new B    from a, what do I put in b.anotherInt?    Let's not do this!  */ b = a; 

In your example, there's no attributes someInt nor anotherInt, so it could work. But the compiler will not allow it anyway.