c++ - 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?

question from:https://stackoverflow.com/questions/6549795/why-is-an-assignment-to-a-base-class-valid-but-an-assignment-to-a-derived-class

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.