Converting Derived** to Base** and Derived* to Base*

Question

Ok, I was reading through this entry in the FQA dealing about the issue of converting a Derived** to a Base** and why it is forbidden, and I got th

Answer 1

*b = Base(3) calls Base::operator=(const Base&), which is actually present in Derived as member functions (inc. operators) are inherited.

What would happen then (calling Derived::operator=(const Base&)) is sometimes called "slicing", and yes, it's bad (usually). It's a sad consequence of the sheer omnipresence of the "become-like" operator (the =) in C++.

(Note that the "become-like" operator doesn't exist in most OO languages like Java, C# or Python; = in object contexts there means reference assignment, which is like pointer assignment in C++;).

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Summing up:

Casts Derived** -> Base** are forbidden, because they can cause a type error, because then you could end up with a pointer of type Derived* pointing to an object of type Base.

The problem you mentioned isn't a type error; it's a different type of error: mis-use of the interface of the derived object, rooting from the sorry fact that it has inherited the "become-like" operator of its parent class.

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(Yes, I call op= in objects contexts "become-like" deliberately, as I feel that "assignment" isn't a good name to show what's happening here.)