Trouble with inheritance of operator= in C++

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If you do not declare copy-assignment operator in a class, the compiler will declare one for you implicitly. The implicitly declared copy-assignment operator will hide any inherited assignment operators (read about “name hiding” in C++), meaning that any inherited assignment operators will become “invisible” to the unqualified name lookup process (which is what happens when you do b = c), unless you take specific steps to “unhide” them.

In your case, class B has no explicitly declared copy-assignment operator. Which mean that the compiler will declare

B& B::operator =(const B&)

implicitly. It will hide the operator inherited from A. The line

b = c;

does not compile, because, the only candidate here is the above implicitly declared B::operator = (the compiler told you about that already); all other candidates are hidden. And since c is not convertible to B&, the above assignment does not compile.

If you want your code to compile, you can use using-declaration to unhide the inherited A::operator = by adding

using A::operator =;

to the definition of class B. The code will now compile, although it won’t be a good style. You have to keep in mind that in this case the b = c assignment will invoke A::operator =, which assigns only the A portions of the objects involved. (But apparently that is your intent.)

Alternatively, in cases like this you can always work around name hiding by using a qualified version of the name 

b.A::operator =(c);

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