Upcast, downcasting, and hiding methods in C++ [closed]

About question #1: yes, if you provide a factory method along with your ‘simple’ interface:

class B {
    static B * create();
    virtual void simple() = 0;

in B’s implementation file (cpp):

B *B::create()
    return new D(); //or whatever 'hidden' implementation 

About your #2: hiding functionality means simplification in the first place, and is intended as a favour toward the library users, not a punishing constraint. So, hide the D class entirely and let the (good) users be happy with it.

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