Why does this virtual destructor trigger an unresolved external?

Question

Consider the following:

In X.h:

class X
{
    X();
    virtual ~X();
};

X.cpp:

#inclu         


        

Answer 1

You may be getting away with this because both constr and destr are private - if there is no other ref to class X in your build then the compiler may be deducing that the destr is not required, so lack of a definition is no biggie.

This does not explain to me why case 1 fails while 2 and 3 build OK though. Wonder what happens if both are made public?