Why can't my object access protected members of another object defined in common base class?

Question

The following code produces a compiler error:

\'BaseTest::_protMember\' : cannot access protected member declared in class \'BaseTest\'

Answer 1

You can access protected members only in the class instance. That is :

class SubTest : public BaseTest
{
    SubTest(const BaseTest &baseTest)
    {
        _protMember = baseTest._protMember;
     // ^^^^^^^^^^^ Is good because you are in the instance of its class
        _protMember = baseTest._protMember;
     //               ^^^^^^^^^^^^^^^^^^^^^ Produce error because you are not in the baseTest instance.              
    };

    // followup question
    SubTest(const SubTest &subTest)
    {
        _protMember = subTest._protMember;
      // Compile because access modifiers work on class level, and not on object level.
    };
};

EDIT for the followup :

The access modifiers work on class level, and not on object level.

That is, two objects of the same class can access each others private members.

This is my source : Why can I access private variables in the copy constructor?