Comments on: Enforcing Inheritance Rules

By: Shriram

Shriram — Sun, 22 Nov 2009 12:57:15 +0000

@ Mikael
In your previous code above, you are the defeating the whole point of having a pure virtual function in a class by overloading it.
I am sure the compiler(s) will give you a compile error for your code.
Pure virtual function should do absolutely nothing in the base class( that is no code or lines , or no implementation at all )
when you mention it in a base class.
Almost as if it acts as an interface, or base pointer for polymorphism purposes.

http://answers.google.com/answers/threadview?id=379269
gives good explanation too.
( in the case of having a another member function with the same name of a pure virtual member function )

By: Mikael Olenfalk

Mikael Olenfalk — Fri, 25 Sep 2009 12:19:42 +0000

Sorry, I found a typo in my last comment:

The body for B::must_implement() should of course call A::must_implement() and not A::do_common_stuff():

class B : public A {
public:
virtual void must_implement()
{ A::must_implement(); do_more_stuff(); }
};

By: Mikael Olenfalk

Mikael Olenfalk — Fri, 25 Sep 2009 12:17:48 +0000

Wouldn’t this solve the issue just as well?

class A {
public:
virtual void must_implement() = 0;
};
void A::must_implement ()
{ do_common_stuff(); }

class B : public A {
public:
virtual void must_implement()
{ A::do_common_stuff(); do_more_stuff(); }
};

By: tglek

tglek — Sat, 19 Sep 2009 17:56:32 +0000

Overriding isn’t only about replacing methods. OO design also involves calling the same method on the superclass to do involve functionality common to all of the method implementations

By: Mikael Olenfalk

Mikael Olenfalk — Sat, 19 Sep 2009 09:01:17 +0000

Hi,

Maybe I missed something but why did he not just make the member function pure-virtual? I mean the function is already virtual and because the new attribute requires the class to be modified I do not really see why one could not just add ‘= 0′.

Of course by making it pure-virtual the base class cannot be instantiated anymore, however instantiating a class with NS_MUST_OVERRIDE cannot (?) be well-defined anyway otherwise the ‘_MUST_’ doesn’t really make any sense.

Please enlighten me,

With kind regards,

Mikael Olenfalk