Re: mandatory/optionally overridable virtual functions

From:
"Dilip" <rdilipk@lycos.com>
Newsgroups:
comp.lang.c++
Date:
2 Mar 2007 10:26:19 -0800
Message-ID:
<1172859979.633538.117100@t69g2000cwt.googlegroups.com>
On Mar 2, 12:15 pm, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

Dilip wrote:

struct AbstractBase
{
  virtual void mandatory_func() = 0;
  virtual void optional_func() { }
};

struct concretebase1 : public AbstractBase
{
  virtual void mandatory_func() { }
};

struct concretebase2 : public AbstractBase
{
  virtual void mandatory_func() { }
  virtual void optional_func() { // do something cool }
};

on the face of it, it does seem natural but somehow I can't get it out
of my mind that I just keep adding virtual functions to AbstractBase
for every *specific* action I want to perform from one of the derived
classes.


Well, *that* doesn't seem sensible. Your motivation here is either
wrong or unclear.


Can I flesh this out a little bit? Lets say I have a client code that
does something like this:

void DoMe(AbstractBase* b1)
{
   b1->mandory_func(); // everything is fine and dandy
   b1->optional_func(); // no op for whichever classes don't need to
do this
}

If DoMe can be called with any of the concrete dervied classes as
parameter and only one of them needs to implement a particular
operation while the others no-op it out, how would I do it?

 > > If I have a lot of such operations I am concerned there

would be a virutal function bloat in AbstractBase.


What's a virtual function bloat? Why are you concerned with it?


I meant metaphorically -- like seeing a lot of unrelated virtual
functions scattered all over AbstractBase with no relation to each
other (making sense only to derived classes that choose to implement
them).

another quick question: in hierarchies like this do you make the dtor
of AbstractBase ordinary or pure virtual (with an empty impl)?

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