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Variant sub-classes in C++

OK, this is a weird one, and a question that I'd put on comp.lang.c++ except that, like all my questions there, it would turn into a pointless flame war/pissing contest, with a teeny bit of useful content from some people who are mostly on this group anyway.

I want to have something that implements a honkin' big class, which has JUST ONE LITTLE ELEMENT that's implemented as one of several different classes, all descended from a parent class.

There is absolutely no reason why this little element needs to have any of it's functionality visible to the outside world -- so, I'd like to implement it's whole class structure as private sub-classes to my big class:

class CBigClass { public: // etcetera

private:

class CSubClassBase {};

class CBlueSubClass : public CSubClassBase {}; class CRedSubClass : public CSubClassBase {};

// etcetera };

If I do this, the only way that I can see to direct any particular instance of CBigClass to have a particular flavor of CSubClass is with some sort of enum passed to the constructor: BLUE, RED, etc.

This seems untidy to me, but I'll do it that way if no other inspiration comes along.

Can any of you see a way to get this functionality in a way that's (a) cleaner than making an enum and passing it by value to the CBigClass constructor, and (b) keeps all the sub-classes entirely private?

Thanks loads.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott
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As an alternative, being able to have the sub-classes only make their constructors available to the outside world would be nice, but I really don't like the "friend" mechanism for then exposing the rest of their functions to CBigClass, because I don't want it to have access to EVERYTHING.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

I'm no C++ wizard and I'm not sure I understand what you want, but it sounds like maybe a template?

template class CBigClass { public: ... private: SomeColorClass thingy; }

You can even use an enum as the template parameter, something like:

enum Color { red, green, blue }; template ColorClass{}; template ColorClass { int stopsign, redcross; } // red things template ColorClass { int grass, lantern; } // green things

In general, I think the C++ world has moved to this style and away from derived classes and inheritance over the past decade or so.

Reply to
Paul Rubin

One thing that I notice is that CBigClass doesn't have any of the subclasses defined in it as members. It has in its namespace a class hierarchy, that is private, so it can make objects of this class in its member functions, but other people can't.

Perhaps you meant that somewhere was a CSubClassBase* pointer inside that it could create an object of one of the derived classes and point to it. (Maybe in a smart pointer so you don't need to recreate all the control code for it).

Reply to
Richard Damon

I presume you've used C++ inheritance before but do not wish to now? You can cast the subclass to the parent and pass it, but it has to be instantiated as the subclass before doing that.

And maybe this helps?

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This also has ideas:

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--
Les Cargill
Reply to
Les Cargill

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