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# Again classes, multiple inheritance, one base class

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Hello its me again. I have one class CMessageHandler, this it the base class for all other classes. This class provide a communication channel between all other classes. It have list of messages, and two methods: PostMessage, GetMessage. Now there is a class CLog: class CLog: public virtual CMessageHandler CLog create log file and using method CLog::Logic() get messages using CMessageHandler::GetMessage() and write them to Log file. Another class is CGraphics: class CGraphics: public virtual CMessageHandler. Both CGraphics and CLog virtually derived from CMessageHandler in order to have only one CMessageHandler. But seems like Messages that was posted by CGraphics not shown to CLog, it looks like I have two CMessageHandler classes anyway. I thought using virtual inheritance will solve this problem, but seems it not. Am I doing something wrong?

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Where is the multiple inheritance here? Do you have a class that inherits from both Graphics and Log? What sense does that make?

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Quote:
 Am I doing something wrong?

Yes.

Consider the class hierarchy below:
      Foo                     class Foo     /   \                    {ChildA   ChildB                 protected:     \   /                        int A;      Bar                     };

This is an appropriate scenario for virtual inheritance. Since a derived type inherits the members of its base, in this example, ChildA and ChildB both contain the members of Foo. But Bar contains the members of both ChildA and ChildB. Which means it has two copies of A, the one it got from ChildA and the one it got from ChildB. Virtual inheritance prevents this -- if Child and ChildB iherited virtuallly from Foo, only one copy of Foo's data members would exist in Bar.

Your scenario is like this:
CMessageHandler      CMessageHandler      |                     |     CLog               CGraphics

This is not the "diamond" inheritance issue virtual inheritance was designed for. You also seem to be confused about the "one instance" thing. Unless CMessageHandler data is all static (making it, effectively, a singleton), CLog and CGraphics instances will have completely isolated copies of the CMessageHandler data members. These are not shared in any way, and virtual inheritance is not designed to make them shared. If you want to share the member variables of CMessageHandler between all instances of CMessageHandler (including instances that are sub-objects of derived types), you need to make those members static. However, this would probably result in a bad design (you'd essentially be making a singleton). There are better ways -- you should look into existing signal/slot implementations, such as the one in Boost.

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Oh... I though I can use virtual inheritance not only with "diamond".
So the only why is to create one more class that will derive from CLog and CGraphics?

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 Oh... I though I can use virtual inheritance not only with "diamond".So the only why is to create one more class that will derive from CLog and CGraphics?

Absolutely not. That would be abusing the language mechanics to achieve a goal that is incorrect because your design is broken and you're too lazy to fix it.

Any object that derives from both CLog and CGraphics must be both a CLog and a CGraphics. There are not very many objects that could realistically exist like that -- a CLoggableGraphics object maybe, but that's just ungainly.

I mentioned the most-direct solution already: make the message handler methods static. This is ugly, and just a band aid that renders the entire system an over engineered hack around a global variable, but it would work. I also mentioned a better solution: using a signal/slot implementation, similar perhaps to the one in Boost's libraries. I suggest you look into that.

Virtual inheritance is not the solution to your problem -- stop trying to apply a solution that solves a problem other than the one you're having. Look up Boost's signal library and study it (or just use it directly; why reinvent the wheel?)

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Hey don't be angry with me, I am just asking questions in order to learn new thing.
No one born programmer.
If there is no solution to my problem (due bad design or other things) so there is no solution. I'll not get with my head into wall.
If my design is bad so I'll redesign it again, and if second time will be bad I'll try again and again and again, until I'll not get the necessary knowledge to do it normally.
But I cant learn without asking questions.
I'm sure you know a lot of thing more than me 101%, but hey don't tell things like "you're too lazy to fix it". I didn't say I'm lazy to fix something and you have no right do judge me, I just asked a question.

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I'm not angry. Your location field is pretty ambiguous, I presume English is not your native language?

In the sentence "That would be abusing the language mechanics to achieve a goal that is incorrect because your design is broken and you're too lazy to fix it," the you is being used as a hypothetical non-specific "you." The sentence is essentially the same as "that would be abusing the language mechanics because one's design is broken," but the usage of "one" is more formal and stilted. I was not directly accusing you, s.kwee, of being lazy; instead I was pointing out the effect of a hypothetical cause.

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Quote:
 Original post by s.kweeHey don't be angry with me, I am just asking questions in order to learn new thing.No one born programmer.If there is no solution to my problem (due bad design or other things) so there is no solution. I'll not get with my head into wall.If my design is bad so I'll redesign it again, and if second time will be bad I'll try again and again and again, until I'll not get the necessary knowledge to do it normally.But I cant learn without asking questions.I'm sure you know a lot of thing more than me 101%, but hey don't tell things like "you're too lazy to fix it". I didn't say I'm lazy to fix something and you have no right do judge me, I just asked a question.

You are reading too much into that statement. I'm 99% positive jpetrie isn't angry at anyone. That is quite a strong emotion to have in response to an anonymous internet post. [grin]

Possibly "you're too lazy" could have been better worded, but it was meant to prompt you into action, preferably action directed towards your class design, not towards an "offended response" post. [smile]

How about you read jpetrie's post again and put more energy into researching the various suggestions he mentioned (Boost, etc), rather than focusing on the least important part of his post.

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So in this case I made a fool of myself.
And yes English is not my native language.
So that I want to apologize about my mistake.

rip-off
Every word of every post is important, if it wasn't people wouldn't post it.
And I'm already reading about Boosts signal lib.

Thanks again

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