undefined reference to a static variable

[m m]

Hi Everybody,

I have 3 classes, A, B and C.

class A {
	private:
	int   N;	

	public:
	int  *VectorInA;		// Size N

	void  SetN_A(int n){ N = n; }   // This is used by the constructor of
                                        // the derived class described above. 

	(... everything else ...)
};

The other classes look more or less the same.

After, I construct 3 derived classes AB, AC and BC. These contain no data, but functions relating the Vectors In A B and C.

class AB: public A, public B{

      (... stuff ...)

};

The size of the vectors N, is the same for all classes.

I have 3 questions.

1:
If I write static int N instead of int N, I get the error message

undefined reference to N

the same number of times I try to use this static variable.
Why?

(I don't mind just int, but I want to know why.)

2:
If I don't include a variable N in the classes AB etc, the compiler says that there are ambiguities when this variable is called. I really don't want to name each one NA NB and NC in the classes A B and C but there has to be a better way to deal with this problem rather than introducing yet more variables N (one in each class AB, AC and BC) whose value will be the same.

3:
In the constructors of the classes AB AC and BC, I write:

VectorInA = new int[N]

If I don't, I get errors

free(): invalid pointer:

etc. However, if I write

delete [] VectorInA; delete [] VectorInB;

in the destructor of the classes AB etc. I get

double free or corruption (fasttop)

etc.

This makes me thing that the destructor of the classes A and B are called in the destructor of the derived classes, but NOT the constructor. (However, my bibliography states the opposite.)
I just want to make sure that I don't have a memory leak. Suggestions?

By the way, I use g++.

Thanks !!

[kempofighter]

Which destructor are you talking about? Class A's destructor should delete only VectorInA and Class B's destructor should only delete VectorInB. Of course if you actually used the std::vector in the first place it is a non-issue.

As far as the multiple inheritance issue goes have you tried the scope resolution operator? You might need to specify which N you wish to use at any time. A::N, B::N, C::N, and so forth. Also see sections 25.8 and 25.9 below. I don't see your entire example so it doesn't sound like you are dealing with the dreaded diamond however some of the info in these FAQ's might help you anyway.
http://www.parashift.com/c++-faq-lite/multiple-inheritance.html#faq-25.8

Making those variables static isn't the answer at all so I'm not going to bother going into detail on that. Just don't do it because it won't solve your problem.

[m m]

Which destructor are you talking about? Class A's destructor should delete only VectorInA and Class B's destructor should only delete VectorInB.

I thought I was clear, but here I go again:

In the constructors of the derived class, AB I allocate the memory for the vectors in A and B, otherwise I get an error. In the destructor of the derived class AB, if I delete the vectors, I get

double free or corruption (fasttop)

.

This makes me think that the constructor of the class A and the class B is NOT called when the constructor of the derived class AB is called. However, the destructor of the classes A and B is called when the destructor of the class AB is called. My bibliography stated the opposite. What is actually going on?

I know that static int will not solve my problem. (Trial and error.)
But, does anybody knows why? or what does the error means?

kempofighter, thanks for the reference.

Cheers,

[kempofighter]

Well it isn't clear because you are trying to describe it instead of just posting the code. That always makes things difficult. static variables are not a part of the object. They are special. You probably forgot to implement them in the .cpp files which is a common mistake.

You really shouldn't be setting be setting the pointers in the most derived class in the first place. I wouldn't know why you are getting those errors unless you post an example. Post the code for class A, B, and AB and make a main function that constructs an AB so that we can see what you are doing and compile it for ourselves. It doesn't have to be fancy - just enough for someone else to duplicate the compiler error.