Static variables in classes

[bmw]

If I have a static variable in a class e.g. a pointer to another class like this: (B is another class)

class A
{
public:
   static B* cB;
};

Then I set that variable and create multiple instances of class A like this:

A::cB = new B;
As = new A[Number];

Then will the value of cB be the same across all instances?

I cannot pass the class pointer in the constructor as I need to create an array of instances. I tried this method but I get linker error.... unresolved external.

[LB]

Static data members are not associated with any specific instance.

You don't need a static variable like this. For one, you can call a different constructor than the default constructor when you make an array, and for two, you should be using std::vector.

std::vector<A> MyVec;
for(blah)
{
    MyVec.push_back(A{B{}});
}

[bmw]

Yeah but in the default constructor it needs the pointer to another class... so I have to somehow set that static variable before I create the instances.

Yeah I guess I could use a vector.

[LB]

Don't use a static variable.

[bmw]

Then how do I pass the class pointer to the other class before I create the instance?

[LB]

You use the constructor:

#include <iostream>
#include <vector>
#include <algorithm>

struct B
{
    int blah;
    void Blah()
    {
        std::cout << blah << std::endl;
    }
};

struct A
{
    B *bp;
    A(B *youmakethewayalittlebetter) : bp(youmakethewayalittlebetter)
    {
    }
    void Meh()
    {
        bp->Blah();
    }
};

int main()
{
    B b {7};
    std::vector<A> av (10, &b);
    std::for_each(av.begin(), av.end(), [](A &a)
    {
        a.Meh();
    });
}

http://ideone.com/pmFBVo

[bmw]

But can I do it with a static variable? Because I could be creating a large number of instances, and having a static variable could save a bit of memory. And IMO it would be easier.

[LB]

No, you cannot do it with a static variable. There would be no way to have different 'groups' of A, each group pointing to a different B.

[bmw]

I don't know if you really understand what I want.

I want to set a static variable in a class at runtime before I create an instance of that class. How can I do this?

So pretty much, this is what I want:

CLASS::StaticPointer = value;
CLASS *a = new CLASS[numberofclasses];

But I get an unresolved external....

Or the question could be rephrased as:
How can I set a static variable in a class during runtime?

[LB]

Your question is not "How can I set a static variable in a class during runtime", it is "Why do I get an unresolved external symbol error when I try to link my program?"

You need to provide a definition for the static variable in one and only one cpp file (because static variables are basically globals in a class scope)

//MyClass.hpp
struct MyClass
{
    static int MyGlobal; //declaration
};

//MyClass.cpp
#include "MyClass.hpp"

int MyClass::MyGlobal {0}; //definition - required 

//Main.cpp
#include "MyClass.hpp"

//... 

[bmw]

Ok, working now.

Thanks so much.

[LB]

Just note that if you change the static variable later it will affect the instances you already created, which not what you want :p

[bmw]

Yup, thanks for the warning. I won't be changing it :D