Multiple classes need access to a static variable - c++

Question

We have classes A, B and C, all of which need access to a static variable staticVariable in class D.
Moreover, class A needs to have an instance of classes B and C like this:
class A{
public:
B instanceB;
C instanceC;
};
Class D hold the static variable of object type T:
class D{
public:
D() {
staticVariable.init();
};
static T staticVariable;
};
In the example classes B and C can be empty placeholder classes:
class B{
};
class C{
};
The main function creates an instance of A:
int main(){
A a;
/*...*/
}
Again, classes A, B and C need access to staticVariable. I've tried multiple approaches including inheritance and friend functions, however I always get dependency issues or linker errors I don't quite understand:
Error LNK2001 unresolved external symbol "public: static class T
D::staticVariable" (?window#D##2VT#sf##A) SortingAlgorithms
C:\Users\Dusan\source\repos\SortingAlgorithms\SortingAlgorithms\B.obj
is being reported in .obj files of classes A, B and C.
I'm not sure if I need an instance of D in main.
How do I implement this error-free?
And how do you do it for static objects that call a function for initialization?
I meant if the function is of the same class as the object you're trying to initialize, and is non-static, I can't seem to be able to call staticVariable.init();

Answer 1

I'm not sure if I need an instance of D in main.
You don't. static variables are global, they are shared by all the instances of the same class and you don't need to instantiate a class to be able to use such variables so they must be initialized regardless of class instantiation. That being the case they need to be initialized outside the class.
How do I implement this error-free?
Something like:
class B{};
class C{};
class A{
public:
B instanceB;
C instanceC;
};
class D{
public:
D(){};
static int staticVariable;
};
int D::staticVariable = 10; //<-- here, outside the class
int main()
{
//no instance of D needed
std::cout << D::staticVariable; // prints 10
D::staticVariable++;
std::cout << D::staticVariable; // prints 11
}
And how do you do it for static objects that call a function for initialization?
I meant if the function is of the same class as the object you're trying to initialize, and is non-static, I can't seem to be able to call staticVariable.init();
It's the same principle, you declare the object static within the class and initialize it outside:
class A{
public:
void init(){std::cout << "Do Something";} //non-static method
};
class D{
public:
static A obj; //declare
};
A D::obj{}; //<-- initialize outside the class
int main()
{
D::obj.init(); // prints 'Do Something'
}

Answer 2

// in D.hpp
class D {
public:
D() { ... }; // staticVariable is static, so the ctor has no business initializing it
void init();
static int staticVariable;
static D staticD;
};
// in D.cpp
#include "D.hpp"
int D::staticVariable = 10;
// Immediatly invoked initializing lambda expression
D D::staticD = []{ D tmp; tmp.init(); return tmp; }();