"undefined reference" to static member of template class accessed from static method - c++

Question

I have a static class method that needs access to a pointer MyTypePointer that therefore has to be declared static. Since it is a template class I have to put the methods inside the header file but I can't define MyTypePointer in the header.
So I get the "undefined Reference" error since MyTypePointer is not declared. How can I make this work / declare MyTypePointer.
myclass.h
template <typename A, typename B>
class PathfindingClass {
typedef std::vector<GenericEdgeClass<A, B>*> MyType;
static MyType *MyTypePointer;
};
template <typename A, B>
void MyClass<A, B>::MyMethod(int x, int y) {
//do something with MyTypePointer
}
Thank you very much.

Answer 1

It is a late answer for full reference, as this question is linked as a reference to another question.
A minimal broken example of a static field declared but not defined could be :
template<typename T>
class A
{
public:
static T val;
static void init()
{
val=0;
}
};
int main()
{
// A::init();
A<double>::init();
return 0;
}
The fix is just to define the static field after the class definition :
template<typename T>
class A
{
public:
static T val;
static void init()
{
val=0;
}
};
template<typename T> T A<T>::val; // only change here
int main()
{
// A::init();
A<double>::init();
return 0;
}

Answer 2

In the template definition, static MyType *MyTypePointer; declares an object. You still have to define it, outside the template class definition:
template <class A, class B>
typename PathfindingClass<A, B>::MyType*
PathfindingClass<A, B>::MyTypePointer;

Answer 3

You can still define the template member and explicitly instantiate it for all needed specializations. If you insist in having a state data member of a class template that's roughly what's needed.
Given that global variables share all sorts of problems, including dependency issues during initialization, you are much better off, wrapping you data member by a static member function:
template <typenane T>
class foo {
// ...
static X*& member() {
static X* rc = ...;
return rc;
}
};
The local variable is initialized the first time the function is called and a reference to it can safely be used. This approach also works for templates.
Note that I still recommend to avoid global variables! They cause many problems in the long-term and short-term benefit of using them is a huge debt which normally doesn't pay off.