How to declare extern class pointers in C++?

Question

The following is declared variable in the cpp file but I get an error, so I have conducted a research and I found that I need to declare it in the header file. therefore how can I declare and an extern class pointer in the header file

 extern AnimationController* g_pAnimationController;

Answer 1

Just like how you have there. Ex:

// In header file:
// Declare the pointer with external linkage.
extern int* my_global_int;

// In source file:
// Define the pointer so that the linker can link stuff together with the code
// referencing the `my_global_int` symbol.
int* my_global_int = 0;

For classes and structs, if the type is unknown, then we need a forward declaration so that the compiler has some idea of what it is. But we can combine it with the declaration, like so:

// In header file:
extern class AnimationController* g_pAnimationController;

Or written more verbosely:

// In header file:
class AnimationController;
extern AnimationController* g_pAnimationController;

Update for comment question:

#include <map>
#include <string>

// Declare AnimationCallback
extern std::map<std::string, AnimationCallback*> g_mCallbackMap;