What happened with this static method? - c++

Question

I have a static method called getSingleton() in a class called Renderer, which I use as a macro like this:
Renderer.h
#define g_Renderer Renderer::getSingleton()
class Renderer {
private:
Renderer();
static Renderer renderer;
public:
Renderer getSingleton();
};
Renderer.cpp
#include "Renderer.h"
Renderer::Renderer() {}
Renderer Renderer::getSingleton() {
#ifndef CREATED_RENDERER
#define CREATED_RENDERER;
renderer = Renderer();
#endif // !CREATED_RENDERER
return renderer;
}
I access the method in main:
Main.cpp
#include "Renderer.h"
int main(){
return 0;
}
But Visual Studio gives me this error:
Main.obj : error LNK2019: unresolved external symbol "public: static class Renderer __cdecl Renderer::getSingleton(void)" (?getSingleton#Renderer##SA?AV1#XZ) referenced in function main
Why is this? Thanks in advance!

Answer 1

Your getRenderer() method is setup all wrong for what you are trying to do.
For one thing, it is not static.
For another, the static variable is only being declared, not defined anywhere, which is why the linker is complaining. You would need to add this line to Renderer.cpp to actually instantiate the variable:
Renderer Renderer::renderer;
Also, getRenderer() is returning a new Renderer object each time it is called, which is useless for a singleton. The #ifdef is also useless at runtime.
Try something more like this instead:
Renderer.h
#ifndef Renderer_H
#define Renderer_H
#define g_Renderer Renderer::getSingleton()
class Renderer {
private:
Renderer();
public:
static Renderer& getSingleton();
};
#endif
Renderer.cpp
#include "Renderer.h"
Renderer::Renderer() {}
Renderer& Renderer::getSingleton() {
static Renderer instance;
return instance;
}
Main.cpp
#include "Renderer.h"
int main(){
// use g_Renderer as needed...
return 0;
}