C++: unresolved external symbol thrown in Application.cpp (solution is at the end of the question for future readers)

Question

So I'm writing an Input handler in C++ (and GLFW, but that doesn't matter), and one function that is not implemented in the header drop this error:

1>Input.obj : error LNK2001: unresolved external symbol "private: static class Input Input::instance" (?instance@Input@@0V1@A)

It is called like this in Application.cpp:

Input::GetInstance().Init(window);
Input::GetInstance().Update();

And the implementation is like this:

Input.h:

#ifndef INPUT_H_INCULDED
#define INPUT_H_INCLUDED

//Some includes

class Input
{
public:
    static Input& GetInstance();
  //some other non-static stuff
};

#endif

And the implementation in Input.cpp:

Input& Input::GetInstance()
{
return instance;
}

instance is a static member of Input.

I have glew, and glfw libs and includes in a folder called Dependencies in the solution directory. I included it in project properties, under C/C++\General\Additional Include Directories, and I also added $(SolutionDir)src, because I've placed every source file under src.

I then linked them, under Linker\General\Additional Library Directories, I added the location of the lib files, and under Linker\Input, I've added glfw3.lib, opengl32.lib, and glew3s.lib to the Additional Dependencies, and finally, added the GLEW_STATIC preprocessor definition. These are the only changes I've made in the project properties, and I use Debug, x86 configuration. The link to the source code:

https://github.com/Andrispowq/Prehistoric-Engine---C-

Edit: for any future readers: my problem was that I had to write

Input Input::instance;

somewhere in the code, in a cpp file, because instance is a static member of Input class. So, if you have a similar problem, make sure to check if you have static variables and if you have initialized them like this!

How do you build your project? The error is somewhere there. — Yksisarvinen
What is an undefined reference/unresolved external symbol error and how do I fix it? — Jesper Juhl
So basically I excluded Input.h and Input.cpp from the project, and included them again, and it got rid of two problems out of 3. I'm gonna update the question — Andrew
Ok the problem's solved, the static Input& Input::GetInstnace() was the one that hadn't disappeared, but the problem was that I forgot to say somewhere Input Input::instance — Andrew

Boki Boki · Accepted Answer · 2020-02-27T18:12:56

Your instance member, obviously need to be static member as well, from many reasons, for example to achieve singleton pattern, and especially cause you are returning by reference. By the way, if your static member function access some members of your class, that member need to be static as well. Exception will be if your static method, for example takes argument which can be particular instance of your class, or cast to it, and further using its parameter, static member function can possibly access or use methods or data members through that known instance, passed as argument to static member, but your GetInstance takes no argument. I hope this helps you to understand what your compiler telling you and that is, that compiler cant find symbol instance.

C++: unresolved external symbol thrown in Application.cpp (solution is at the end of the question for future readers)

1 Answers