CMake doesn't link correctly?

Question

I am currently trying to debug a program, which is rather determined to not run. I guess the mistake is rather simple, I am really new to c++ and using cmake.

Here is the error message:

Scanning dependencies of target viewer

[ 25%] Building CXX object CMakeFiles/viewer.dir/Main.cpp.o
[ 50%] Building CXX object CMakeFiles/viewer.dir/TriangleMesh.cpp.o
[ 75%] Building CXX object CMakeFiles/viewer.dir/Camera.cpp.o
[100%] Building CXX object CMakeFiles/viewer.dir/MainWindow.cpp.o
Linking CXX executable viewer
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mousePressed(int, int, int, int): error: undefined reference
to 'MainWindow::mouse_button'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mousePressed(int, int, int, int): error: undefined reference
to 'MainWindow::mouse_state'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::render(): error: undefined reference to 'MainWindow::cam'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::render(): error: undefined reference to 'MainWindow::mesh'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::mouse_button'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::old_y'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::old_x'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::cam'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::cam'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::mouse_button'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::old_x'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::old_y'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::mouse_button'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::mouseMove(int, int): error: undefined reference to
'MainWindow::cam'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::MainWindow(Camera*, int, char**): error: undefined reference
to 'MainWindow::old_x'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::MainWindow(Camera*, int, char**): error: undefined reference
to 'MainWindow::old_y'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::MainWindow(Camera*, int, char**): error: undefined reference
to 'MainWindow::mesh'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::MainWindow(Camera*, int, char**): error: undefined reference
to 'MainWindow::mesh'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::MainWindow(Camera*, int, char**): error: undefined reference
to 'MainWindow::mesh'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::getInstance(Camera*, int, char**): error: undefined reference
to 'MainWindow::instance'
CMakeFiles/viewer.dir/MainWindow.cpp.o:MainWindow.cpp:function
MainWindow::getInstance(Camera*, int, char**): error: undefined reference
to 'MainWindow::instance'
collect2: ld returned 1 exit status
make[2]: *** [viewer] Error 1
make[1]: *** [CMakeFiles/viewer.dir/all] Error 2
make: *** [all] Error 2 

So, the code compiles just fine, it just cannot really link one of the 4 source files..

Edit:

Here is the gist of MainWindow.hpp

class MainWindow
{
        private:
                MainWindow() {}
                MainWindow(const MainWindow&) {}
                MainWindow(Camera *cam, int argc, char* argv[]);
                static MainWindow *instance;
                static Camera *cam;
                static TriangleMesh mesh;
                static int old_x;
                static int old_y;
                static int mouse_button;
                static int mouse_state;
                static void mousePressed(int button, int state, int x, int y);
                static void initGlut(int argc, char* argv[]);
                static void initCallbacks();
                static void render();
                static void reshape(int w, int h);
                static void keyPressed(unsigned char key, int x, int y);
                static void mouseMove(int x, int y);

        public:
                static MainWindow* getInstance(Camera *cam, int argc, char* argv[]);
                static void setCamera(Camera* cam);
                static void setMesh(TriangleMesh* mesh);
                static void destroy();
};

Answer 1

If you have static member variables, they need to be defined as well, not only declared:

In a header file:

class Foo
{
    ...

    // This declares a static member variable
    static int static_member_variable;

    ...
};

In a source file:

// This defines the variable
int Foo::static_member_variable;