inline function error

[TinyTertle]

I'm using a inlined function that when called elsewhere causes an undefined reference error.

//inlined function
SDL_Renderer* Sol_Window::get_renderer() {
    return renderer;
}

//elsewhere in code (causes error)
//win is a Sol_Window object.
win.get_renderer();
//... 

Why does this happen? What is it that I'm missing about inlined functions?

[Peter87]

Inline functions must be defined in every translation unit where it's used.