c++, Identifier is undefined when part of the code is put in another cpp file -

i'm sorry if question layout weird or first time asking question. have started learning c++ week ago , know how create window glfw clean code , make nicer. tried put "create window" stuff inside .cpp file named "window.cpp" , import main file. when take out glfwcreatewindow function doesn't recognize window name "w_gill" in swapbuffer, windowshouldclose , destroywindow functions. can please me?

here main.cpp file:

#include <iostream> #include <windows.h> #include "window.cpp"  int main(){     do{         createwindow();         glfwswapbuffers(w_gill);         glfwpollevents();     } while (!glfwwindowshouldclose(w_gill));      glfwdestroywindow(w_gill);     glfwterminate();     return 0; } 

and window.cpp file:

#include <gl\glew.h> #include <glfw\glfw3.h>  int windowwidth = 1920 / 2; int windowheight = 1080 / 2;  int createwindow(){     if (!glfwinit())     {         glfwterminate();         return -1;     }     glfwwindowhint(glfw_samples, 4);     glfwwindowhint(glfw_version_major, 3);     glfwwindowhint(glfw_version_minor, 4);     glfwwindowhint(glfw_resizable, gl_true);      glfwwindow* w_gill;     w_gill = glfwcreatewindow(windowwidth, windowheight, "gillow", null, null);      if (!w_gill)     {         glfwterminate();         return -1;     }     glfwmakecontextcurrent(w_gill);     glfwsetinputmode(w_gill, glfw_sticky_keys, gl_true);     } 

nevermind got it, had put

glfwwindow* w_gill; int createwindow(); 

inside of main.cpp file linked. responding anyway.