Seemingly Random LNK2005 Errors With GLFW3 - c++

Question

I'm getting the error LNK2005 for seemingly no reason, or at least none that can I can identify. Essentially, using the following code I can compile without issue.
test.h
#define GLEW_STATIC
#include <GL\glew.h>
#include <GLFW\glfw3.h>
namespace test
{
//Objects and variables
GLFWwindow* window;
//Function prototypes
bool Initialize();
}
test.cpp
#include "test.h"
#include <iostream>
bool test::Initialize()
{
std::cout << "Initializing GLFW: OpenGL version 3.3 \n";
//Initialize GLFW
glfwInit();
//Set window properties (version 3.3, core profile, not resizeable)
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);
//Create Window
window = glfwCreateWindow(800, 800, "Learn OpenGL", nullptr, nullptr);
if (window = nullptr)
{
std::cout << "Failed to create GLFW window \n";
glfwTerminate();
return false;
}
return true;
}
int main()
{
test::Initialize();
return 0;
}
However, when compiling nearly the exact same thing (http://pastebin.com/VpPep9pM), along with some other code, it gives the errors:
Error LNK2005 "struct GLFWwindow * window" (?window##3PAUGLFWwindow##A) already defined in Main.obj OpenGL D:\Users\Matthew\documents\visual studio 2015\Projects\OpenGL\OpenGL\System.obj
Error LNK2005 "struct GLFWwindow * System::window" (?window#System##3PAUGLFWwindow##A) already defined in Main.obj OpenGL D:\Users\Matthew\documents\visual studio 2015\Projects\OpenGL\OpenGL\System.obj
Error LNK1169 one or more multiply defined symbols found OpenGL D:\Users\Matthew\documents\visual studio 2015\Projects\OpenGL\Debug\OpenGL.exe
So, I would like to know what causes the errors, I'm assuming it has something to do with the "context".

Answer 1

In your header file, you are defining you variable, when it needs to be declared in the header and defined in one source file.
In test.h
namespace test {
extern GLFWwindow* window;
}
and in main.cpp
namespace test {
GLFWwindow* window;
}
Without the extern in the header, you create one instance of test::window in every source file that includes it, which is a problem if there are two or more definitions.