LNK2005 Already Defined error when linking my library [duplicate] - c++

Question

This question already has answers here:
error LNK2005, already defined?
(8 answers)
Closed 3 years ago.
I've made a library when i try and use it in another project i get a LNK2005 error.
Structs.h
#pragma once
#ifndef STRUCTS_H
#define STRUCTS_H
#include <vector>
#include <string>
namespace Structs
{
struct GameObject
{
std::string name;
struct PosPoints
{
double px;
double py;
};
std::vector<PosPoints> Points;
double centroid[2];
bool active;
bool init;
};
std::vector<GameObject> objects;
}
#endif
Structs.cpp
#include "stdafx.h"
#include "Structs.h"
struct Structs::GameObject gameObject;
std::vector<Structs::GameObject> objects;
CoreFuncs.cpp
#include "stdafx.h"
#include "CoreFuncs.h"
#include "Structs.h"
#include <GLFW/glfw3.h>
//render
extern struct Structs::GameObject gameObject;
void RenderShapes()
{
for (int i = 0; i < Structs::objects.size(); i++)
{
for (int j = 0; j < Structs::objects[i].Points.size()-1; j++)
{
glBegin(GL_LINES);
glVertex3f(Structs::objects[i].Points[j].px, Structs::objects[i].Points[j].py, 0);
glVertex3f(Structs::objects[i].Points[j + 1].px, Structs::objects[i].Points[j + 1].py, 0);
glVertex3f(0.1, 0.1, 0);
glVertex3f(0.9, 0.9, 0);
glEnd();
}
}
}
The issue stops happening when i comment out the #include "Structs.h" in CoreFuncs.cpp So I assume that inlcude is what is causing the issues. I have looked around and found many sources using extern but i cant seem to get it working here.

Answer 1

Your header file is declaring an actual variable instance named objects. Every translation unit that #includes your header file will get its own copy of that variable. So when you link multiple such units together into a single executable, you will get errors about all of the duplicate variables.
It looks like you want the variable to be instantiated only once in your Structs.cpp file and then shared by your other units. So the variable needs to be declared as extern in your header file. You should also move the extern declaration of the gameObject variable from out of your CoreFuncs.cpp file and place it in your header file, too.
Structs.h
#pragma once
#ifndef STRUCTS_H
#define STRUCTS_H
#include <vector>
#include <string>
namespace Structs
{
struct GameObject
{
std::string name;
struct PosPoints
{
double px;
double py;
};
std::vector<PosPoints> Points;
double centroid[2];
bool active;
bool init;
};
extern std::vector<GameObject> objects;
extern GameObject gameObject;
}
#endif
Structs.cpp
#include "stdafx.h"
#include "Structs.h"
namespace Structs {
std::vector<GameObject> objects;
GameObject gameObject;
}
CoreFuncs.cpp
#include "stdafx.h"
#include "CoreFuncs.h"
#include "Structs.h"
#include <GLFW/glfw3.h>
//render
void RenderShapes()
{
for (size_t i = 0; i < Structs::objects.size(); ++i)
{
Structs::GameObject &go = Structs::objects[i];
for (size_t j = 0; j < go.Points.size()-1; ++j)
{
PosPoints &pt1 = go.Points[j];
PosPoints &pt2 = go.Points[j + 1];
glBegin(GL_LINES);
glVertex3f(pt1.px, pt1.py, 0);
glVertex3f(pt2.px, pt2.py, 0);
glVertex3f(0.1, 0.1, 0);
glVertex3f(0.9, 0.9, 0);
glEnd();
}
}
}