Already Defined / Multiple Object Definitions / Visual Studio

Question

I'm following along with a video tut on SDL. When I do exactly as he has done, by putting an SDL_Texture* in the Game.h and use the object in Game.cpp, I get an error about multiple definitions. Yet, the video creator has no error of such. I can, however, copy and paste the SDL_Texture* to the .cpp and it work just fine.

Why is it that the same code will work on one, but not another machine, in Visual Studio

Game.cpp

    #include "Game.h"


Game::Game()
{


}

Game::~Game()
{


}

void Game::init(const char* title, int xpos, int ypos, int width, int height, bool fullscreen)
{
    int flags = 0;
    if (fullscreen)
    {
        flags = SDL_WINDOW_FULLSCREEN;
    }

    if (SDL_Init(SDL_INIT_EVERYTHING) == 0)
    {
        std::cout << "Successfully Initialized Subsystem . . . " << std::endl;

        window = SDL_CreateWindow(title, xpos, ypos, width, height, flags);
            if (window)
            {
                std::cout << "Successfully Created Window . . . " << std::endl;
            }
            else
            {
                std::cout << "Something went wrong. Error: " << SDL_GetError() << std::endl;
            }

            renderer = SDL_CreateRenderer(window, -1, 0);
            if (renderer)
            {
                SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
                std::cout << "Successfully Created Renderer . . . " << std::endl;
            }
            else
            {
                std::cout << "Something went wrong. Error: " << SDL_GetError() << std::endl;
            }

            is_running = true;
    }
    else
    {
        is_running = false;
    }

    SDL_Surface* surf_temp = IMG_Load("assets/player.png");
    tex_player = SDL_CreateTextureFromSurface(renderer, surf_temp);
    SDL_FreeSurface(surf_temp);


}


void Game::event_handle()
{
    SDL_Event event;
    SDL_PollEvent(&event);
    switch (event.type)
    {
    case SDL_QUIT:
        is_running = false;

    }

}

void Game::update()
{
    cnt++;
    std::cout << cnt << std::endl;


}



void Game::render()
{
    SDL_RenderClear(renderer);
    SDL_RenderCopy(renderer, tex_player, NULL, NULL); 
    SDL_RenderPresent(renderer);

}

void Game::clean()
{
    SDL_DestroyWindow(window);
    SDL_DestroyRenderer(renderer);
    SDL_Quit();
    std::cout << "Game cleaned . . . " << std::endl;
}

Game.h

#include "SDL.h"
#include "SDL_image.h"
#include <iostream>
#include <stdio.h>
#include <string>



SDL_Texture* tex_player;

class Game 
{

public:
    Game();
    ~Game();

    void init(const char* title, int xpos, int ypos, int width, int height, bool fullscreen);

    void event_handle();
    void update();
    void render();
    void clean();

    bool running() { return is_running; }


private:

    int cnt;

    bool is_running;
    SDL_Window *window;
    SDL_Renderer *renderer;


};

main.cpp

#include "Game.h"



Game *game = nullptr;

int main(int argc, char *argv[])
{
    game = new Game();

    game->init("Yeah", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600, false);
    while (game->running())
    {
        game->event_handle();
        game->update();
        game->render();
    }

    game->clean();

    return 0;
}

Error:

1>------ Build started: Project: SDL_Template, Configuration: Debug Win32 ------
1>main.cpp
1>LINK : C:\Users\Onion\documents\visual studio 2017\Projects\SDL_Template\Debug\SDL_Template.exe not found or not built by the last incremental link; performing full link
1>main.obj : error LNK2005: "struct SDL_Texture * tex_player" (?tex_player@@3PAUSDL_Texture@@A) already defined in Game.obj
1>C:\Users\Onion\documents\visual studio 2017\Projects\SDL_Template\Debug\SDL_Template.exe : fatal error LNK1169: one or more multiply defined symbols found
1>Done building project "SDL_Template.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

Think of #include as "please copy-paste the content of this file here."

Since you #includeGame.h in both main.cpp and Game.cpp, the line SDL_Texture* tex_player; which is a global variable definition (as opposed to declaration) appears in both files, which confuses the linker.

The immediate fix is to declare the variable in game.h:

extern SDL_Texture* tex_player;

and define it in game.cpp

SDL_Texture* tex_player;

The better fix would be to make tex_player a member variable of Game (or some other class).