c++ - SDL C++ 多重定义错误

Question

在我的文件中，我无法弄清楚为什么会出现多个定义错误。我正在为名为“screen”的“SDL_Surface”获取它们。

screen 位于 render.h 中。我已经尝试将其设置为 extern，但它仍然会引发错误。我查了很多资料，但找不到解决方案。

这里是有问题的文件:

main.cpp

//The headers
#include <iostream>
#include <string>
#include <vector>
#include "SDL/SDL.h"
#include "render.h"
#include "Entities.h"

//The surfaces that will be used
SDL_Surface *civilian = NULL;
SDL_Surface *background = NULL;

int main( int argc, char* args[])
{
    //initialize all of SDL systems
    INIT();

    //Load the images
    civilian = load_image("civilian.png");
    background = load_image("background.png");

    //Apply the background to the screen
    apply_surface(0,0, background, screen);

    //apply the civilian to the screen
    apply_surface(180, 140, civilian, screen);

    //update the screen
    if(SDL_Flip( screen ) == -1)
    {
        return 1;
    }

    //Wait 2 seconds
    SDL_Delay(2000);

    //free the surfaces
    SDL_FreeSurface(civilian);
    SDL_FreeSurface(background);

    //Quit SDL
    SHUTDOWN();

    return 0;
}

render.cpp

#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include <string>
#include "render.h"

bool INIT()
{
    //start SDL
    if (SDL_Init(SDL_INIT_EVERYTHING) == -1)
    {
        return false;
    };

    //Set up the screen
    screen = SDL_SetVideoMode(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE);

    //if there was an error setting up the screen
    if(screen == NULL)
    {
        return false;
    }

    //set the window caption
    SDL_WM_SetCaption("Zombie!", NULL);

    //if everything initialized fine
    return true;
}

void SHUTDOWN()
{
    //Free images eventually an if statement and array that stores all the images that need to be loaded
    // SDL_FreeSurface(image);

    //Quit SDL
    SDL_Quit();


}

//Optimized image loading function
SDL_Surface *load_image(std::string filename)
{
    //temporary storage for the image that's loaded
    SDL_Surface* loadedImage = NULL;
    //The optimized image that will be used
    SDL_Surface* optimizedImage = NULL;

    //Load the image
    loadedImage = IMG_Load(filename.c_str());

    //If nothing went wrong in loading the image
    if(loadedImage != NULL)
    {
        //Create an optimized image
        optimizedImage = SDL_DisplayFormat(loadedImage);

        //Free the old image
        SDL_FreeSurface(loadedImage);
    }
    return optimizedImage;
}


void apply_surface(int x, int y, SDL_Surface* source, SDL_Surface* destination)
{
    //Make a temporary rect to hold the offsets
    SDL_Rect offset;
    //Give the offsets to the rectangle
    offset.x = x;
    offset.y = y;

    //Blit the surface
    SDL_BlitSurface(source, NULL, destination, &offset);
}

/*bool load_files()
{
    //load the image
    image = load_image( "civilian.png");

    //If there was an error in loading the image
    if (image == NULL)
    {
        return false;
    }
    //if everything loaded fine
    return true;
}
*/

//Event handling
SDL_Event event;

render.h

#ifndef RENDER_H_INCLUDED
#define RENDER_H_INCLUDED

const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;
const int SCREEN_BPP = 32;

SDL_Surface *screen = NULL;

struct Images
{
    std::string filename; //The name of the file to be loaded
    SDL_Surface* image; //set to null when created
    bool optimized; //set to false when created

};

extern Images ImageFiles[10];

//initialize all of SDL subsystems
bool INIT();

//quit SDL
void SHUTDOWN();

void apply_surface(int x, int y, SDL_Surface* source, SDL_Surface* destination);

//Optimized image loading function
SDL_Surface *load_image(std::string filename);

#endif // RENDER_H_INCLUDED

这是错误:

obj\Debug\render.o||In function Z4INITv':|
C:\Users\Joshua\Documents\CodeBlocks\Simulation\Zombie Survival\render.cpp|11|multiple definition ofscreen'| obj\Debug\main.o:C:\Users\Joshua\Documents\CodeBlocks\Simulation\Zombie Survival\main.cpp|20|first defined here| ||=== Build finished: 2 errors, 0 warnings (0 minutes, 1 seconds) ===|

Answer 1

从 render.h 中删除 SDL_Surface *screen = NULL;。
在 render.cpp 中声明 SDL_Surface *screen = NULL;。
在 main.cpp 中声明 extern SDL_Surface *screen;。

因为，一个commentor要求解释，我正在解释 C++ 编译器的行为方式。

C++ 编译器将每个源文件 (.cpp) 编译为一个编译单元。如果在 render.h 中声明了 SDL_Surface* screen = NULL;，则每个编译单元将拥有一份全局变量 screen(参见 external linkage)。现在，当链接器组合这些编译单元时，它会看到相同全局变量名称的两个拷贝并出错。