c ++未定义的静态变量引用

Question

我不知道为什么这段代码不起作用.所有的源文件都编译好了,但是当我尝试链接它们时,编译器会因为未定义的引用错误而对我大喊大叫.这是代码:

main.cpp中:

#include "SDL/SDL.h"
#include "Initilize.cpp"

int main(int argc, char* args[]) 
{
    //Keeps the program looping
    bool quit = false;
    SDL_Event exit;
    //Initilizes, checks for errors
    if(Initilize::Start() == -1) 
    {
        SDL_Quit();
    }
    //main program loop
    while(quit == false) 
    {
        //checks for events
        while(SDL_PollEvent(&exit)) 
        {
            //checks for type of event;
            switch(exit.type) 
            {
                case SDL_QUIT:
                quit = true;
                break;
            }
        }
    }
    return 0;
}

Initilize.h:

#ifndef INITILIZE_H
#define INITILIZE_H
#include "SDL/SDL.h"

/* Declares surface screen, its attributes, and Start(); */
class Initilize {
protected:
    static SDL_Surface* screen;
private:
    static int SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP;
public:
    static int Start();
};

#endif

Initilize.cpp:

#include "Initilize.h"
#include "SDL/SDL.h"

/* Initilizes SDL subsystems, sets the screen, and checks for errors */
int Initilize::Start() 
{
    //screen attributes
    SCREEN_WIDTH = 640;
    SCREEN_HEIGHT = 480;
    //Bits per pixel
    SCREEN_BPP = 32;
    //Inits all subsystems, if there's an error, return 1   
    if(SDL_Init(SDL_INIT_EVERYTHING) == -1) {
            return 1;
    }
    //sets screen
    screen = SDL_SetVideoMode(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE);
    //Returns 1 if there was in error with setting the screen
    if(screen == NULL) {
            return 1;
    }
    SDL_WM_SetCaption("Game", NULL);
    return 0;
}

很抱歉,如果代码格式化很奇怪,插入四个空格放入代码块会搞砸一些东西.

Answer 1

将以下内容添加到cpp文件中:

SDL_Surface* Initilize::screen = 0; // or nullptr
int Initilize::SCREEN_WIDTH = 640;
int Initilize::SCREEN_HEIGHT = 480;
int Initilize::SCREEN_BPP = 32;

而且,如果这些价值永远不会改变,那么制作它们会很好const.您需要将上述内容添加到cpp文件中的原因是因为需要在类外部定义静态成员变量.static SDL_Surface* screen;在你的类里面只是一个声明,而不是一个定义.static成员被认为是特殊的,与全局变量非常相似.

这样做的原因是因为静态成员在类的所有实例之间共享.这意味着它们只能被定义一次,并且允许类中的定义会导致多个定义发生,因此C++标准强制您在类之外定义它(并且还暗示您应该将定义放在cpp文件中).

Answer 2

看来您从未初始化过您的 vairables。您在 Initialize start 方法中分配它们，但没有初始化它们。尝试在源文件中分配它之前添加 a ，int SCREENWIDTH;而不仅仅是头文件