SDL2 C ++，创建单例时出错

Question

We're following a book called SDL Game Development by Shaun Mitchell which uses SDL2

We're at the part where we have to create a singleton for a "Texture Manager".

TextureManager.h

#include <SDL.h>
#include <SDL_image.h>
#include <string.h>
#include <iostream>
#include <map>
class TextureManager
{
    public:

        bool load(std::string fileName,std::string id,SDL_Renderer* pRenderer);
        void draw(std::string id, int x, int y, int width, int height, SDL_Renderer* pRenderer, SDL_RendererFlip flip = SDL_FLIP_NONE);
        void drawFrame(std::string id, int x, int y, int width, int height, int currentRow, int currentFrame, SDL_Renderer* pRenderer, SDL_RendererFlip flip = SDL_FLIP_NONE);
        std::map <std::string, SDL_Texture*> m_textureMap;



    protected:
    private:
        TextureManager(){}
        static TextureManager* Instance()
        {
            if(s_pInstance == 0)
            {
                s_pInstance = new TextureManager();
                return s_pInstance;
            }
            return s_pInstance;
        }
        typedef TextureManager TheTextureManager;


};

TextureManager.cpp

#include "TextureManager.h"


TextureManager* TextureManager::s_pInstance = 0;
bool TextureManager::load(std::string fileName, std::string id, SDL_Renderer* pRenderer)
{
    SDL_Surface* pTempSurface = IMG_Load(fileName.c_str());
    if(pTempSurface == 0)
    {
        return false;
    }
    SDL_Texture* pTexture =
    SDL_CreateTextureFromSurface(pRenderer, pTempSurface);
    SDL_FreeSurface(pTempSurface);
    // everything went ok, add the texture to our list
    if(pTexture != 0)
    {
        m_textureMap[id] = pTexture;
        return true;
    }
// reaching here means something went wrong
    return false;
}

void TextureManager::draw(std::string id, int x, int y, int width, int height, SDL_Renderer* pRenderer, SDL_RendererFlip flip)
{
    SDL_Rect srcRect;
    SDL_Rect destRect;

    srcRect.x = 0;
    srcRect.y = 0;
    srcRect.w = destRect.w = width;
    srcRect.h = destRect.h = height;
    destRect.x = x;
    destRect.y = y;
    SDL_RenderCopyEx(pRenderer, m_textureMap[id], &srcRect,
    &destRect, 0, 0, flip);
}

void TextureManager::drawFrame(std::string id, int x, int y, int width, int height, int currentRow, int currentFrame, SDL_Renderer *pRenderer, SDL_RendererFlip flip)
{
    SDL_Rect srcRect;
    SDL_Rect destRect;
    srcRect.x = width * currentFrame;
    srcRect.y = height * (currentRow - 1);
    srcRect.w = destRect.w = width;
    srcRect.h = destRect.h = height;
    destRect.x = x;
    destRect.y = y;
    SDL_RenderCopyEx(pRenderer, m_textureMap[id], &srcRect,
    &destRect, 0, 0, flip);
}

The book said to make the constructor a private which we did.
Then followed by declaring static TextureManager* Instance(), which we don't know where to properly declare so we just put it right after the constructor, then it also told us to typedef TextureManager. Finally it told us to declare TextureManager* TextureManager::s_pInstance = 0; on TextureManager.cpp

After following the instruction, we came across an error that says

||=== Build: Debug in Game (compiler: GNU GCC Compiler) ===|
include\TextureManager.h|22|error: 's_pInstance' was not declared in this scope|
include\TextureManager.h|27|error: 's_pInstance' was not declared in this scope|
||=== Build failed: 2 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

Answer 1

The problem is this:

//
//TextureManager.cpp
//
TextureManager* TextureManager::s_pInstance = 0;

I don't know why you'd want to do this for any class variable. Usually, for classes, you would want to put all the variables in the header files that are not local to functions to make it visible to all files that use the header.

It also makes sense that since the variable exists only in the .cpp file, none of the other files that make use of the header will be able to see it. Therefore, it's as if you never declared it at all in other files, and therefore, you get an undefined reference error:

||=== Build: Debug in Game (compiler: GNU GCC Compiler) ===|
include\TextureManager.h|22|error: 's_pInstance' was not declared in this scope|
include\TextureManager.h|27|error: 's_pInstance' was not declared in this scope|
||=== Build failed: 2 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|