（SDL渲染問題）C ++異常拋出：讀取訪問沖突。 這是nullptr

Question

每當我運行我的程序時，我會在兩個單獨的代碼行上拋出異常，我應該將其中一個注釋掉，問題跳轉到另一個但是兩個都引用相同的變量。

GameScreenManager.cpp：

#pragma once
#include <SDL.h>
#include "Commons.h"
#include "GameScreenManager.h"
#include "GameScreenLevel1.h"
#include "GameScreen.h"

GameScreenManager::GameScreenManager(SDL_Renderer* renderer, SCREENS startScreen)
{
    SDL_Renderer* mRenderer;
    mRenderer = NULL;
    ChangeScreen(startScreen);
}

GameScreenManager::~GameScreenManager()
{
    mRenderer = NULL;
    delete mCurrentScreen;
    mCurrentScreen = NULL;
}

void GameScreenManager::Render()
{
    mCurrentScreen->Render();
}

void GameScreenManager::update(float deltaTime, SDL_Event e)
{
    mCurrentScreen->Update(deltaTime, e);
}

void GameScreenManager::ChangeScreen(SCREENS newScreen)
{
    //Clear up the old screen
    if (mCurrentScreen != NULL)
    {
        delete mCurrentScreen;
    }

    GameScreenLevel1* tempScreen;


    switch (newScreen)
    {
    case SCREEN_INTRO:
        break;
    case SCREEN_MENU:
        break;
    case SCREEN_LEVEL1:
        tempScreen = new GameScreenLevel1(mRenderer);
        mCurrentScreen = (GameScreen*)tempScreen;
        tempScreen = NULL;
        break;
    case SCREEN_LEVEL2:
        break;
    case SCREEN_GAMEOVER:
        break;
    case SCREEN_HIGHSCORES:
        break;
    default:
            break;
    }
}

GameScreenManager.h：

#pragma once
#ifndef _GAMESCREENMANAGER_H
#define _GAMESCREENMANAGER_H
#include <SDL.h>
#include "Commons.h"

class GameScreen;

class GameScreenManager
{
    private:
        SDL_Renderer*    mRenderer;
        GameScreen*      mCurrentScreen;

    public:
        GameScreenManager(SDL_Renderer* renderer, SCREENS startScreen);
        ~GameScreenManager();

        void Render();
        void update(float deltaTime, SDL_Event e);
        void ChangeScreen(SCREENS newScreen);
};

#endif //_GAMESCREENMANAGER_H

目前問題出現在第24行;

    mCurrentScreen->Render();

但是，如果我注釋掉該行，那么它將出現在第29行：

    mCurrentScreen->Update(deltaTime, e);

“拋出未處理的異常：讀取訪問沖突。 這是nullptr。發生”

Answer 1

顯然，當您調用Render（）或update（）時，mCurrentScreen字段為NULL。 通過檢查mCurrentScreen是否為NULL來保護呼叫：

void GameScreenManager::Render()
{
    if (mCurrentScreen != NULL)
        mCurrentScreen->Render();
}

void GameScreenManager::update(float deltaTime, SDL_Event e)
{
    if (mCurrentScreen != NULL)
        mCurrentScreen->Update(deltaTime, e);
}

或者確保在調用Render（）或Update（）之前設置了mCurrentScreen。

Answer 2

實際上，我實際上錯過了設置mCurrentScreen的一行代碼，並且switch語句未正確傳遞，因此它沒有觸發。 另外，在我的任何cpp文件中都沒有正確設置mRenderer變量，因此每當我嘗試使用它時都會返回“Invalid Renderer”。 以下是工作代碼，感謝所有人幫助我朝着正確的方向前進;

#pragma once
#include <SDL.h>
#include "Commons.h"
#include "GameScreenManager.h"
#include "GameScreenLevel1.h"
#include "GameScreen.h"

GameScreenManager::GameScreenManager(SDL_Renderer* renderer, SCREENS startScreen)
{
    mRenderer = renderer;
    GameScreen* mCurrentScreen = NULL;
    ChangeScreen(startScreen);
}

GameScreenManager::~GameScreenManager()
{
    mRenderer = NULL;
    delete mCurrentScreen;
    mCurrentScreen = NULL;
}

void GameScreenManager::Render()
{
    mCurrentScreen->Render();
}

void GameScreenManager::Update(float deltaTime, SDL_Event e)
{
    mCurrentScreen->Update(deltaTime, e);
}

void GameScreenManager::ChangeScreen(SCREENS newScreen)
{
    //Clear up the old screen
    if (mCurrentScreen != NULL)
    {
        delete mCurrentScreen;
    }

    GameScreenLevel1* tempScreen;

    switch (newScreen)
    {
    case SCREEN_INTRO:
        break;
    case SCREEN_MENU:
        break;
    case SCREEN_LEVEL1:
        tempScreen = new GameScreenLevel1(mRenderer);
        mCurrentScreen = (GameScreen*)tempScreen;
        tempScreen = NULL;
        break;
    case SCREEN_LEVEL2:
        break;
    case SCREEN_GAMEOVER:
        break;
    case SCREEN_HIGHSCORES:
        break;
    default:
        break;
    }
}