如何在 SDL2 中呈现同一结构的多个实例

Question

我正在使用 SDL2 构建 C++ 游戏。一切进展顺利，我能够让玩家射出一颗子弹。后来我修改了我的代码以呈现多个项目符号，如下所示。

主游戏循环

void Game::gameLoop(){
    Entity player;
    Entity bulletHead;
    Entity *bulletTail = new Entity();

    bulletTail = &bulletHead;

    player.x = 500;
    player.y = 640;
    player.dx = 0;
    player.dy = 0;
    player.texture = loadTexture((char*)"res/images/player.png");
    SDL_QueryTexture(player.texture, NULL, NULL, &player.w, &player.h);

    SDL_Texture *bulletTexture = loadTexture((char*)"res/images/bullet.png");

    while(gameState != GameState::EXIT){
        prepareScene(); // sets up rendering
        handleEvents(); // collects and precesses user input

        player.x += player.dx;
        player.y += player.dy;

        // Player key input
        if(player.reload > 0) player.reload--;

        if (up)
        {
            player.y -= 4;
        }

        if (down)
        {
            player.y += 4;
        }

        if (left)
        {
            player.x -= 4;
        }

        if (right)
        {
            player.x += 4;
        }

        // allow fire bullet every 8 frames
        if(fire && player.reload == 0){
            player.reload = 8;

            // Create bullet
            Entity *bullet = new Entity();
            memset(bullet, 0, sizeof(Entity));

            bulletTail->next = bullet;
            bulletTail = bullet;

            bullet->x = player.x + 8;
            bullet->y = player.y;
            bullet->dx = 0;
            bullet->dy = -PLAYER_BULLET_SPEED;
            bullet->health = 1;
            bullet->texture = bulletTexture;
            SDL_QueryTexture(bullet->texture, NULL, NULL, &bullet->w, &bullet->h); 
        }

        // handle physics and render for each bullet
             Entity *b = new Entity();
             Entity *prev = new Entity();
             prev = &bulletHead;
// Error starts here 👇 🚫🚫🚫🚫🚫🚫🚫🚫🚫🚫🚫
             for (b = bulletHead.next ; b != NULL ; b = b->next){
                 b->x += b->dx;
                 b->y += b->dy;

                 blit(b->texture, b->x, b->y);
            
                 if(b->y < 0){
                    if (b == bulletTail)
                    {
                        bulletTail = prev;
                    }
                    prev->next = b->next;
                    free(b);
                    b = prev;
                 }

                 prev = b;

             }
            
// Error stops here 🚫🚫🚫🚫🚫🚫🚫🚫🚫🚫🚫

        blit(player.texture, player.x, player.y); // display image


        presentScene(); // displays scene
        SDL_Delay(16); // limits fps to around 62fps
    }

    SDL_DestroyWindow(window);
    SDL_Quit();
}

如上所示，我可以成功创建项目符号，但每当我包含 for 循环以遍历我的项目符号链接列表并移动/绘制它们时，程序都会意外关闭。没有显示错误，我一运行它就关闭了。删除 for 循环是有效的，但我需要一种方法来遍历项目符号的链接列表并绘制/动画它们。

任何帮助将不胜感激

Answer 1

标准库有很多容器供你使用，例如

std::vector

。下面的例子假设你有一个

std::vector<Entity> bullets

。 如果你想遍历一个向量并且可以选择在你走的时候删除项目，有两个选项，你可以使用

erase

方法，它从向量中删除一个项目并返回一个新迭代器。

使用：

for (auto it = bullets.begin(); it != bullets.end(); /* no-op, see below */) {
  // do stuff with *it
  if (/* bullet needs to be removed */) {
    it = bullets.remove(it);
  } else {
    // advance to the next item
    it++;
}