从数组访问 SDL_Rect 成员时出现垃圾值？

Question

一直在关注LazyFoo的SDL tutorials (and also adding my own organization and coding style). When I got to his animation tutorial我决定单独做一个class来存放动画算法相关的变量和方法，而不是全局变量。他用一个SDL_Rects的数组来定义一个精灵sheet上不同精灵的边界，所以我用一个SDL_Rect的指针来存储我自定义的数组class。当我编译所有东西时我没有看到动画，当我编译原始源代码时我看到了。当我开始调试时，我发现当我渲染精灵时，矩形实际上充满了垃圾，即使当我初始化它们时矩形也很好。我曾多次尝试简化问题，但我在更简单的环境中重新创建错误的每一种方法实际上都按预期工作！因此，考虑到这一点，我为大量代码道歉，因为我似乎无法减少问题。

texture.h

#ifndef TEXTURE_H
#define TEXTURE_H

#include <SDL2/SDL.h>
#include <string>

class Animation {
public:
    Animation(SDL_Renderer* renderer);
    ~Animation();

    void load(std::string path, int frames, SDL_Rect* clips),
         free(),
         render(int x, int y),
         next_frame();
private:
    SDL_Renderer* _renderer=NULL;
    SDL_Rect* _clips=NULL;
    SDL_Texture* _texture=NULL;
    int _frame=0, _frames=0, _width=0, _height=0;
};

#endif

texture.cpp

#include <stdio.h>
#include <SDL2/SDL_image.h>
#include "texture.h"
#include "error.h"

Animation::Animation(SDL_Renderer* renderer) {
    _renderer = renderer;
}

Animation::~Animation() {
    free();
    _renderer = NULL;
}

void Animation::load(std::string path, int frames, SDL_Rect* clips) {
    free();
    SDL_Texture* texture = NULL;

    SDL_Surface* surface = IMG_Load(path.c_str());
    if (!surface)
        throw ErrorIMG("Could not load image "+path);
    SDL_SetColorKey(surface, SDL_TRUE,
                    SDL_MapRGB(surface->format, 0, 0xFF, 0xFF));

    texture = SDL_CreateTextureFromSurface(_renderer, surface);
    if (!texture)
        throw ErrorSDL("Could not create texture from image "+path);

    _width = surface->w;
    _height = surface->h;
    SDL_FreeSurface(surface);

    _frames = frames;
    _clips = clips;
    printf("clips[%d]: w: %d h: %d\n", 0, _clips[0].w, _clips[0].h);
}

void Animation::free() {
    if (_texture) {
        SDL_DestroyTexture(_texture);
        _texture = NULL;
        _clips = NULL;
        _frames = 0;
        _frame = 0;
        _width = 0;
        _height = 0;
    }
}

void Animation::render(int x, int y) {
    SDL_Rect crect = _clips[_frame/4];
    printf("in render (clips[%d]): w: %d, h: %d\n", _frame/4, crect.w, crect.h);
    SDL_Rect render_space = {x, y, crect.w, crect.h};
    SDL_RenderCopy(_renderer, _texture, &_clips[_frame], &render_space);
}

void Animation::next_frame() {
    SDL_Rect crect = _clips[_frame/4];
    printf("in next frame (clips[%d]): w: %d, h: %d\n", _frame/4, crect.w, crect.h);
    ++_frame;
    if (_frame/4 >= _frames)
        _frame = 0;
}

game.h

#ifndef GAME_H
#define GAME_H

#include "texture.h"

class Game {
public:
    Game();
    ~Game();
    void main();
private:
    void load_media();

    SDL_Window* _window=NULL;
    SDL_Renderer* _renderer=NULL;
    Animation* _anim=NULL;

    const int SCREEN_WIDTH=640, SCREEN_HEIGHT=480;
};

#endif

game.cpp

#include <SDL2/SDL_image.h>
#include "game.h"
#include "error.h"

void Game::main() {
    load_media();

    bool has_quit = false;
    SDL_Event event;

    while (!has_quit) {
        while (SDL_PollEvent(&event))
            if (event.type == SDL_QUIT)
                has_quit = true;

        SDL_SetRenderDrawColor(_renderer, 0xff, 0xff, 0xff, 0xff);
        SDL_RenderClear(_renderer);

        _anim->render(100, 100);
        _anim->next_frame();
        SDL_RenderPresent(_renderer);
    }
}

Game::Game() {
    if (SDL_Init(SDL_INIT_VIDEO))
        throw ErrorSDL("SDL could not initialize");

    _window = SDL_CreateWindow("SDL Tutorial", SDL_WINDOWPOS_UNDEFINED,
                               SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH,
                               SCREEN_HEIGHT, SDL_WINDOW_SHOWN);
    if (!_window)
        throw ErrorSDL("Window could not be created");

    Uint32 render_flags = SDL_RENDERER_ACCELERATED;
    render_flags |= SDL_RENDERER_PRESENTVSYNC;
    _renderer = SDL_CreateRenderer(_window, -1, render_flags);
    if (!_renderer)
        throw ErrorSDL("Renderer could not be created");
    SDL_SetRenderDrawColor(_renderer, 0xff, 0xff, 0xff, 0xff);

    if (!(IMG_Init(IMG_INIT_PNG) & IMG_INIT_PNG))
        throw ErrorIMG("SDL_image could not initialize");
}

Game::~Game() {
    delete _anim;

    SDL_DestroyRenderer(_renderer);
    SDL_DestroyWindow(_window);
    _renderer = NULL;
    _window = NULL;

    IMG_Quit();
    SDL_Quit();
}

void Game::load_media() {
    const int nclips = 4;
    SDL_Rect clips[nclips];

    for (int i=0; i < nclips; i++) {
        clips[i].x = i*64;
        clips[i].y = 0;
        clips[i].w = 64;
        clips[i].h = 164;
    }

    _anim = new Animation(_renderer);
    _anim->load("sheet.png", nclips, &clips[0]);
}

Answer 1

堆栈分配的 Game::load_media()::clips 数组在超出范围时消失。在 Animation::load() 中创建一个副本，而不是只存储一个指针。

Answer 2

您正在存储一个指向临时文件的指针。您传递给 Animation::load 的 SDL_Rect* clips 指针被分配给函数 returns 之后使用的成员 _clips。为了使其正常工作，指向 的数据需要 在 Animation class 使用它时一直存在。问题出在这里：

void Game::load_media() {
    const int nclips = 4;
    SDL_Rect clips[nclips];
    ...
    _anim->load("sheet.png", nclips, &clips[0]);
}

在这段代码中，clips是一个local变量。这意味着它在 load_media() 结束时被销毁，并且该位置的内存内容将变成垃圾。

您可以通过多种方式解决此问题。一个简单的方法是使用 std::vector<SDL_Rect> 而不是 SDL_Rect*。 std::vector 可以安全地复制并为您管理其内部结构。您的新代码可能如下所示：

class Animation {
    ...
    std::vector<SDL_Rect> _clips;
    ...
}


void Animation::load(std::string path, int frames, std::vector<SDL_Rect> clips) {
    ...
    _clips = clips;
    ...
}

void Game::load_media() {
    const int nclips = 4;
    std::vector<SDL_Rect> clips;
    clips.resize(nclips);
    ...
    _anim->load("sheet.png", nclips, clips);
}

别忘了 #include <vector>。 std::vector 的文档是 here。请注意 std::vector 有一个 size() 方法，它可能会在任何出现的地方替换 frames。

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