SDL2 - Visual Studio 2017 SDL_FreeWAV访问冲突

Question

我正在使用SDL2，并创建了一个Audio类来处理游戏音乐和声音效果。声音完美无缺，但每当Audio类被破坏时，SDL_FreeWAV()调用都会引发访问冲突：

Program.exe中的0x000000006C7A8737（SDL2.dll）抛出异常：0xC0000005：访问冲突读取位置0x00007FF4A9080008

Audio.h：

#pragma once

namespace Audio {

    class Audio {

    public:

        Audio ();
        Audio (char*, char*);
        ~Audio ();

        void pause (int);

    private:

        Uint32 wav_length;
        Uint8 *wav_buffer = NULL;
        SDL_AudioSpec wav_spec;
        SDL_AudioDeviceID device_id;
        int success;

    };

    class Music {

    public:

        Music ();
        Music (char*);
        ~Music ();

    private:

        Audio *audio = NULL;

    };

    class Effect {

    public:

        Effect ();
        Effect (char*);
        ~Effect ();

    private:



    };

};

Audio.cpp

#include "stdafx.h"
#include "Audio.h"

#include "SDL_audio.h"

namespace Audio {

    Audio::Audio () {

        //Default constructor

    }

    Audio::Audio (char *filename, char *channelName) {

        if (SDL_LoadWAV (filename, &this->wav_spec, &this->wav_buffer, &this->wav_length) == NULL) {

            std::cout << "[-] SDL: " << SDL_GetError () << "\n";

            exit (ERROR_SDL_AUDIO_WAV_LOAD);

        }

        this->device_id = SDL_OpenAudioDevice (channelName, 0, &this->wav_spec, NULL, 0);

        this->success = SDL_QueueAudio (this->device_id, this->wav_buffer, this->wav_length);

    }

    Audio::~Audio () {

        SDL_CloseAudioDevice (this->device_id);
        SDL_FreeWAV (this->wav_buffer); // <-- access violation here

    }

    void Audio::pause (int on) {

        SDL_PauseAudioDevice (this->device_id, on);

    }

};

Answer 1

只要您有一个默认构造函数并且没有定义自己的构造函数，C ++编译器就会免费为您提供move constructor和copy constructor。他们浅层复制对象的所有成员。

当您使用operator=（将对象赋值给另一个对象）时，它使用copy构造函数。当您返回临时对象（rvalue）时，它使用move构造函数。

遗憾的是，在这种情况下，默认的复制和移动构造函数不适合您。

考虑一下：

Audio a = Audio("filename", "channel");

在这个看似简单的代码行中，您将：

1. 制作临时（右值）Audio对象
2. 打电话给Audio::operator=
3. 使用move constructor
4. 删除临时对象

所以在这一行有效的C ++之后，a有：

1. 被关闭的device_id。
2. 释放的wav_buffer。

---

那么我们如何解决这个问题呢？

Audio::Audio (char *filename, char *channelName) {
    if (SDL_LoadWAV (filename, &this->wav_spec, &this->wav_buffer, &this->wav_length) == NULL) {
        std::cout << "[-] SDL: " << SDL_GetError () << "\n";
        exit (ERROR_SDL_AUDIO_WAV_LOAD);
    }
    this->device_id = SDL_OpenAudioDevice (channelName, 0, &this->wav_spec, NULL, 0);
    this->success = SDL_QueueAudio (this->device_id, this->wav_buffer, this->wav_length);
}

Audio::Audio(Audio&& other) : // move constructor
    wav_length(other.wav_length),
    wav_buffer(other.wav_buffer),
    wav_spec(other.wav_spec),
    device_id(other.device_id),
    success(other.success)
{
    other.wav_buffer = nullptr;
}

Audio::~Audio () {
    if(wav_buffer != nullptr) {
        SDL_CloseAudioDevice (this->device_id);
        SDL_FreeWAV (this->wav_buffer); // Gives access violation
    }
}

现在当移动Audio对象时，它的wav_buffer无效，因此当它被破坏时不会被清除。