SDL2-Visual Studio 2017 SDL_FreeWAV访问冲突

Question

I'm using SDL2, and created an Audio class to handle game music and sound effects. The sound works perfectly, but whenever the Audio class is destructed, the SDL_FreeWAV() call throws an access violation:

Exception thrown at 0x000000006C7A8737 (SDL2.dll) in Program.exe: 0xC0000005: Access violation reading location 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

As long as you have a default constructor and didn't define your own, the C++ compiler gives you a move constructor and a copy constructor for free. They shallow-copy all members of the object.

When you use operator= (assignment of the object to another object), it uses the copy constructor. When you return a temporary object (rvalue), it uses the move constructor.

Sadly the default copy and move constructors don't fit you in this case.

Consider this:

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

In this deceptively simple line of code you're:

1. Making a temporary (rvalue) Audio object
2. Calling Audio::operator=
3. Using the move constructor
4. Deleting the temporary object

So after this line of valid C++, a has:

1. device_id that was closed.
2. wav_buffer that was freed.

---

So how do we fix this?

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
    }
}

Now when an Audio object is moved, its wav_buffer is nullified so that it won't be cleaned up when it's destroyed.