What is the best way to get a callback from an instance of a native C++ class to a C++ ref class?

Question

I am writing a C++ WinRT Component DLL for use in my .NET-based WinRT application. The DLL defines a SoundSample ref class that creates an XAudio voice by calling IXAudio2::CreateSourceVoice. CreateSourceVoice takes a "IXAudio2VoiceCallback *pCallback" parameter to enable callbacks on various audio events. Now I am trying to implement that callback based on this article. XAudio will supposedly just call back into methods of my SoundCallback class defined as:

#pragma once
#include "xaudio2.h"
#include "pch.h"

class SoundCallback
    : public IXAudio2VoiceCallback
{
private:
    //SoundSample^ sample; //does not compile

public:
    SoundCallback(void);
    ~SoundCallback(void);

    //Called when the voice has just finished playing a contiguous audio stream.
    void OnStreamEnd();
    void OnVoiceProcessingPassEnd();
    void OnVoiceProcessingPassStart(UINT32 SamplesRequired);
    void OnBufferEnd(void * pBufferContext);
    void OnBufferStart(void * pBufferContext);
    void OnLoopEnd(void * pBufferContext);
    void OnVoiceError(void * pBufferContext, HRESULT Error);
};

Everything is fine until I try to figure out how to call back from an instance of my native callback class to the parent SoundSample object. I was thinking I could pass an instance of the SoundSample class to the SoundCallback object, but it seems like it does not allow me to declare a ref class field in the native class:

SoundCallback.h(9): error C2143: syntax error : missing ';' before '^'

SoundCallback.h(9): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

SoundCallback.h(9): error C3699: '^' : cannot use this indirection on type 'int'

I looked back at implementing callbacks in native C++ and I could not find a reasonable solution so far. What is the best/easiest way to do this?

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Filip Skakun

Answer 1

Solved it (thanks to Jeremiah Morrill) - the problem is not with any barrier blocking the use of ref classes in basic classes. C4430 means that SoundSample is an unrecognized type, which was hidden by Intellisense - since that seemed to indicate that SoundSample is known. What needs to be added is a declaration of the SoundSample type and this all starts working fine.

I just added

namespace MyNamespace { ref class SoundSample; }

before the SoundCallback class declaration and then SoundCallback class could declare:

MyNamespace::SoundSample^ sample;

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Filip Skakun

Answer 2

Thanks for sharing the solution.

Best regards,
Jesse

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Jesse Jiang [MSFT]
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