programming problem - pointer to pointer question

docyoung83 · 2023-02-05T16:23:38

I am working with a function called CreateSourceVoice that takes a pointer to a pointer as its first argument. The definition is: HRESULT CreateSourceVoice(IXAudio2SourceVoice **ppSourceVoice, ...); The following code does not work. I get an error at the spot designated by the carets stating that the expression must be an l-value or a function designator. class SourceVoice { public: IXAudio2SourceVoice* GetSourceVoice() { return sourceVoice; } private: IXAudio2SourceVoice* sourceVoice; }; SourceVoice musicVoice; void SoundSystem::Play(Sound& sound) { XAudio2->CreateSourceVoice(&(musicVoice.GetSourceVoice()), ...); ^^^^^ } I can easily fix this by creating another function called GetSourceVoiceAddress like below and it works fine. class SourceVoice { public: IXAudio2SourceVoice* GetSourceVoice() { return sourceVoice; } IXAudio2SourceVoice** GetSourceVoiceAddress() { return &sourceVoice; } private: IXAudio2SourceVoice* sourceVoice; }; SourceVoice musicVoice; void SoundSystem::Play(Sound& sound) { XAudio2->CreateSourceVoice(musicVoice.GetSourceVoiceAddress(), ...); } Or, if I store it in another pointer variable, it works fine. void SoundSystem::Play(Sound& sound) { IXAudio2SourceVoice* tempVoice = musicVoice.GetSourceVoice(); XAudio2->CreateSourceVoice(&tempVoice, ...); } My question is what is fundamentally wrong with the top example, and why do the bottom two examples work when it seems like they are all doing the same thing. I am vaguely familiar with r-values and l-values, but obviously not enough.