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.

For Beginners C++

Started by docyoung83 January 31, 2023 05:08 PM

10 comments, last by docyoung83 1 year, 3 months ago

docyoung83

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February 05, 2023 04:23 PM

Very cool! Thanks for the info. I'll have to look into it.

programming problem - pointer to pointer question

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