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Getting in the RAII

, 17 Jun 2010 CPOL
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Looking at a simple MIDI handle wrapping class

One of Web Biscuit's long-term secret projects is the MidiWrapper, an object-oriented interface for MIDI. A cornerstone of MIDI operations is the HMIDIOUT handle, which requires opening before use, and closing when we're done with it.

This can be done in a RAII class. RAII stands for Resource Acquisition is Initialization, not the snappiest acronym in the whole wide world and not that self explanatory. Basically, we use the guarantee that creating an object always calls a constructor and eventually, a destructor. Played smartly, these two opposing calls can give us automatic Open/Close functionality.

Here's our class:

class CMidiOut
{
public:
    CMidiOut() {}
    ~CMidiOut() {}
private:
    HMIDIOUT m_hMidiOutHandle;
};

We could add Open and Close as public members, but there really is no need. We don't want to burden the creator of our object with things they might forget to do. So let's do it for them:

class CMidiOut
{
public:
    CMidiOut()  :
        m_hMidiOutHandle(nullptr)
    {
        midiOutOpen(
            &m_hMidiOutHandle,  	// Handle
            MIDI_MAPPER,  		// Default device
            NULL,  		// No callback
            NULL,  		// No callback parameters
            CALLBACK_NULL);  	// Flags
    }
    ~CMidiOut()
    {
        if(m_hMidiOutHandle != nullptr)
        {
            midiOutClose(m_hMidiOutHandle);
            m_hMidiOutHandle = nullptr;
        }
    }
private:
    HMIDIOUT m_hMidiOutHandle;
}; 

This is very cute. For a full solution, you'll want an accessor to your handle, and you'll need to prevent or handle copying of your class. These are all exciting subjects we can discuss next time.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

WebBiscuit
Software Developer Web Biscuit
United Kingdom United Kingdom
At Web Biscuit, you can find software, articles, a good dollop of quality and an unhealthy obsession over biscuits.
Website: http://www.webbiscuit.co.uk
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Comments and Discussions

 
GeneralRAII is not about initialisation... PinmemberAescleal18-Jun-10 6:58 
... it's all about destruction. The idea of RAII is make your code exception resistant and not leak resources when something goes wrong and not have to do the Java dance of try/finally.
 
Cheers,
 
Ash
 
PS: I'm also interested in seeing how you intend to handle errors from midiOutOpen.
GeneralRe: RAII is not about initialisation... PinmemberWebBiscuit18-Jun-10 7:10 
GeneralRe: RAII is not about initialisation... PinmemberAescleal18-Jun-10 9:26 

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