
Title	COM with smart pointers (Used across several functions)
Summary	Use this method to instantiate a COM object without having to call CoCreateInstance. It also allows the COM object to be declared in the class declaration and created in InitialDialog() or simular, then used throughout the class.
Contributor	John McTainsh
Published	10-Sep-2000
Last updated	2-Aug-2001

Page rating	90% for 2 votes	Useless Brilliant

Description.

Smart pointers are a relatively simple method of accessing COM objects. They encapsulate much of the tricky code required to instantiate and use a COM object.

Where do the names come from?

The name of the items in this example can be found in the tlh file and class factory as follows;

Name here	Purpose	How to find
`TrickyCom.exe`	Name of the file containing the object. May be a DLL or EXE.	Path and name of file.
`TRICKYCOMLib`	This is the Library name space	The namespace is located in `.tlh` and begins with `namespace` and ends with `...Lib {`
`ICtrlFlasherPtr`	Smart pointer name	Look in `.tlh` and add `Ptr` to the end of the first parameter in `_COM_SMARTPTR_TYPEDEF(ICtrlFlasher,`
`CtrlFlasher`	The coclass name	Look in `.tlh` and use the name defines in `struct /* coclass */ CtrlFlasher;`
`Increment()` and `Count`	The methods and properties	Look in `.tlh` for the `// Property data` and `// Wrapper methods for error-handling` sections

Solution.

In StdAfx.h add.

//Reference Com object to create *.tlh and *.tli
#import "C:\Temp\TrickyCom\Debug\TrickyCom.exe"
//The namespace is located in *.tlh and ends in ...Lib {
using namespace TRICKYCOMLib;

In Gizzmo.cpp in BOOL CGizzmoApp::InitInstance(); or any simular start-up function that is called only once.

    //Initialise the application for use with COM objects
    HRESULT hr = CoInitialize( NULL );
    if( FAILED( hr ) )
    {
        AfxMessageBox( "CoInitialize failed\n");
        return FALSE;
    }

    //TODO Call CoUninitialize(); in the Destructor or other location
    //when all COM activity is completed.

In the header of the class place the object reference variable. UseItHere.h

    ICtrlFlashPtr m_pFlash;                 //Smart pointer

In class contructor initialise the variable. UseItHere.cpp Constructor.

    m_pFlash = NULL;

In class destructor release the object. UseItHere.cpp Destructor.

    m_pFlash.Release();
    m_pFlash = NULL;

In an initialisation routine such as InitDialog instantiate the object. UseItHere.cpp

    //Instantiate the smart pointer
    hr = m_pFlash.CreateInstance( __uuidof( CtrlFlash ) );

    if( FAILED( hr ) )
    {
        TRACE( _T("ERROR : Instanciate FLASH FAILED %x!!!\n"), hr );
        AfxMessageBox( "Flash load failed\n");
        return FALSE;
    }

Place this code where you want to access the object. ie In a button handler.

    try
    {
        TRACE( _T("Inital count = %d\n"), pFlash->nBeerCount );
        pFlash->MoreBeer();
    }
    catch( _com_error e )
    {
        //Catch and display errors
        TRACE( _T("Com Error %x, %s\n"), e.Error, e.ErrorMessage() );
    }

Comments

Date