Windows NT Systems Programming: Spring 2000

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Previous Simulated Example (ex24a)

• Components (server) don't live with clients, they live in DLLs or separate processes

Real Com  Example (ex24bc)

• Component server is placed in DLL
• DLL is registered.
• Component user must find DLL by looking in registry using functions found in the
• COM DLL which is loaded by "coinitialize" (so there are at least 2 DLLs involved - The com library and the specific COM object we wish to access)
• COM support is built into CCmdTarget Object
• COM is an evolving standard with an evolving set of tools and support libraries.
• Let's look at the client first - notice similarity to simulated one.

void CEx24cView::OnTestSpaceship( ) 
{
    CLSID clsid;
    LPCLASSFACTORY pClf; 
    LPUNKNOWN pUnk;
    IMotion* pMot;
    IVisual* pVis;
    HRESULT hr;

    if ((hr = ::CLSIDFromProgID(L"Spaceship", &clsid)) != NOERROR) {
        return;
    }
    if ((hr = ::CoGetClassObject(clsid, CLSCTX_INPROC_SERVER,
        NULL, IID_IClassFactory, (void **) &pClf)) != NOERROR) {;
        return;
    }

    pClf->CreateInstance(NULL, IID_IUnknown, (void**) &pUnk);
    pUnk->QueryInterface(IID_IMotion, (void**) &pMot); // All three
    pMot->QueryInterface(IID_IVisual, (void**) &pVis); //  pointers
                                                       //  should work
    // Test all the interface virtual functions
    pMot->Fly();
    int nPos = pMot->GetPosition();
    pVis->Display();

    pClf->Release();
    pUnk->Release();
    pMot->Release();
    pVis->Release();
}

• Converts user friendly name into UUID
• This creates a class factory object and returns its vtable
• creates a spaceship object and returns its IUNKNOWN interface
• Gets its two other interface vtables
• this is what the object does for a living

Looking at the Component Implementation

CSpaceship::CSpaceship()
{
    m_nPosition = 100;
    m_nAcceleration = 101;
    m_nColor = 102;
    // To keep the application running as long as an OLE automation
    //   object is active, the constructor calls AfxOleLockApp.
    
    AfxOleLockApp();
}

BEGIN_MESSAGE_MAP(CSpaceship, CCmdTarget)
    //{{AFX_MSG_MAP(CSpaceship)
    // NOTE - the ClassWizard will add and remove mapping macros here.
    //}}AFX_MSG_MAP
END_MESSAGE_MAP()

BEGIN_INTERFACE_MAP(CSpaceship, CCmdTarget)
    INTERFACE_PART(CSpaceship, IID_IMotion, Motion)
    INTERFACE_PART(CSpaceship, IID_IVisual, Visual)
END_INTERFACE_MAP()

STDMETHODIMP_(void) CSpaceship::XMotion::Fly()
{
    METHOD_PROLOGUE(CSpaceship, Motion)
    m_nPosition++; // move the spaceship
    return;
}

STDMETHODIMP_(int&) CSpaceship::XMotion::GetPosition()
{
    METHOD_PROLOGUE(CSpaceship, Motion)
    return pThis->m_nPosition;
}

STDMETHODIMP_(void) CSpaceship::XVisual::Display()
{
    METHOD_PROLOGUE(CSpaceship, Visual)
    TRACE("m_nPosition = %d\n", pThis->m_nPosition);
    TRACE("m_nColor = %d\n", pThis->m_nColor);
}

Copyright chris wild 1999/2000.
For problems or questions regarding this web contact [Dr. Wild].
Last updated: March 16, 2000.