ArcGIS Engine Developer Guide

VISUAL C++
4. To create the noncreatable object inside a method, use the CComObject
template to supply the implementation of CreateInstance.
// Get NonCreatable object Bar (implementing IBar) from COM object Foo.
STDMETHODIMP CFoo::CreateBar(IBar **pVal)
{
if (pVal==0) return E_POINTER;
// Smart pointer to noncreatable object Bar
IBarPtr ipBar = 0;
// C++ Pointer to Bar, with ATL template to supply CreateInstance
implementation
CComObject* pBar = 0;
HRESULT hr = CComObject::CreateInstance(&pBar);
if (SUCCEEDED(hr))
{
// Increment the ref count from 0 to 1 to protect the object
// from being released in any initialization code.
pBar->AddRef();
// Call C++ methods (not exposed to COM) to initialize the Bar object.
pBar->InitialiseBar(10);
// QI to IBar and hold a smart pointer reference to the object Bar.
hr = pBar->QueryInterface(IID_IBar, (void**)&ipBar);
}
pBar->Release();
// Return IBar pointer to the caller.
*pVal = ipBar.Detach();
}
return S_OK;
ATL in Visual C++ .NET
Visual C++ version 6 is used for the majority of this help. However, with the
release of Visual C++ .NET, there are enhancements and changes that are relevant
to the ArcGIS ATL developer. Some of these are summarized below.
Attribute-based programming—This is a major change introduced in VC7.
Attributes are inserted in the source code enclosed in square brackets—for example,
[ coclass ]. Attributes are designed to simplify COM programming and
.NET framework common language runtime development. When you include
attributes in your source files, the compiler works with provider DLLs to insert
code or modify the code in the generated object files. There are attributes that aid
in the creation of .idl files, interfaces, type libraries, and other COM elements. In
the integrated development environment (IDE), attributes are supported by the
wizards and by the Properties window. The ATL wizards make extensive use of
attributes to inject the ATL boilerplate code into the class. Consequently, typical
108 • ArcGIS Engine Developer Guide