Smart Pointers
Microsoft's Active Template Library (ATL) contains helper classes that
simplify the use of commonly used COM interfaces, components, and structures,
such as BSTRs, interface pointers, and variants.
The ATL CComPtr
helper class provides a smart pointer wrapper around COM interface pointers.
In this context, a smart pointer is a pointer that properly releases its
reference counts toward COM interfaces when it goes out of scope.
To instantiate the interface types that you will be accessing, you use
CComPtr
with the interface type. For example:
CComPtr<ISldWorks> swApp
CComPtr<IModelDoc2> swModel
CComPtr<ISelectionMgr> swSelMgr
You can assign an interface pointer to your smart pointer in any of
the following ways:
Call the smart pointer's CoCreateInstance
member function.
Pass the interface's raw interface pointer to
CoCreateInstance.
Call the smart pointer's Attach
member function to assign a previously created interface pointer to the
smart pointer. This does not cause the smart pointer to call the smart
pointer's AddRef
function through the new interface pointer.
Simple assignment through the smart pointer's
assignment operator causes the smart pointer to call AddRef
through the new interface pointer.
Calling the smart pointer's CoCreateInstance
member function is the simplest way to create a new interface pointer.
For example, to attach to the currently running instance of the sldworks.exe COM server on your local
computer, use:
hres = swApp.CoCreateInstance(__uuidof(SldWorks),
NULL, CLSCTX_LOCAL_SERVER);
Like most other smart pointers, CComPtr
overrides C++ operators to make the smart pointers act like normal C++
pointers.
To set a smart pointer to the value of a pointer returned from a function,
you can use the &
operator, which works just like a non-smart pointer. If you need to decrement
a reference count, use the smart pointer's Release
member function.
To use ATL helper classes in your program, include atlbase.h
in stdafx.h. For an example of
using smart pointers, see Access Assembly Example (C++ COM).
For more information about using smart pointers with container classes, see
STL Container Classes and Smart Pointers.
For more information about CComPtr
and smart pointers, see MSDN.
Source: Williams, Mickey and Bennett,
David. Visual C++ Unleashed.
Sams Publishing, 2000.