Sunday, December 2, 2012

Automation for C++ COM pointers

This week I was working on a C++ program which used a lot of COM pointers. I remembered I’ve seen an automation class somewhere, and after some searching I found it, it’s Microsoft’s ComPtr template class. Although I liked the concept, it had too much fuss to me, so I was displeased in using it.

Oh well, here we go again: I wrote my own COM pointer automation class. Here it goes the full code of it, which I now share with the world:
#pragma once
#include <Windows.h>
#include <ObjBase.h>

template<typename T> class ComPtr {
	ComPtr()                    : _ptr(NULL) { }
	ComPtr(const ComPtr& other) : _ptr(NULL) { operator=(other); }
	~ComPtr()                   { this->release(); }
	ComPtr& operator=(const ComPtr& other) {
		if(this != &other) {
			_ptr = other._ptr;
			if(_ptr) _ptr->AddRef();
		return *this;
	void release() {
		if(_ptr) {
			_ptr = NULL;
	bool coCreateInstance(REFCLSID rclsid) {
		return _ptr ? false :
			SUCCEEDED(::CoCreateInstance(rclsid, 0, CLSCTX_INPROC_SERVER, IID_PPV_ARGS(&_ptr)));
	bool coCreateInstance(REFCLSID rclsid, REFIID riid) {
		return _ptr ? false :
			SUCCEEDED(::CoCreateInstance(rclsid, 0, CLSCTX_INPROC_SERVER, riid, (void**)&_ptr));
	template<typename COM_INTERFACE> bool queryInterface(REFIID riid, COM_INTERFACE **comPtr) {
		return !_ptr ? false :
			SUCCEEDED(_ptr->QueryInterface(riid, (void**)comPtr));
	bool isNull() const { return _ptr == NULL; }
	T&   operator*()    { return *_ptr; }
	T*   operator->()   { return _ptr; }
	T**  operator&()    { return &_ptr; }
	operator T*() const { return _ptr; }
	T *_ptr;
Basically, by using this class I don’t have to worry about calling the Release method. Most interestingly, I added two shorhand methods to ease my life, coCreateInstance and queryInterface. Here are an example illustrating the use of the ComPtr class, along with the cited methods:
	ComPtr<IGraphBuilder> graph;

	ComPtr<IMediaControl> mediaCtrl;
	if(!graph.queryInterface(IID_IMediaControl, &mediaCtrl))

	graph->RenderFile(L"foo.avi", NULL); // sweet!
If you’re wondering about the usefulness of this ComPtr class, try to write the code above without it.