declaration/definition and instantiation of COM objects with visual c++?

Question

i need to instantiate com object which is .dll and on local machine in visual c++,i know that it can be done by using CoCreateInstance("clsid") ,but i am confused about declaration.so can anyone explain all steps involved? for late binding as well as early binding

1. is any import/inclusion required
2. how to declare com object ?
3. any other steps required before createinstance (e.g CoInitialize?)

or provide any specific reference involving step by step code

Answer 1

First you have to call CoInitialize and don't forget to callCoUnitialize if initialization was successful.

So your code will have the following structure:

HRESULT hr = CoInitialize(NULL); 
if (SUCCEEDED(hr))
{
    try
    {
        CoCreateInstance(...)
        // ...
    }
    catch (_com_error &e)
    {
        //...
    }
    CoUninitialize();
}

For more information visit MSDN. I recommend you to start with The COM Library and then you should read something about CoInitialize and CoCreateInstance functions before you use them.

This tutorial could help you too: Introduction to COM - What It Is and How to Use It.

Answer 2

1. #import is very much recommended. if you import the typelib with #import, you'll be using the Native COM framework, which isolates some gritty details and makes life generally easier.

2. In Native COM, something like this:

   LibName::IMyInterfacePtr pInterface;

In raw C++:

IMyInterface *pInterface;

But see above.

3. Call CoInitialize() in the beginning of the program, CoUninitialize() in the end. if running inside a DLL, then it's much more complicated.