释放COM接口的正确方法

Question

We have some legacy software code, which uses COM. I've noticed that at one place, we perform a queryInterface on an IInspectable pointer, but dont bother calling release.

For ex:

void foo(IInspectable* myInterface)
{
    ComPtr<OBJTYPE> pObj;
    auto hr = myInterface->QueryInterface(__uuidof(SomeType),&pObj);
    if (hr != S_OK)
    {
        return hr;
    }
    //some code that uses pObj
}

I added Release to the above code like below :

auto hr = myInterface->QueryInterface(__uuidof(SomeType),&pObj);
if (hr != S_OK)
{
    return hr;
}
myInterface->Release(); //<-- Is this the correct way to release ?
//some code that uses pObj;

But I see that the above code crashes at times in the above function during release. Is there something wrong in the way I'm releasing the interface ? I can confirm that myInterface is not being used anywhere else in the function after the Release statement.

Apologies for not being able to copy/paste the actual code, but the above code summarizes pretty much the suspect code that I'm debugging. Basically what I want to know is, in the above scenario, do I need to call Release() ? And is this is the right place/way to call Release() ? Do I need to add any other safety checks in place ?

Answer 1

In the first foo function you should NOT call myInterface->Release() . Every call to Release should be paired with an adding of a reference. In that function you don't do myInterface->AddRef , so if you add myInterface->Release then your function will decrement the reference count by one, which does not seem like correct behaviour.

Conceptually, you should call AddRef on entry to the function and Release on exit of the function. Even though the reference count is "intrusive" in COM, the recommended coding style is to treat each interface pointer like it has its own reference count.

This is normally wrapped up in smart pointer classes. But when working with a function that receives a raw "in" interface pointer, this Add and Release can be "optimized" by just omitting both of those calls in the function, and treating the pointer like an observer pointer.

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The call to QueryInterface will implicitly do pObj->AddRef() but this is nothing to do with myInterface . When you are finished with pObj a pObj->Release() should occur -- but this is managed by the ComPtr wrapper, you shouldn't add an explicit call.