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How can I check that [out] parameters in COM can be used?

Officially, you shouldn't use parameters [out]

from COM functions if the function fails, which means there are (at least) three ways to see if a parameter can be used [out]

.

Consider the following interface

interface IFoo : IUnknown {
    HRESULT GetOtherFoo([out] IFoo** ppFoo);
    HRESULT Bar();
};

Which of the following methods would you recommend using it?

1. Check the return value

CComPtr<IFoo> other;
HRESULT hr = foo->GetOtherFoo(&other);
if (SUCCEEDED(hr)) 
    other->Bar();

This makes me a little bit nervous, as a mistake in IFoo

could lead to dereferencing of the pointer NULL

.

2. Check the output parameter

It depends on the fact that if the method fails, it should not change any of the parameters [out]

(if the parameter is changed <==> it is safe to use it).

CComPtr<IFoo> other;
foo->GetOtherFoo(&other);
if (other) 
    other->Bar();

Note that this kind of thing doesn't matter, CComPtr

destructor will call Release

if the pointer is not NULL

, so it cannot be garbage.

3. Paranoid way, check how

CComPtr<IFoo> other;
HRESULT hr = foo->GetOtherFoo(&other);
if (SUCCEEDED(hr) && other) 
    other->Bar();

It's a little wordy in my opinion.

PS See related question .

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2 answers


If you're willing to write more checks and make your code a little slower to make it more robust, Option 3 is for you. Since you expect the COM server to have bugs, it makes sense to check for them.



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COM server methods that return an HRESULT but some of their output parameters are NULL are not very common. There are several cases (IClientSecurity :: QueryBlanket comes to mind) where this is used, but usually the client can expect all output parameters to be non-NULL if the method returns successfully.



It is, after all, a question of how this document is documented. However, in the default case, I believe that 1. be the safe way to go.

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