(MSVC/COM) Allocating BSTR strings?

[BeanDog]

Who is responsible for allocating strings in COM calls?

For instance, if I have a method with an [out] BSTR* parameter, do I have to call AllocSysString and/or SysFreeString to pass it out? Same question for [out,retval]'s. i.e.:

STDMETHODIMP CESValhalla::GetFolderById(long lFolderId, BSTR *bsFolderName/*out*/, BSTR *bsFolderDescription/*out,retval*/)
{
*bsFolderName = CString("Name&quot.AllocSysString();
*bsFolderDescription = CString("Description&quot.AllocSysString();
return(S_OK);
}

Is this correct, or should I be doing something else? Sometimes unexpected things happen once in a while, like both BStr's will end up saying "Name" or both will end up saying "Description". How should I be doing this?


Same question for firing COM events. Should I do this:

Fire_OnFileScoutProgress(csPath.AllocSysString());

or what? Who's responsible for doing what for COM parameter strings?



~BenDilts( void );

 

[jpoggio]

Hello,

I noticed that the best translation from the IDL declaration
[out] BSTR *pbstrEtc... was BSTR **pbstrEtc in the C++ method declaration.

The drawback is that, in the class view tree, the method is no longer related to the interface node, but surprisingly, it suits the actual parameter passing scheme.

Maybe some marshalling issue ?
(Did anyone else experience this ?)

Hope it helps.

 

[Mike42]

There is a nice article in the MSDN library:

The Rules of the Component Object Model:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dncomg/html/msdn_therules.asp