Returning a struct from COM

Question

I am working on an ATL project in Visual Studio which exposes a COM object which defines a structure.

I have the following COM method defined in my *.idl:

[id(2)] HRESULT CheckOut([in] CHAR* feature, [in] CHAR* version, [out] CerberusCheckoutDetails* details, [out] CerberusErrorDetails* error);

My *.idl defines this structure:

[
    uuid(527568A1-36A8-467A-82F5-228F7C3AC926),
    version(1.0)
]
typedef struct CerberusErrorDetails {
    INT ErrorCode;
    BSTR ErrorMessage;
} CerberusErrorDetails;

My implementation returns this structure at some point:

int result = checkout(feature, version, 1, 0, remoteServerName);
if (result != 0)
{
    error = new CerberusErrorDetails();
    error->ErrorCode = result;
    error->ErrorMessage = _bstr_t(errstring()).Detach();
    return result;
}

I call it like this:

CerberusNativeLib::CerberusErrorDetails *error = new CerberusNativeLib::CerberusErrorDetails();
if (session->CheckOut(feature, version, details, error) != S_OK)
            std::wcout << error->ErrorCode << ": " << error->ErrorMessage << std::endl;

My issue is that error->ErrorCode is 0 but it should be -95 (stepping through the code, I can see that result is -95) and error->ErrorMessage is always NULL but it should contain an actual error message as errstring() returns a char * which is filled, such as Error trying to connect.. Why is that? I want to return a structure. Do I have to allocate it in some special way from the calling side or from the COM object side? Can someone provide an example of how to do this please?

Ðаn Ðаn · Accepted Answer · 2017-03-27T15:59:52

Without a complete example, it's hard to know exactly what you're trying to do. But it would appear that you may want to write the following

CerberusNativeLib::CerberusErrorDetails error;
if (session->CheckOut(feature, version, details, &error) != S_OK)

(no new, address of struct on the stack passed) and

int result = checkout(feature, version, 1, 0, remoteServerName);
if (result != 0)
{
    error->ErrorCode = result;
    error->ErrorMessage = _bstr_t(errstring()).Detach();
    return result;
}

(again, no new). Notice this has little to do with COM.

Returning a struct from COM

1 Answers