Convert VARIANT to...?

Question

Note:

• Attempting to invoke a method of an interface, of which the return type is _variant_t

Code:

_variant_t resultsDataString;

_bstr_t simObjectNames;

simObjectNames = SysAllocString (L"TEST example 3");

resultsDataString = pis8->GetSimObject (simObjectNames);

inline function illustrated below, contained in .tli FILE:

inline _variant_t IS8Simulation::GetSimObject ( _bstr_t Name ) {
    VARIANT _result;
    VariantInit(&_result);
     HRESULT _hr = get_SimObject(Name, &_result);
   if (FAILED(_hr)) _com_issue_errorex(_hr, this, __uuidof(this));
    return _variant_t(_result, false);
}

Note:

• resultsDataString is of struct tagVARIANT:
  • VARTYPE vt is 9 (unsigned short)
  • IDispatch IDispatch interface pointer

Question

• How can I then convert or extract the value?
• Possible using VariantChangeType?
• Another way?

Edit:

Note:

• Looking to transform the following, Visual Basic code to Visual C++

• MFC or ATL, if need be

• Ideally, pure C++

Visual basic equivalent:

Public example1, example2 As SIMUL8.S8SimObject
Dim numberOfexamples As Variant
Dim resultString As Variant

Set example1 = MySimul8.SimObject("Example 1")
Set example2 = MySimul8.SimObject("Example 2")

numberOfexamples = example1.CountContents + example2.CountContents

resultString = CStr(numberOfexamples) & "*"

Answer 1

Take a look at the MSDN docs for _variant_t

There is a ChangeType method as well as Extractors.

Edit: Once you have an IDispatch pointer, you need to use QueryInterface to get a specific object type with members and methods, or use IDispatch::Invoke. You can look at the MSDN Docs for IDispatch. Either way, you need to know about the object type that is being returned by the IS8Simulation::GetSimObject call.

Edit #2: Based on your VB code update, you want to use the same kind of code for C++, i.e. use the S8SimObject type in its C++ form (look in the generated .tlh file for _COM_SMARTPTR_TYPEDEF). Then you can directly do the same thing with CountContents. Otherwise, you would need to look up the CountContents DISPID with IDispatch::GetIDsOfNames and then call invoke.

Answer 2

It appears you are using C++ as a COM client by relying on the VC++ compiler's built-in COM support. To make coding the client "easier" you've used #import to generate C++ wrapper classes that attempt to hide all the COM details from you - or at least make the COM details simpler. So you're not using the COM SDK directly, but are using a client-side framework (I think of it like a light-weight COM-only framework akin to ATL or MFC).

Your example code, however, seems to be mixing the direct low-level COM SDK (VARIANTs, BSTR, SysAllocString) with the #import COM framework (_variant_t, _bstr_t, XXXXPtr). COM from C++ is complicated at first - so in a perfect world I would suggest getting to know the basics of COM before going too far forward.

However, if you just want something to work I would guess this is #import-style-of-COM-clients version of the VB code you provided:

_variant_t example1Var;
_variant_t example1Var;
SIMUL8::S8SimObjectQIPtr example1; // I'm guessing at this type-name from the VB code
SIMUL8::S8SimObjectQIPtr example2;

example1Var = pis8->GetSimObject(_bstr_t(L"Example 1"));
example2Var = pis8->GetSimObject(_bstr_t(L"Example 2"));
if (example1Var.vt == VT_DISPATCH && example2Var.vt == VT_DISPATCH)
{
   // **UPDATE** to try to spoon feed the QI ptr...
   example1 = IDispatchPtr((IDispatch*)example1Var);
   example2 = IDispatchPtr((IDispatch*)example2Var);
   // Does this screw-up reference counting?

   int numberOfexamples = example1->CountContents + example2->CountContents;

}

UPDATE:
Documentation on #import This makes using COM from C++ much easier, but is yet one other thing to learn...

Answer 3

You don't have to convert the _variant_t. It contains an IDispatch interface pointer, so you can get it out like this:

if (v.vt == VT_DISPATCH)
{
    IDispatchPtr pDispatch = v.pdispVal;
    // Do something with pDispatch.
    // e.g. assign it to a strongly-typed interface pointer.
}

Answer 4

There is a concept of "value" for an object (DISPID_VALUE).

Here is a codeproject.com article on resolving VARIANTs that might help.