InvokeHelper, VARIANT memory leak

I placed an ActiveX control into a visual C++ dialog, a interface file was build. I modified the interface from:

void CVGraph::DrawXYGraph(LPCTSTR Data)
{
     static BYTE parms[] =     VTS_BSTR;
     InvokeHelper(0x2, DISPATCH_METHOD, VT_EMPTY, NULL, parms, Data);
}

to:

HRESULT CVGraph::DrawXYGraph(LPCTSTR Data)
{
     static BYTE parms[] =     VTS_BSTR;
     VARIANT varhr;
     InvokeHelper(0x2, DISPATCH_METHOD, VT_VARIANT, (void*)&varhr, parms, Data);
     return varhr.lVal;
}

because I need to know the return value. But it causes a memory leak. How can I get rid of the memory leak or even better what is the correct way of doing this.

The function in the ActiveX control is as follow:
STDMETHODIMP CVGraph::DrawXYGraph(BSTR bstrData)
{
     if( !processString( bstrData ) )
          return S_FALSE;
     DrawGraph();
     return S_OK;
}

Thanks very much.
Daniel
LVL 1
danielj040300Asked:
Who is Participating?
 
peterchen092700Connect With a Mentor Commented:
a) How do you see it's an HRESULT?

the declaration does not indicate a dispatch return value (otherwise there would be something like STDMETHODIMP CVGraph::DrawXYGraph(BSTR bstrData, /* [out,retval] */ RETTYPE * retval). HRESULT's are (almost) never returned as dispatch return value.

Before you call InvokeHelper, call VariantInit(&varhr);
This will clear the variable (which ciontains "almost random" data) before the function is called. So you see if there's actually something written into it. If so, please post the vartype (varhr.vt) after InvokeHelper returned


b) Shaun: MFC uses it's own exception handling mechanism, so it's

TRY
{
  DrawXYGraph(LPCTSTR Data)
}
CATCH (COleException,xcept)
{
  // xcept is a COleException *
  ....
}
END_CATCH


Peter
0
 
migelCommented:
Hi!
IMHO you need check what variant type method returns and if it BSTR you need destroy it by SysFreeString
0
 
peterchen092700Commented:
You wo't get the HRESULT this way.

If the method implementation returns an error code, InvokeHelper will throw an exception (COleException). You need to use the MFC Style exception handling macros (TRY, CATCH, AND_CATCH, END_CATCH) for this.

Note that, for IDispatch interfaces, "success codes" other than S_OK are not defined (or valid).

Peter
0
The 14th Annual Expert Award Winners

The results are in! Meet the top members of our 2017 Expert Awards. Congratulations to all who qualified!

 
danielj040300Author Commented:
Hi,

I do get a HRESULT!! You see the problem is that it leaks memory.

Are there any other or better ways of getting the HRESULT?

Could I use something else than InvokeHelper.

Could you please give an example on how to use the TRY, CATCH, AND_CATCH, END_CATCH.

Thanks.

Daniel.
0
 
ShaunWildeCommented:
how do you have amemory leak - I have use InvokeHelper for a long time and I have never known it to leak - it tidies up any BSTRs it creates before throwing the exception

also it iwll thriough an exception on an erro and terefore will not fill in your variant

see what happens if you

VARIANT varhr;
VariantInit(&varhr);
InvokeHelper(...);



example of try catch

try
{
  InvokeHelper(...);
}
catch(COleException *e)
{
  // hresult is in e->m_sc
}
0
 
danielj040300Author Commented:
Thanks it worked!!
It was what I was looking for.

Daniel.
0
 
ShaunWildeCommented:
> MFC uses it's own exception handling mechanism, so it's

but you can use C++ exception handling - the MFC handling is just a MACRO wrapper around the C++ exception handling  
0
 
peterchen092700Commented:
Yes I know they changed the implementaiton to use C++ exception handling in I think MFC 3.x, but as much as I remember the docs, MFC still does some "extra behind the scenes".
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.