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danielj040300

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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
Avatar of migel
migel

Hi!
IMHO you need check what variant type method returns and if it BSTR you need destroy it by SysFreeString
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
Avatar of danielj040300

ASKER

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.
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
}
ASKER CERTIFIED SOLUTION
Avatar of peterchen092700
peterchen092700

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Thanks it worked!!
It was what I was looking for.

Daniel.
> 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  
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".