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LPVARIANT by ref parameter setting: First-chance exception

Posted on 2002-07-26
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Last Modified: 2007-12-19
Hi,  (VC++ 6.0 ATL project)

i'm getting a First-chance exception in OLEAUT32.DLL when trying to execute the VariantCopy statement below.  In MSDN, it says VariantCopy should be used on initialised VARIANT objects.  However, this code was in CodeGuru and MSDN online.  The parameter passed in a LPVARIANT which has not been initialised.  I wanted it to point to the
objects held in Map &pIt->second variable (which incidentally is a VARIANT).  I cannot initialise the pointer as it is returned by the calling function (i.e. i don't want to return a locally initialised variable.

Basically, I want a way of setting the pointer to the address of the &pIt->second object.

I have tried this as well:

*pCopy = pIt->second;

but get a First-chance exception thrown as well.

Thanks


// VarMap::iterator to the external representation, VARIANT
class _CopyVarMapToVariant
{
public:
     static HRESULT copy(LPVARIANT pCopy, std::pair<const CAdapt<CString>, VARIANT>* pIt)
     {
          VariantCopy(pCopy, &pIt->second);
          return S_OK;
     }
     static void init(VARIANT* p) {VariantInit(p);}
     static void destroy(VARIANT* p) {VariantClear(p);}
};
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Question by:ossentoo
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8 Comments
 
LVL 30

Expert Comment

by:Axter
ID: 7180686
Stry changing your std::pair to the following:
std::pair<const CAdapt<CString>, LPVARIANT>*
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Expert Comment

by:Axter
ID: 7180694
Actually, I think your problem is that you have the arguments switched around.

HRESULT VariantCopy(
  VARIANTARG FAR*  pvargDest,  
  VARIANTARG FAR*  pvargSrc  
);
 

So try the following:
VariantCopy(&pIt->second, pCopy);
0
 
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Expert Comment

by:Axter
ID: 7180704
Disregard last comment.

If you're trying to get pCopy to point to pIt->second, then you should use the following syntax:

pCopy = &pIt->second;

Continue.....
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LVL 30

Expert Comment

by:Axter
ID: 7180737
pCopy = &pIt->second;

Although the above method will get pCopy to point to the value of pIt->second, this change will not get reflected on the calling function.

In order to change the variable of the calling function you need to modify your function argument to the following:

static HRESULT copy(LPVARIANT &pCopy, std::pair<const CAdapt<CString>, VARIANT>* pIt)

Notice the "&" next to pCopy.
0
 
LVL 30

Accepted Solution

by:
Axter earned 50 total points
ID: 7180754
The following is an example showing why you need the "&".
Both functions try to change the pointer being pass to it.
If the functions are successful, the pointer will be non-zero.
If you run this code, you'll notice only the second function is able to change the pointer.

void Funct1(LPVARIANT pCopy)
{
     ++pCopy;
}
void Funct2(LPVARIANT &pCopy)
{
     ++pCopy;
}

void CallingFunction()
{
     LPVARIANT data1 = NULL;
     LPVARIANT data2 = NULL;
     Funct1(data1);
     Funct2(data2);

     CString Msg;
     Msg.Format("Funct1 = %i, Funct2 = %i", data1, data2);
     AfxMessageBox(Msg);
}
0
 
LVL 30

Expert Comment

by:Axter
ID: 7180760
One last point.
You should avoid using names that beging with an underscore.
class _CopyVarMapToVariant //Not a portable name

Using names that begin with an underscore can make your code non-portable.  Not only that, you could have your code working in one version of the compiler, and not work on a new version of the compiler.

According to the C++ standard, names that begin with underscore are reserved to the implementation for any use.
0
 
LVL 30

Expert Comment

by:Axter
ID: 7181043
Check out the following link:

http://www.codeproject.com/shell/surfhelper.asp

The program is free, and the source code is available on the same link.

I hate to see you reinvent the wheel.
0
 

Author Comment

by:ossentoo
ID: 7181855
thx a lot Axter for all your answers.  
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