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Howto fill a variant structure

Posted on 1998-10-27
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Last Modified: 2013-11-20
I have a function that use a variant for parameter. I need to fill it with a buffer of char. How do I create, fill and destroy correcly this type of structure ?
I never used variant structure, so please be precise.
Thank you very much
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Question by:kharandir
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Norbert earned 200 total points
ID: 1323886
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by:Norbert
ID: 1323887
You are using MFC so the simplest way to use variants is to use
COleVariant
You say You will fill it with a buffer of char.
Is it a string or is it simple an array of byte data
if it is a string I would recommend to use - it is the simplest-
CString Str=YourBuffer;
COleVariant OleVar=Str.AllocSysString();
The class COlevariant has an assignment operator that does all for you
if it is an array you can use the class COleArray or directly fill the variant
For using COleArray have a look to the online help
Direct filling the Variant do the following:
VARIANT Var;
VariantInit(Var)
      
SAFEARRAY* SafeArray=NULL;
SAFEARRAYBOUND ArrayBoundaries[1]; //one dimensional array
ArrayBoundaries[0].lLbound = 0; /* First index of limits array is always 0 */
ArrayBoundaries[0].cElements=SizeOfYourArray;
SafeArray=SafeArrayCreate(VT_UI1,1,ArrayBoundaries)
if(SafeArray==NULL)
     ErrorHandling
else
{
     char* ArrayData=NULL;
     HRESULT hr=SafeArrayAccessData(SafeArray,(void**)&ArrayData);
     if(FAILED(hr)
       ErrorHandling
     else
     {
         memcpy(ArrayData,YourArray,YourArraySize);
         SafeArrayUnaccessData(SafeArray);
         Var.vt=VT_ARRAY|VT_UI1
         Var.parray=SafeArray;
       }
}
now the variant is filled and can be passed.
To clear the variant:
If you use COleVariant or COleArray it is cleared automatically
if the COleVariant/COleArray variable goes out of scope.
If you use  the Variant directly
simple call VariantClear(&Var).
Attention Note:
If Using something like
VARIANT Func()
{
       VARIANT Var;
       VariantInit(&Var);
       SAFEARRAY* SafeArray=NULL;
       SAFEARRAYBOUND ArrayBoundaries[1]; //one dimensional array
       ArrayBoundaries[0].lLbound = 0; /* First index of limits array is always 0 */
      ArrayBoundaries[0].cElements=SizeOfYourArray;
      SafeArray=SafeArrayCreate(VT_UI1,1,ArrayBoundaries)
      if(SafeArray==NULL)
           ErrorHandling
      else
      {
          char* ArrayData=NULL;
          HRESULT hr=SafeArrayAccessData(SafeArray,(void**)&ArrayData);
           if(FAILED(hr)
             ErrorHandling
           else
          {
               memcpy(ArrayData,YourArray,YourArraySize);
               SafeArrayUnaccessData(SafeArray);
               Var.vt=VT_ARRAY|VT_UI1
                Var.parray=SafeArray;
          }
       }
       return Var;
}
It is nice to use it like
COleVariant OleVar=Func();
_BUT_ this will produce a memory leak because
COleVariant has an assignmen operator that copies the array
and so the array created inside Func is lost.
in the case you want to use the functionality of COleVariant beter use
     COleVariant *OleVar=(COleVariant*)&(Func());
     and after done
     OleVar->Clear();
To access the array inside the variant
first check the datatype inside the variant:
      switch(Var.vt)
      {
       case VT_ARRAY|VT_I1:
              DoYorArrayHandlingStuff using SafeArrayAccessData(Var.parray,(void**)&YourPointerToData)
               Do with *pointer what ever you want
               SafeArrayUnaccessData(Var.parray);
        CASE other supported VT_TYPES
        default:
              Handle error for unsupported types
       
     }

I hope now something about variants is more clear.
Norbert
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