Using VARIANT !!! 2

[HadiRezaee]

Hi all,
Please explain to me, what is VARIANT structure and how can i use of it in my programs. (Please write source code)

Thanks.

 

[Cagliostro]

Typical OLE variant you should use like:
VARIANT x;
VariantInit(&x);//standard initialization
V_VT(&x)=VT_BOOL;//assigning some tpes to x, in this
//exapmple will be BOOL
VT_BOOL(&x)=1;//initializing the boolean x
VariantChangeType(&x,&x,0,VT_BSTR);//if you whant you may
//change type
....use variant there
VariantClear(&x);//freeing the x, you must do it

In ATL variant is more simple. Just use _variant_t. John Fill

ivfmd@mail.md

 

[HadiRezaee]

Thanks, John.
It was useful example.
Thanks.

 

[MSamoila]

In my opinion the above code unfortunately contains a subtile error and an inexact statement.
1. The BOOL type for VARIANT follows the VB conventions: 0 for false and -1 for true. For our convenience in VC++ there are two macros:

#define VARIANT_TRUE -1
#define VARIANT_FALSE 0

This type is a short, therefore -1 will be 0xffff (two bytes).

This line:

VT_BOOL(&x)=1;//initializing the boolean x

is not correct (the VT_BOOL is a macro without arguments) and value 1 will NOT set the BOOL as TRUE ! The corect line is:

x.boolVal = VARIANT_TRUE;//initializing the boolean x

2. The

_variant_t

is not an ATL class ! Is part of VC++ library. Just

#include <comdef.h>

and you can use it, as well as

_bstr_t

and

_com_error

ATL has another wrapper class for VARIANT:

CComVariant

. The MFC also has

COleVariant

. I prefer to use

CComVariant

and ATL because does not throw exceptions.

Marius Samoila
Brainbench MVP for Visual C++

http://www.brainbench.com

 

[HadiRezaee]

Hi Samoila.
Now, i understand ...
Thanks alooooooooot for your excelent reply.

 

[Cagliostro]

MSamoila, you are right, more correct is to put -1 instead of 1. But isn't an error because ProxyStub codes change it to correct value. Correct is to put every values what are not equal with 0 as true and 0 as false. John Fill

ivfmd@mail.md