How to extract an integer from the VARIANT structure?

This is a discussion on How to extract an integer from the VARIANT structure? within the Networking/Device Communication forums, part of the General Programming Boards category; I am trying serial port communications between two computers. 1.To send data,one computer runs a DOS-based program and use _outp() ...

  1. #1
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    How to extract an integer from the VARIANT structure?

    I am trying serial port communications between two computers.

    1.To send data,one computer runs a DOS-based program and use _outp() to send an integer.

    2.To receive data,the other runs a windows-based program developed using MFC of VC,and MSComm is used to receive data.

    Now I have trouble in extract the integer from the VARIANT structure after trying all bVal,iVal,dblVal,lVal....the retrieved data from these members are not correct.

    Please give a hand to me.Thank you very much!

  2. #2
    &TH of undefined behavior Fordy's Avatar
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    As you are in MFC, try the COleVariant wrapper or the VC++ version _variant_t.

    Both have good extraction & conversion member functions

  3. #3
    Super Moderator VirtualAce's Avatar
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    VARIANTs from VB do not always return correct values back to C. Data misalignment is probably the reason but I've not researched it enough to know. I simply re-coded the other app in C because I was getting a serious headache trying to pass data back and forth between C and VB.

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    COleVariant wrapper?

    I have tried the COleVariant structure,the vt and iVal,bVal,lVal,dblVal...all these members doesn't work.

    Would you tell me how to use COleVariant wrapper?

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    it is a short integer

    Yes,I just send a short integer from the DOS-based program using _outp().For example,I sent 2,3,4....But I failed to extract it from the received VARIANT structure of VC program.

  6. #6
    &TH of undefined behavior Fordy's Avatar
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    Originally posted by onon
    COleVariant wrapper?

    I have tried the COleVariant structure,the vt and iVal,bVal,lVal,dblVal...all these members doesn't work.

    Would you tell me how to use COleVariant wrapper?
    I tend to use _variant_t more...but both are available with VC++ and both are pretty similar

    Code:
    #include <comdef.h>
    
    void foo(VARIANT* lpVar)
    {
    	_variant_t cVar(lpVar);
    	cVar.ChangeType(VT_I2);//assuming you want a 16 bit value
    	WORD wd = cVar;
    	//do whatever with wd
    }

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    Thanks,but it still exists some problem

    I have tried your code,there is a compiling error in the sentence:
    WORD wd = cVar;

    I also tried COleVariant instead of _variant_t,the similiar error occurs.

    the error message is:
    cannot convert from 'class COleVariant' to 'unsigned short'. No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

  8. #8
    &TH of undefined behavior Fordy's Avatar
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    It should work with _variant_t as shown as that object has an operator unsigned short() as well as an operator short().

    What compiler?

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    the compiler is MS Visual C++

    I use MS Visual C++ as the compiler.

    the code cannot pass the compile because of the error I mentioned above.

    Do you use the same compiler?

  10. #10
    &TH of undefined behavior Fordy's Avatar
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    Re: the compiler is MS Visual C++

    Originally posted by onon
    I use MS Visual C++ as the compiler.

    the code cannot pass the compile because of the error I mentioned above.

    Do you use the same compiler?
    I'm using Visual C++.NET. and it works on that, but suprise suprise.....I tried it on VC++6 (I assume you have this one) and it didnt compile.

    I think version 6's implementation of _variant_t lacked an operator short() function (now added on VC++.NET).

    This will work on VC++6

    Code:
    void foo(VARIANT* lpVar)
    {
    	_variant_t cVar(lpVar);
    	cVar.ChangeType(VT_I2);//assuming you want a 16 bit value
    	SHORT wd = cVar;
    	//do whatever with wd
    }

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