Guide to BSTR and C String Conversions

• Download demo project - 11 Kb

Introduction

One of the confusing aspects of Windows programming is managing the conversion of Visual Basic style strings to/from C language style strings.  It isn't that it is so difficult, it is just difficult to remember the details, it is usually not done often, the MSDN documentation so voluminous that it is difficult to find answers to your questions.  But the worst part is that you could perform some typecast that compiles fine, but doesn't work the way you expect.  This results code that doesn't work, and the bugs are hard to track down.  After some experience, you learn to make sure your string conversions are doing what you expect.

C strings are arrays of characters terminated by a NULL character.  Visual Basic strings differ in that the length of the string preceded the characters in the string.  So a VB string knows it's own length.  In addition, all VB strings are Unicode (16 bits per character).

String Types

BSTR/C String conversions are required if:

• You are doing COM programming in C/C++
• You are writing multiple language applications, such as C++ DLL's accessed by Visual Basic applications. 

C Language String Types and Classes

This article deals with the following C/MFC/ATL string types:

• char/wchar/TCHAR -- The C strings for ANSI and Unicode
• CString -- The C++/MFC class wrapper for C strings
• BSTR -- The Visual Basic string type
• _bstr_t -- A C++ class wrapper for the Visual Basic string type
• CComBSTR -- Yet another C++ class wrapper for the Visual Basic string type used predominately in ATL code

Demo Project

The demo project is just an MFC dialog based application with buttons for each type of conversion.  It is built using VC++ 6.0.  It uses a couple of support functions you may find helpful:

BSTR GetBSTR()
{
    _bstr_t bstr1(_T("This is the test string."));
    
    BSTR bstr;

    bstr = bstr1.copy();

    return bstr;
}




CComBSTR GetComBSTR()
{
    CComBSTR bstr("This is the test string.");

    return bstr;
}


void CVbsDlg::ShowBSTR(BSTR bstr)
{
    _bstr_t bstrStart(bstr); 
    
    CString s;

    s.Format(_T("%s"), (LPCTSTR)bstrStart);

    AfxMessageBox(s);

}

Conversions

So let's get to it.  Here are the conversion techniques:

Converting BSTR to _bstr_t

    // BSTR to _bst_t

    BSTR bstrStart = GetBSTR();

    // use the constructor
    _bstr_t bstrFinal(bstrStart);

    ShowBSTR(bstrFinal);

    // Use the = operator
    bstrFinal = bstrStart;

    ShowBSTR(bstrFinal);

 Converting a _bstr_t to BSTR

You may want to get a BSTR from a _bstr_t class.

    // _bstr_t to BSTR

    _bstr_t bstrStart(_T("This is the test string."));

    BSTR bstrFinish;

    // use _bstr_t::copy member function
    bstrFinish = bstrStart.copy();

    ShowBSTR(bstrFinish);

    // use = operator
    bstrFinish = bstrStart;

    ShowBSTR(bstrFinish);

 

Converting a CComBSTR to BSTR

You may want to get a BSTR from a CComBSTR class.

    // CComBSTR to BSTR
    CComBSTR bstrStart(_T("This is the test string."));

    BSTR bstrFinish;

    // use the = operator
    bstrFinish = bstrStart;

    ShowBSTR(bstrFinish);

    // use the Copy member function
    bstrFinish = bstrStart.Copy();

    ShowBSTR(bstrFinish);
 

 Converting _bstr_t to CComBSTR

    // _bstr_t to CComBSTR
    _bstr_t bstrStart(_T("This is the test string."));

    CComBSTR bstrFinish;  

    bstrFinish.AppendBSTR(bstrStart);

    ShowBSTR(bstrFinish);

 Converting BSTR to C String

(Note :-  conversion that only works in Unicode)

    // BSTR to C String

    BSTR bstrStart;

    bstrStart = GetBSTR();

    TCHAR szFinal[255];

    // direct conversion from BSTR to LPCTSTR only works in Unicode
    _stprintf(szFinal, _T("%s"), (LPCTSTR)bstrStart);
    AfxMessageBox(szFinal);

    _bstr_t bstrIntermediate(bstrStart); // convert to _bstr_t
    CString strFinal;

    // you have to go through _bstr_t to have it work in ANSI and Unicode    
    _stprintf(szFinal, _T("%s"), (LPCTSTR)bstrIntermediate);

    // Or using MFC

    strFinal.Format(_T("%s"), (LPCTSTR)bstrIntermediate);

    AfxMessageBox(strFinal);

Converting _bstr_t to C String

(this works in both ANSI and Unicode)

    _bstr_t bstrStart(_T("This is the test string.")); 
    TCHAR szFinal[255];

    _stprintf(szFinal, _T("%s"), (LPCTSTR)bstrStart);

    AfxMessageBox(szFinal);
    

Converting CComBSTR to LPCTSTR

(not possible, must go through _bstr_t )

    // CComBSTR to C String
    CComBSTR bstrStart("This is the test string.");

    _bstr_t bstrIntermediate(bstrStart);

    TCHAR szFinal[255];

    _stprintf(szFinal, _T("%s"), (LPCTSTR)bstrIntermediate);

    AfxMessageBox(szFinal);

Converting LPCTSTR to _bstr_t

(Use a constructor or = operator)

    // LPCTSTR to _bstr_t

    LPCTSTR szStart = _T("This is the text string");

    // Use the constructor
    _bstr_t bstrFinal(szStart);

    ShowBSTR(bstrFinal);

    // or use = operator
    bstrFinal = szStart;
    
    ShowBSTR(bstrFinal);

 Converting LPCTSTR to CComBSTR

Use a constructor or CComBSTR::Append function

    // LPCTSTR to CComBSTR

    // Use a constructor    

    LPCTSTR szStart = _T("This is the text string");

    // Use the constructor
    CComBSTR bstrFinal(szStart);

    ShowBSTR(bstrFinal);

    // Or use the Append function
    bstrFinal.Empty();
    bstrFinal.Append(szStart);

    ShowBSTR(bstrFinal);

Conclusion

Well I tested all of the conversion in the demo project.  If you need to try others, download the demo for easy modification.  I am sure I will hear if there are any mistakes!