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!