C++/COM string conversion

char* <--> wchar_t*
1. sprintf(desWChars, "%s", srcChars);
   swprintf(desChars, "%S", srcWChars);
2. wchar_t *desWChars = A2W(srcChars); // ATL convention.
   char *desChars = W2A(srcWChars);    // ATL convention.
   Note: A2W&W2A uses "alloca" to alloc memory from stack. So don't release the pointer.
3. Safe way:
   WideCharToMultiByte
4. CW2A pszConverted (bstrSrc);

5. int converted = wcstombs(pChars, pWChars, MB_CUR_MAX);
    int converted = mbstowcs(pWChars, pChars, MB_CUR_MAX);

STL string <--> char*
   char *pChars = stlString.c_str();
   string stlString(pChars)  // Construct Func

char* --> OLECHAR*
1. Simple but Not safe:
   #include <atlconv.h>
   USES_CONVERSION; // This is necessary.
   BSTR str = A2OLE(pChar);
   char *pChar = OLE2A(str); // pChar can't be freed by user. It is in stack.
        // Pay attention to "stack overflow" exception.
2. Safe:
   WideCharToMultiByte

STL wstring <--> BSTR
   std::wstring wstrAnsiVersion (bstrMyBeaster);

BSTR --> _bstr_t
    _bstr_t bstrDes(bstrSrc); // Constructor.
    bstrDes = bstrSrc;       // Operator "=" function
_bstr_t --> BSTR
    BSTR bstrDes = bstrSrc.copy();
    BSTR bstrDes = bstrSrc;   // Operator "="

//////////////////////////////////////////////////////////////////
Properties -> Configuration Properties -> General -> Use of ATL.

LPSTR = char*
LPCSTR = const char*
LPWSTR = wchar_t*
LPCWSTR = const wchar_t*
LPTSTR = char* or wchar_t* depending on _UNICODE
LPCTSTR = const char* or const wchar_t* depending on _UNICODE

////////////////////////////////////////////////////////////////////////////////////////////

C Language String Types and Classes

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

String Type	Description
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:

1. Converting BSTR to _bstr_t
2. Converting a _bstr_t to BSTR
3. Converting a CComBSTR to BSTR
4. Converting _bstr_t to CComBSTR
5. 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); 

    

   (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);
   

    

   (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);
   

    

   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);
   

    

   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);
   

6. Converting _bstr_t to C String
7. Converting CComBSTR to LPCTSTR
8. Converting LPCTSTR to _bstr_t
9. Converting LPCTSTR to CComBSTR

  // BSTR to _bst_t

  BSTR bstrStart = GetBSTR();

  // use the constructor
  _bstr_t bstrFinal(bstrStart);

  ShowBSTR(bstrFinal);

  // Use the = operator
  bstrFinal = bstrStart;

  ShowBSTR(bstrFinal);

 

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);

 

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);

 

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

  CComBSTR bstrFinish;

  bstrFinish.AppendBSTR(bstrStart);

  ShowBSTR(bstrFinish);