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
   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 "="

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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

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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:

Converting BSTR to _bstr_t Converting a _bstr_t to BSTR Converting a CComBSTR to BSTR Converting _bstr_t to CComBSTR Converting BSTR to C String

 

(Note: conversion that only works in Unicode)

  // BSTR to C String

  BSTR bstrStart;

  bstrStart = GetBSTR();

  TCHAR szFinal[255];

  

 

(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);

Converting _bstr_t to C String Converting CComBSTR to LPCTSTR Converting LPCTSTR to _bstr_t 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);