Convert CString to ANSI string in UNICODE projects

Posted 2020-07-13 人最大的荣耀不在于从未失败，而在于每次失败以后都能东山再起

Convert CString to ANSI string in UNICODE projects

Quick Answer: use an intermediate CStringA.

• Normally, this is not something that should be done. *It is technically unreliable, unless you can guarantee that the source CString to be converted does not contain any 2-byte characters.
• This will work fine if you are using the English language without any special 2-byte symbols or accented letters.
• This article is for educational use, and explains how it can easily be done, without relying on the USES_CONVERSION macro with W2A, or ridiculous WideCharToMultiByte API functions.
• If you are using a language that actually requires Unicode (Asian languages, etc), or if the source CString contains any 2-byte character, this cannot be done. This is because there is no ANSI equivalent of any 2-byte character.
• It is the responsibility of the programmer to ensure that the source CString does not contain any 2-byte characters.

  Use intermediate CStringA (highly recommended):

• Pros: this is the easiest to use.
• Cons: you cannot specify a code page.

  CString LastNameW(L"Smith");

  CStringA LastNameA(LastNameW);

  FunctionForAnsi(LastNameA.GetString());

• Or an even simpler example:

  CString LastNameW(L"Smith");

  FunctionForAnsi(CStringA(LastNameW).GetString());

  Here are some other ways that either do not work or are not recommended. I list them here to document things to avoid. What not to do.

  WideCharToMultiByte API function (not recommended):

• Pros: you can specify the desired code page.
• Cons: too much code to write, test, debug.

  CString LastNameW(L"Smith");

  int nLen = WideCharToMultiByte(CP_ACP, 0, (LPCWSTR)LastNameW, -1, NULL, NULL);

  LPSTR lpszA = new CHAR[nLen];

  WideCharToMultiByte(CP_ACP, 0, (LPCWSTR)LastNameW, -1, lpszA, nLen);

  FunctionForAnsi(lpszA);

  delete[] lpszA; // free the string

  W2A ATL 3.0 macros (not recommended):

• Cons: not safe inside loops.
• Cons: you cannot specify a code page.

  USES_CONVERSION;

  CString LastNameW(L"Smith");

  FunctionForAnsi(W2A(LastNameW.GetString()));

  CW2A ATL 7.0 conversion template classes (not recommended):

• There are 3 ways you can use the CW2A template class. Only one of them is the right way.
• Cons: too difficult to remember the correct usage
• Cons: too easy to use improperly.

  CString LastNameW(L"Smith");

  CW2A pszA(LastNameW.GetString()); // this is the right way

  FunctionForAnsi(pszA);

  CString LastNameW(L"Smith");

  FunctionForAnsi(CW2A(LastNameW.GetString())); // improper usage, do not do this

  CString LastNameW(L"Smith");

  LPCSTR pszA = CW2A(LastNameW.GetString()); // improper usage, do not do this

  FunctionForAnsi(pszA);

  (LPCSTR)(LPCTSTR) cast:

• Do not use this!
• You cannot use (LPCSTR)(LPCTSTR) to cast a CString to LPCSTR in Unicode projects.
• It does compile, but it does not properly convert the CString to an LPCSTR.
• The resulting string will either be 0 or 1 length, or filled with garbage characters of unknown length, because the cast just changes the pointer type without any conversion.
• You end up with a CHAR* pointing to a WCHAR array, a very bad thing.

  CString LastName(L"Smith");

  FunctionForAnsi((LPCSTR)(LPCTSTR)LastName); // improper usage, do not to this

  REF:

  ATL String: What‘s wrong with the USES_CONVERSION macros? How to avoid using them?

  Using MFC MBCS/Unicode Conversion Macros

  ATL and MFC String Conversion Macros

  CString Management