How can I convert a Unicode string to null terminated byte string and vise versa?

Multibyte string is the name for ASCII strings and wide character is the name for unicode. We can convert these wide character to multibyte or vice versa. We can do it with Win32 API or C standard library or by ATL/MFC macros. We can choose any of the mechanism depending on our application or component framework.

Win32 API

UNICODE String => [WideCharToMultiByte] => ASCII string
ASCII string => [MultiByteToWideChar] => UNICODE String

MultiByteToWideChar
The MultiByteToWideChar function maps a character string to a wide-character (Unicode) string. The character string mapped by this function is not necessarily from a multibyte character set.

int MultiByteToWideChar(
  UINT CodePage,
  DWORD dwFlags,
  LPCSTR lpMultiByteStr,
  int cchMultiByte,
  LPWSTR lpWideCharStr,
  int cchWideChar
);

Parameters:
CodePage - code page
dwFlags - character-type options
lpMultiByteStr - address of string to map
cchMultiByte - number of bytes in string
lpWideCharStr - address of wide-character buffer
cchWideChar - size of buffer

WideCharToMultiByte
The WideCharToMultiByte function maps a wide-character string to a new character string. The new character string is not necessarily from a multibyte character set.

int WideCharToMultiByte(
  UINT CodePage, 
  DWORD dwFlags, 
  LPCWSTR lpWideCharStr, 
  int cchWideChar, 
  LPSTR lpMultiByteStr, 
  int cchMultiByte, 
  LPCSTR lpDefaultChar,
  LPBOOL lpUsedDefaultChar
);

Parameters:
CodePage - code page
dwFlags - performance and mapping flags
lpWideCharStr - address of wide-character string
cchWideChar - number of characters in string
lpMultiByteStr - address of buffer for new string
cchMultiByte - size of buffer
lpDefaultChar - address of default for unmappable characters
lpUsedDefaultChar - address of flag set when default char. used

C STDIO library

UNICODE String => [wcstombs] => ASCII string
ASCII string => [mbstowcs] => UNICODE String

ATL/MFC macros

UNICODE String => [W2A/W2CA] => ASCII string
ASCII string => [A2W/A2CW] => UNICODE String

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Further readings

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