Windows SDK String: How to convert between ANSI and UNICODE strings?

[Gabriel Fleseriu]

Q: How to convert between ANSI and UNICODE strings?

A:

The quick and dirty way

This way of working is correct for codepages that are single-byte and Unicode strings that are UCS2. This applies to most cases, but if your program should run correctly on Japanese, Chinese, Taiwanese and other systems which have DBCS codepages then use the "correct way" described further below.

ANSI to UNICODE:

The conversion is done using the 'MultiByteToWideChar()' function:

char *ansistr = "Hello";
int a = lstrlenA(ansistr);
BSTR unicodestr = SysAllocStringLen(NULL, a);
::MultiByteToWideChar(CP_ACP, 0, ansistr, a, unicodestr, a);
//... when done, free the BSTR
::SysFreeString(unicodestr);

UNICODE to ANSI:

The UNICODE string mostly is returned by some COM function, like this one:

HRESULT SomeCOMFunction(BSTR *bstr)
{
   *bstr = ::SysAllocString(L"Hello");
   return S_OK;
}

The conversion is done using the 'WideCharToMultiByte()' function:

BSTR unicodestr = 0;
SomeCOMFunction(&unicodestr);
int a = SysStringLen(unicodestr)+1;
char *ansistr = new char[a];
::WideCharToMultiByte(CP_ACP, 
                        0, 
                        unicodestr, 
                        -1, 
                        ansistr, 
                        a, 
                        NULL, 
                        NULL);
//...use the strings, then free their memory:
delete[] ansistr;
::SysFreeString(unicodestr);

The correct way

If you want to handle DBCS codepages and UTF-16 Unicode strings then you should do things this way. The idea is to call 'MultiByteToWideChar()' resp. 'WideCharToMultiByte()' twice. First you get the length of the result, then you allocate the resulting string and call it again to convert.

ANSI to Unicode

char *ansistr = "Hello"
int lenA = lstrlenA(ansistr);
int lenW;
BSTR unicodestr;

lenW = ::MultiByteToWideChar(CP_ACP, 0, ansistr, lenA, 0, 0);
if (lenW > 0)
{
  // Check whether conversion was successful
  unicodestr = ::SysAllocStringLen(0, lenW);
  ::MultiByteToWideChar(CP_ACP, 0, ansistr, lenA, unicodestr, lenW);
}
else
{
  // handle the error
}

// when done, free the BSTR
::SysFreeString(unicodestr);

Unicode to ANSI

BSTR unicodestr = 0;
char *ansistr;
SomeCOMFunction(&unicodestr);
int lenW = ::SysStringLen(unicodestr);
int lenA = ::WideCharToMultiByte(CP_ACP, 0, unicodestr, lenW, 0, 0, NULL, NULL);
if (lenA > 0)
{
  ansistr = new char[lenA + 1]; // allocate a final null terminator as well
::WideCharToMultiByte(CP_ACP, 0, unicodestr, lenW, ansistr, lenA, NULL, NULL);
  ansistr[lenA] = 0; // Set the null terminator yourself
}
else
{
  // handle the error
}

//...use the strings, then free their memory:
delete[] ansistr;
::SysFreeString(unicodestr);