How to properly use MultiByteToWideChar

Question

I am using MultiByteToWideChar to convert my string to a wstring. I am first trying to get the required size for my wstring. According to the documentation passing 0 as the last argument should accomplish this. Using MultiByteToWideChar(CP_UTF8, MB_COMPOSITE, str.c_str(), -1, nullptr, 0); returns 0 as the required size of the wstring buffer. I have verified that str is a non empty string as well. What am I doing wrong here?

If the function returns 0, you need to call GetLastError to get the error code. What error code is returned? — Harry Johnston, Jul 29 '14 at 21:33
Is `str` properly UTF-8 encoded, or might it be in a different encoding? — Mark Ransom, Jul 29 '14 at 22:15
It is an error return. Always assert() such values, use GetLastError() to find out what went wrong. — Hans Passant, Jul 29 '14 at 22:39

score 7 · Accepted Answer · answered Jul 29 '14 at 22:30

From the MSDN documentation:

For UTF-8 or code page 54936 (GB18030, starting with Windows Vista), dwFlags must be set to either 0 or MB_ERR_INVALID_CHARS. Otherwise, the function fails with ERROR_INVALID_FLAGS.

You're using CP_UTF8 but also passing the MB_COMPOSITE flag, so that's why it's failing.

How to properly use MultiByteToWideChar

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