How to know if wstring can be safely (no data loss) converted to string? - string

Question

So I already know how to convert wstring to string (How to convert wstring into string?).
However, I would like to to know whether it is safe to make the conversion, meaning, the wstring variable does not contain any characters that are not supported in string type.

Answer 1

strings can hold any data, if you use the right encoding. They are just sequences of bytes. But you need to check with your particular encoding / conversion routine.
Should be simply a matter of round-tripping. An elegant solution to many things.
Warning, Pseudo-code, there is no literal convert_to_wstring() unless you make it so:
if(convert_to_wstring(convert_to_string(ws)) == ws)
happy_days();
If what goes in comes out, it is non-lossy (at least for your code points).
Not that its the most efficient solution, but should allow you to build from your favorite conversion routines.
// Round-trip and see if we lose anything
bool check_ws2s(const std::wstring& wstr)
{
return (s2ws(ws2s(str)) == wstr);
}
Using #dk123's conversions for C++11 at How to convert wstring into string? (Upvote his answer here https://stackoverflow.com/a/18374698/257090)
wstring s2ws(const std::string& str)
{
typedef std::codecvt_utf8<wchar_t> convert_typeX;
std::wstring_convert<convert_typeX, wchar_t> converterX;
return converterX.from_bytes(str);
}
string ws2s(const std::wstring& wstr)
{
typedef std::codecvt_utf8<wchar_t> convert_typeX;
std::wstring_convert<convert_typeX, wchar_t> converterX;
return converterX.to_bytes(wstr);
}
Note, if your idea of conversion is truncating the wide chars to chars, then it is simply a matter of iterating and checking that each wide char value fits in a char. This will probably do it.
WARNING: Not appropriate for multibyte encoding.
for(wchar_t& wc: ws) {
if(wc > static_cast<char>::(wc))
return false;
}
return true;
Or:
// Could use a narrowing cast comparison, but this avoids any warnings
for(wchar_t& wc: ws) {
if(wc > std::numeric_limits<char>::max())
return false;
}
return true;
FWIW, in Win32, there are conversion routines that accept a parameter of WC_ERR_INVALID_CHARS that tells the routine to fail instead of silently dropping code points. Non-standard solutions, of course.
Example: WideCharToMultiByte()
http://msdn.microsoft.com/en-us/library/windows/desktop/dd374130(v=vs.85).aspx