How to convert std::string to wchar_t* - c++

Question

std::regex regexpy("y:(.+?)\"");
std::smatch my;
regex_search(value.text, my, regexpy);
y = my[1];
std::wstring wide_string = std::wstring(y.begin(), y.end());
const wchar_t* p_my_string = wide_string.c_str();
wchar_t* my_string = const_cast<wchar_t*>(p_my_string);
URLDownloadToFile(my_string, aDest);
I'm using Unicode, the encoding of the source string is ASCII, UrlDownloadToFile expands to UrlDownloadToFileW (wchar_t*) the code above compiles in debug mode, but with a lot of warnings like:
warning C4244: 'argument': conversion from 'wchar_t' to 'const _Elem', possible loss of data
So do I ask, how I could convert a std::string to a wchar_t?

Answer 1

First off, you don't need the const_cast, as URLDownloadToFileW() takes a const wchar_t* as input, so passing it wide_string.c_str() will work as-is:
URLDownloadToFile(..., wide_string.c_str(), ...);
That being said, you are constructing a std::wstring with the individual char values of a std::string as-is. That will work without data loss only for ASCII characters <= 127, which have the same numeric values in both ASCII and Unicode. For non-ASCII characters, you need to actually convert the char data to Unicode, such as with MultiByteToWideChar() (or equivilent), eg:
std::wstring to_wstring(const std::string &s)
{
std::wstring wide_string;
// NOTE: be sure to specify the correct codepage that the
// str::string data is actually encoded in...
int len = MultiByteToWideChar(CP_ACP, 0, s.c_str(), s.size(), NULL, 0);
if (len > 0) {
wide_string.resize(len);
MultiByteToWideChar(CP_ACP, 0, s.c_str(), s.size(), &wide_string[0], len);
}
return wide_string;
}
URLDownloadToFileW(..., to_wstring(y).c_str(), ...);
That being said, there is a simpler solution. If the std::string is encoded in the user's default locale, you can simply call URLDownloadToFileA() instead, passing it the original std::string as-is, and let the OS handle the conversion for you, eg:
URLDownloadToFileA(..., y.c_str(), ...);

Answer 2

There is a cross-platform solution. You can use std::mbtowc.
std::wstring convert_mb_to_wc(std::string s) {
std::wstring out;
std::mbtowc(nullptr, 0, 0);
int offset;
size_t index = 0;
for (wchar_t wc;
(offset = std::mbtowc(&wc, &s[index], s.size() - index)) > 0;
index += offset) {
out.push_back(wc);
}
return out;
}
Adapted from an example on cppreference.com at https://en.cppreference.com/w/cpp/string/multibyte/mbtowc .