C++ Convert string (or char*) to wstring (or wchar_t*)

Question

Assuming that the input string in your example (おはよう) is a UTF-8 encoded (which it isn't, by the looks of it, but let's assume it is for the sake of this explanation :-)) representation of a Unicode string of your interest, then your problem can be fully solved with the standard library (C++11 and newer) alone.

The TL;DR version:

#include <locale>
#include <codecvt>
#include <string>

std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
std::string narrow = converter.to_bytes(wide_utf16_source_string);
std::wstring wide = converter.from_bytes(narrow_utf8_source_string);

Longer online compilable and runnable example:

(They all show the same example. There are just many for redundancy...)

• http://ideone.com/KA1oty
• http://ide.geeksforgeeks.org/5pRLSh
• http://rextester.com/DIJZK52174

Note (old):

As pointed out in the comments and explained in https://stackoverflow.com/a/17106065/6345 there are cases when using the standard library to convert between UTF-8 and UTF-16 might give unexpected differences in the results on different platforms. For a better conversion, consider std::codecvt_utf8 as described on http://en.cppreference.com/w/cpp/locale/codecvt_utf8

Note (new):

Since the codecvt header is deprecated in C++17, some worry about the solution presented in this answer were raised. However, the C++ standards committee added an important statement in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0618r0.html saying

this library component should be retired to Annex D, along side , until a suitable replacement is standardized.

So in the foreseeable future, the codecvt solution in this answer is safe and portable.

int StringToWString(std::wstring &ws, const std::string &s)
{
    std::wstring wsTmp(s.begin(), s.end());

    ws = wsTmp;

    return 0;
}

Your question is underspecified. Strictly, that example is a syntax error. However, mbstowcs is probably what you're looking for.

It is a C-library function and operates on buffers, but here's an easy-to-use idiom, courtesy of Mooing Duck:

std::wstring ws(s.size(), L' '); // Overestimate number of code points.
ws.resize(::mbstowcs_s(&ws[0], ws.size(), s.c_str(), s.size())); // Shrink to fit.

If you are using Windows/Visual Studio and need to convert a string to wstring you could use:

#include <AtlBase.h>
#include <atlconv.h>
...
string s = "some string";
CA2W ca2w(s.c_str());
wstring w = ca2w;
printf("%s = %ls", s.c_str(), w.c_str());

Same procedure for converting a wstring to string (sometimes you will need to specify a codepage):

#include <AtlBase.h>
#include <atlconv.h>
...
wstring w = L"some wstring";
CW2A cw2a(w.c_str());
string s = cw2a;
printf("%s = %ls", s.c_str(), w.c_str());

You could specify a codepage and even UTF8 (that's pretty nice when working with JNI/Java). A standard way of converting a std::wstring to utf8 std::string is showed in this answer.

// 
// using ATL
CA2W ca2w(str, CP_UTF8);

// 
// or the standard way taken from the answer above
#include <codecvt>
#include <string>

// convert UTF-8 string to wstring
std::wstring utf8_to_wstring (const std::string& str) {
    std::wstring_convert<std::codecvt_utf8<wchar_t>> myconv;
    return myconv.from_bytes(str);
}

// convert wstring to UTF-8 string
std::string wstring_to_utf8 (const std::wstring& str) {
    std::wstring_convert<std::codecvt_utf8<wchar_t>> myconv;
    return myconv.to_bytes(str);
}

If you want to know more about codepages there is an interesting article on Joel on Software: The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets.

These CA2W (Convert Ansi to Wide=unicode) macros are part of ATL and MFC String Conversion Macros, samples included.

Sometimes you will need to disable the security warning #4995', I don't know of other workaround (to me it happen when I compiled for WindowsXp in VS2012).

#pragma warning(push)
#pragma warning(disable: 4995)
#include <AtlBase.h>
#include <atlconv.h>
#pragma warning(pop)

Edit: Well, according to this article the article by Joel appears to be: "while entertaining, it is pretty light on actual technical details". Article: What Every Programmer Absolutely, Positively Needs To Know About Encoding And Character Sets To Work With Text.

Windows API only, pre C++11 implementation, in case someone needs it:

#include <stdexcept>
#include <vector>
#include <windows.h>

using std::runtime_error;
using std::string;
using std::vector;
using std::wstring;

wstring utf8toUtf16(const string & str)
{
   if (str.empty())
      return wstring();

   size_t charsNeeded = ::MultiByteToWideChar(CP_UTF8, 0, 
      str.data(), (int)str.size(), NULL, 0);
   if (charsNeeded == 0)
      throw runtime_error("Failed converting UTF-8 string to UTF-16");

   vector<wchar_t> buffer(charsNeeded);
   int charsConverted = ::MultiByteToWideChar(CP_UTF8, 0, 
      str.data(), (int)str.size(), &buffer[0], buffer.size());
   if (charsConverted == 0)
      throw runtime_error("Failed converting UTF-8 string to UTF-16");

   return wstring(&buffer[0], charsConverted);
}

Here's a way to combining string, wstring and mixed string constants to wstring. Use the wstringstream class.

This does NOT work for multi-byte character encodings. This is just a dumb way of throwing away type safety and expanding 7 bit characters from std::string into the lower 7 bits of each character of std:wstring. This is only useful if you have a 7-bit ASCII strings and you need to call an API that requires wide strings.

#include <sstream>

std::string narrow = "narrow";
std::wstring wide = L"wide";

std::wstringstream cls;
cls << " abc " << narrow.c_str() << L" def " << wide.c_str();
std::wstring total= cls.str();