c++ can't convert string to wstring

问题:

I would like to convert a string variable to wstring due to some german characters that cause problem when doing a substr over the variable. The start position is falsified when any these special characters is present before it. (For instance: for "ä" size() returns 2 instead of 1)

I know that the following conversion works:

wstring ws = L"ä";

Since, I am trying to convert a variable, I would like to know if there is an alternative way for it such as

wstring wstr = L"%s"+str //this is syntaxically wrong, but wanted sth alike

Beside that, I have already tried the following example to convert string to wstring:

string foo("ä"); 
wstring_convert<codecvt_utf8<wchar_t>> converter;
wstring wfoo = converter.from_bytes(foo.data());
cout << foo.size() << endl;
cout << wfoo.size() << endl;

, but I get errors like

‘wstring_convert’ was not declared in this scope

I am using ubuntu 14.04 and my main.cpp is compiled with cmake. Thanks for your help!

回答1:

The solution from "hahakubile" worked for me:

std::wstring s2ws(const std::string& s) {
    std::string curLocale = setlocale(LC_ALL, ""); 
    const char* _Source = s.c_str();
    size_t _Dsize = mbstowcs(NULL, _Source, 0) + 1;
    wchar_t *_Dest = new wchar_t[_Dsize];
    wmemset(_Dest, 0, _Dsize);
    mbstowcs(_Dest,_Source,_Dsize);
    std::wstring result = _Dest;
    delete []_Dest;
    setlocale(LC_ALL, curLocale.c_str());
    return result;
}

But the return value is not 100% correct:

string s = "101446012MaßnStörfall   PAt  #Maßnahme Störfall                      00810000100121000102000020100000000000000";
wstring ws2 = s2ws(s);
cout << ws2.size() << endl; // returns 110 which is correct
wcout << ws2.substr(29,40) << endl; // returns #Ma�nahme St�rfall with symbols

I am wondering why it replaced german characters with symbols.

Thanks again!

回答2:

If you are using Windows/Visual Studio and need to convert a string to wstring you should 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).

CA2W ca2w(str, CP_UTF8);

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.

回答3:

The main point is that

string foo("ä")

Is already an error. Start from here and read all answers. And beware, one is very wrong :)