Why does std::codecvt not work as defined?

Question

#include <iostream>

using namespace std;

void f1()
{
    wcout.imbue(locale("chs"));
    wcout << L"您" << endl;
}

void f2()
{
    locale loc(wcout.getloc(), new codecvt<wchar_t, char, mbstate_t>());

    wcout.imbue(loc);
    wcout << L"好" << endl;
}

int main()
{
    f1(); // OK
    f2(); // Error. There is no output as expected.
}

According to cplusplus.com's online documentation:

codecvt<wchar_t,char,mbstate_t>: 

    converts between native wide and narrow character sets.

This program is compiled with VC++, and runs on Windows.

In this program, the internal character set is UCS-2, which is defined by the VC++ compiler; the external character set, i.e. the narrow character set, is GBK (a Chinese character set) in console environment. If the documentation is true, then wcout can convert the unicode string from UCS-2 to GBK as f1() does; however, it does not. Why?

Better documentation: [`std::codecvt`](http://en.cppreference.com/w/cpp/locale/codecvt). — IInspectable, Sep 21 '13 at 11:41

Cubbi · Accepted Answer · 2013-09-21T12:03:22.080

You've default-constructed an std::codecvt, with no particular conversion rules. It has no way of knowing that you want GBK and not GB18030 or UTF-8.

Ways to get a codecvt that converts wchar_t to GBK:

Construct an std::locale for GBK just use that with your stream, no need to pull out a facet

wcout.imbue(std::locale("")); // this uses the current user settings,
wcout.imbue(std::locale("zn_CN.gbk")); // or name the locale explicitly,
                                       // by whatever name Windows calls it

Construct the facet directly with std::codecvt_byname

wcout.imbue(std::locale(wcout.getloc(),
            new std::codecvt_byname("zh_CN.gbk")); // explict name

Write your own conversion routine and derive from std::codecvt, so you could use it with
```
wcout.imbue(std::locale(wcout.getloc(), new yourcodecvt);
```

Windows support for C++ locales is very poor, though, WinAPI may have a more appropriate conversion function.

Why does std::codecvt not work as defined?

1 Answers1