Is codecvt not a std header?

The reason why GCC rejects this code is simple: libstdc++ doesn't support <codecvt> yet.

The C++11 support status page confirms this:

22.5 Standard code conversion facets N

The question was asked almost 3 years ago and I was surprised that I am also having same issue using Ubuntu 14.04 with fresh updates.

Second surprise, the link provided by @Fanael shows now:

22.5 Standard code conversion facets Y

So I searched which version of GCC would fully implement C++11. Turns out that full support was added in GCC 5:

https://gcc.gnu.org/gcc-5/changes.html

Full support for C++11, including the following new features:

...

locale facets for Unicode conversion;

...

I would have been happy to put a comment on the answer if I had enough reputation :)

A workaround using Boost.Locale:

#include <boost/locale/encoding_utf.hpp>
#include <string>

using boost::locale::conv::utf_to_utf;

std::wstring utf8_to_wstring(const std::string& str)
{
    return utf_to_utf<wchar_t>(str.c_str(), str.c_str() + str.size());
}

std::string wstring_to_utf8(const std::wstring& str)
{
    return utf_to_utf<char>(str.c_str(), str.c_str() + str.size());
}