How to encode UTF-8 string in JSON

json_write.cpp

#include <iostream>
#include <stdexcept>
#include <vector>
#include <string>
#include <cstring>
#include <locale>
#include <boost/property_tree/json_parser.hpp>
using namespace std;

class CodeCvt
{
public:
	explicit CodeCvt(const char *locale="en_US.UTF-8")
	: locale_(locale), codecvt_(use_facet<codecvt_type>(locale_)) {}

	wstring widen(const string &s)
	{
		const char *fb = s.data();
		const char *fe = fb+s.size();
		const char *fn;
		vector<wchar_t> ws(s.size());
		wchar_t *tb = &ws.front();
		wchar_t *te = tb+ws.size();
		wchar_t *tn;
		mbstate_t st;
		bzero(&st, sizeof(st));

		const codecvt_base::result res = codecvt_.in(st, fb, fe, fn, tb, te, tn);
		assert_result(res, fn, fe);
		return wstring(tb, tn);
	}

	string narrow(const wstring &ws)
	{
		const wchar_t *fb = ws.data();
		const wchar_t *fe = fb+ws.size();
		const wchar_t *fn;
		vector<char> s(ws.size()*4);
		char *tb = &s.front();
		char *te = tb+s.size();
		char *tn;
		mbstate_t st;
		bzero(&st, sizeof(st));

		const codecvt_base::result res = codecvt_.out(st, fb, fe, fn, tb, te, tn);
		assert_result(res, fn, fe);
		return string(tb, tn);
	}

private:
	template<class CharT>
	static void assert_result(codecvt_base::result res, const CharT *fn, const CharT *fe)
	{
		switch(res)
		{
			case codecvt_base::ok:
				if(fn != fe)
				{
					throw invalid_argument("Conversion failure");
				}
				break;
			case codecvt_base::partial:
				throw invalid_argument("partial");
			case codecvt_base::error:
				throw invalid_argument("error");
			case codecvt_base::noconv:
				throw invalid_argument("noconv");
		}
	}

	typedef codecvt<wchar_t, char, mbstate_t> codecvt_type;
	const locale locale_;
	const codecvt_type &codecvt_;
};


int main(int argc, char *argv[])
{
	try
	{
		CodeCvt cvt;
		boost::property_tree::wptree pt;
		pt.put(L"message", cvt.widen(argv[1]));
		boost::property_tree::write_json(wcout, pt, false);
		return 0;
	}
	catch(const exception &e)
	{
		cerr << e.what() << endl;
		return 1;
	}
}

See also boost::property_treeで日本語を含むJSONが文字化けする

Thank you! I spent many hours in trying to solve the problem with saving UTF-8 strings in JSON file, and this method really helped me :)