Fix building on Mac and Linux due to deprecated C++ std api

rd-cpp/src/rd_core_cpp/src/main/std/to_string.h

@@ -9,7 +9,12 @@
 #include <atomic>
 #include <future>
 #include <locale>
+#if defined(_MSC_VER) || defined(__APPLE__)
 #include <codecvt>
+#else
+#include <limits>
+#include <iconv.h>
+#endif
 
 #include <thirdparty.hpp>
 
@@ -29,12 +34,63 @@ inline std::string to_string(const char* val)
 	return val;
 }
 
+#if defined(_MSC_VER) || defined(__APPLE__)
+template<class I, class E, class S>
+struct codecvt : std::codecvt<I, E, S>
+{
+	~codecvt()
+	{ }
+};
+
 inline std::string to_string(std::wstring const& val)
 {
-	using convert_type = std::codecvt_utf8<wchar_t>;
+#if defined(__APPLE__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+#endif
+	using convert_type = codecvt<wchar_t, char, std::mbstate_t>;
 	std::wstring_convert<convert_type> converter;
 	return converter.to_bytes(val);
+#if defined(__APPLE__)
+#pragma clang diagnostic pop
+#endif
+}
+#else
+const std::string conv_error("Conversion Error");
+inline std::string to_string(const std::wstring& wstr) {
+	std::string result;
+	if (wstr.empty()) {
+		return result;
+	}
+	// Order of arguments is to, from
+	auto icvt = iconv_open("UTF-8", "WCHAR_T");
+	// CentOS is not happy with -1
+	if (std::numeric_limits<iconv_t>::max() == icvt) {
+		return conv_error;
+	}
+
+	// I hope this does not modify the incoming buffer
+	wchar_t* non_const_in = const_cast<wchar_t*>(wstr.c_str());
+	char* iconv_in = reinterpret_cast<char*>(non_const_in);
+	size_t iconv_in_bytes = wstr.length() * sizeof(wchar_t);
+	// Temp buffer, assume every code point converts into 3 bytes, this should be enough
+	// We do not convert terminating zeros
+	const size_t buffer_len = wstr.length() * 3;
+	auto buffer = std::make_unique<char[]>(buffer_len);
+
+	char* iconv_out = buffer.get();
+	size_t iconv_out_bytes = buffer_len;
+	auto ret = iconv(icvt, &iconv_in, &iconv_in_bytes, &iconv_out, &iconv_out_bytes);
+	if (static_cast<size_t>(-1) == ret) {
+		result = conv_error;
+	} else {
+		size_t converted_len = buffer_len - iconv_out_bytes;
+		result.assign(buffer.get(), converted_len);
+	}
+	iconv_close(icvt);
+	return result;
 }
+#endif
 
 inline std::string to_string(std::thread::id const& id)
 {