Replace deprecated wstring_convert (#290)

Co-authored-by: Jonas Borchelt <jonas.borchelt@icloud.com>

Author: JonasBorchelt

include/jwt-cpp/jwt.h

@@ -24,6 +24,7 @@
 #include <algorithm>
 #include <chrono>
 #include <cmath>
+#include <cstring>
 #include <functional>
 #include <iterator>
 #include <locale>
@@ -3016,18 +3017,37 @@ namespace jwt {
 			}
 
 			static std::string to_lower_unicode(const std::string& str, const std::locale& loc) {
-#if __cplusplus > 201103L
-				std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t> conv;
-				auto wide = conv.from_bytes(str);
+				std::mbstate_t state = std::mbstate_t();
+				const char* in_next = str.data();
+				const char* in_end = str.data() + str.size();
+				std::wstring wide;
+				wide.reserve(str.size());
+
+				while (in_next != in_end) {
+					wchar_t wc;
+					std::size_t result = std::mbrtowc(&wc, in_next, in_end - in_next, &state);
+					if (result == static_cast<std::size_t>(-1)) {
+						throw std::runtime_error("encoding error: " + std::string(std::strerror(errno)));
+					} else if (result == static_cast<std::size_t>(-2)) {
+						throw std::runtime_error("conversion error: next bytes constitute an incomplete, but so far "
+												 "valid, multibyte character.");
+					}
+					in_next += result;
+					wide.push_back(wc);
+				}
+
 				auto& f = std::use_facet<std::ctype<wchar_t>>(loc);
 				f.tolower(&wide[0], &wide[0] + wide.size());
-				return conv.to_bytes(wide);
-#else
-				std::string result;
-				std::transform(str.begin(), str.end(), std::back_inserter(result),
-							   [&loc](unsigned char c) { return std::tolower(c, loc); });
-				return result;
-#endif
+
+				std::string out;
+				out.reserve(wide.size());
+				for (wchar_t wc : wide) {
+					char mb[MB_CUR_MAX];
+					std::size_t n = std::wcrtomb(mb, wc, &state);
+					if (n != static_cast<std::size_t>(-1)) out.append(mb, n);
+				}
+
+				return out;
 			}
 		};
 	} // namespace verify_ops

tests/HelperTest.cpp

@@ -18,9 +18,8 @@ TEST(HelperTest, Base64DER2PemCert) {
 }
 
 TEST(HelperTest, DER2PemCert) {
-	auto decoded = jwt::base::decode<jwt::alphabet::base64>(
-		jwt::base::pad<jwt::alphabet::base64>(google_cert_base64_der)
-	);
+	auto decoded =
+		jwt::base::decode<jwt::alphabet::base64>(jwt::base::pad<jwt::alphabet::base64>(google_cert_base64_der));
 	auto cert_pem = jwt::helper::convert_der_to_pem(decoded);
 	ASSERT_EQ(google_cert, cert_pem);
 }