Use from_chars when supported in GCC and VS

Author: danielaparker

include/jsoncons/config/compiler_support.hpp

@@ -114,6 +114,24 @@
    #endif
 #endif
 
+#if !defined(JSONCONS_HAS_STD_FROM_CHARS)
+#  if defined(__GNUC__)
+#   if (__GNUC__ >= 11)
+#    if (__cplusplus >= 201703)
+#     define JSONCONS_HAS_STD_FROM_CHARS 1
+#    endif // (__cplusplus >= 201703)
+#   endif // (__GNUC__ >= 11)
+#  endif // defined(__GNUC__)
+#  if defined(_MSC_VER)
+#   if (_MSC_VER >= 1924 && _MSVC_LANG >= 201703)
+#    define JSONCONS_HAS_STD_FROM_CHARS 1
+#   endif // (_MSC_VER >= 1924 && MSVC_LANG >= 201703)
+#  endif // defined(_MSC_VER)
+#endif
+#if defined(JSONCONS_HAS_STD_FROM_CHARS)
+#include <charconv>
+#endif
+
 #if !defined(JSONCONS_HAS_2017)
 #  if defined(__clang__)
 #   if (__cplusplus >= 201703)

include/jsoncons/detail/parse_number.hpp

@@ -909,7 +909,43 @@ base16_to_integer(const CharT* s, std::size_t length, T& n)
     return to_integer_result<T,CharT>(s, to_integer_errc());
 }
 
-#if defined(JSONCONS_HAS_MSC_STRTOD_L)
+
+#if defined(JSONCONS_HAS_STD_FROM_CHARS)
+
+class to_double_t
+{
+public:
+
+    char get_decimal_point() const
+    {
+        return '.';
+    }
+
+    template <class CharT>
+    typename std::enable_if<std::is_same<CharT,char>::value,double>::type
+    operator()(const CharT* s, std::size_t len) const
+    {
+        double val = 0;
+        std::from_chars(s, s+len, val);
+        return val;
+    }
+
+    template <class CharT>
+    typename std::enable_if<std::is_same<CharT,wchar_t>::value,double>::type
+    operator()(const CharT* s, std::size_t len) const
+    {
+        std::string input(len,'0');
+        for (size_t i = 0; i < len; ++i)
+        {
+            input[i] = static_cast<char>(s[i]);
+        }
+
+        double val = 0;
+        std::from_chars(input.data(), input.data() + len, val);
+        return val;
+    }
+};
+#elif defined(JSONCONS_HAS_MSC_STRTOD_L)
 
 class to_double_t
 {