Add generic impl of conversion from string

Author: mborland

include/boost/decimal/string.hpp

@@ -13,6 +13,38 @@
 namespace boost {
 namespace decimal {
 
+namespace detail {
+
+template <typename DecimalType>
+auto from_string_impl(const std::string& str, std::size_t* idx = nullptr)
+    BOOST_DECIMAL_REQUIRES(detail::is_decimal_floating_point_v, DecimalType)
+{
+    DecimalType val;
+    const auto r {from_chars(str, val)};
+
+    if (r.ec == std::errc::result_out_of_range)
+    {
+        val = std::numeric_limits<DecimalType>::signaling_NaN();
+        BOOST_DECIMAL_THROW_EXCEPTION(std::out_of_range("Conversion is outside the range of the type"));
+    }
+    else if (r.ec != std::errc{})
+    {
+        val = std::numeric_limits<DecimalType>::signaling_NaN();
+        BOOST_DECIMAL_THROW_EXCEPTION(std::invalid_argument("Conversion could not be performed"));
+    }
+    else
+    {
+        if (idx != nullptr)
+        {
+            *idx = static_cast<std::size_t>(r.ptr - str.data());
+        }
+    }
+
+    return val;
+}
+
+} // namespace detail
+
 BOOST_DECIMAL_EXPORT template <typename DecimalType>
 auto to_string(const DecimalType value)
     BOOST_DECIMAL_REQUIRES_RETURN(detail::is_decimal_floating_point_v, DecimalType, std::string)