Prevent compiler error when using std::experimental::optional

Author: stephen-webb

src/main/cpp/loggingevent.cpp

@@ -206,7 +206,7 @@ bool LoggingEvent::getNDC(LogString& dest) const
 	// Otherwise use the NDC that is associated with the thread.
 	if (m_priv->dc)
 	{
-		result = m_priv->dc->ctx.has_value();
+		result = bool(m_priv->dc->ctx);
 		if (result)
 			dest.append(NDC::getFullMessage(m_priv->dc->ctx.value()));
 	}

src/main/include/log4cxx/helpers/optional.h

@@ -53,8 +53,9 @@ class Optional : private std::pair<bool, T>
 		this->second = value;
 		return *this;
 	}
-	bool has_value() const { return this->first; }
-	const T& value() const { return this->second; }
+	constexpr explicit operator bool() const noexcept { return this->first; }
+	constexpr bool has_value() const noexcept { return this->first; }
+	constexpr const T& value() const noexcept { return this->second; }
 };
 } // namespace LOG4CXX_NS
 #endif