Define relative comparison operators for fly::Json

This allows fly::Json to be used as the template parameter of other
containers like std::set which require a comparator function.

Author: trflynn89

fly/types/json/json.cpp

@@ -777,6 +777,61 @@ bool operator!=(Json::const_reference json1, Json::const_reference json2)
     return !(json1 == json2);
 }
 
+//==================================================================================================
+bool operator<(Json::const_reference json1, Json::const_reference json2)
+{
+    auto visitor = [&json1, &json2](const auto &value1, const auto &value2) -> bool
+    {
+        using S1 = decltype(value1);
+        using S2 = decltype(value2);
+
+        if constexpr (JsonNull<S1> && JsonNull<S2>)
+        {
+            return false;
+        }
+        else if constexpr (
+            (JsonFloatingPoint<S1> && JsonNumber<S2>) || (JsonFloatingPoint<S2> && JsonNumber<S1>))
+        {
+            const auto fvalue1 = static_cast<json_floating_point_type>(value1);
+            const auto fvalue2 = static_cast<json_floating_point_type>(value2);
+
+            return fvalue1 < fvalue2;
+        }
+        else if constexpr (JsonNumber<S1> && JsonNumber<S2>)
+        {
+            return value1 < static_cast<S1>(value2);
+        }
+        else if constexpr (fly::SameAsAny<S1, S2>)
+        {
+            return value1 < value2;
+        }
+        else
+        {
+            return json1.m_value.index() < json2.m_value.index();
+        }
+    };
+
+    return std::visit(std::move(visitor), json1.m_value, json2.m_value);
+}
+
+//==================================================================================================
+bool operator<=(Json::const_reference json1, Json::const_reference json2)
+{
+    return !(json2 < json1);
+}
+
+//==================================================================================================
+bool operator>(Json::const_reference json1, Json::const_reference json2)
+{
+    return !(json1 <= json2);
+}
+
+//==================================================================================================
+bool operator>=(Json::const_reference json1, Json::const_reference json2)
+{
+    return !(json1 < json2);
+}
+
 //==================================================================================================
 json_string_type Json::validate_string(json_string_type &&value)
 {

fly/types/json/json.hpp

@@ -1385,6 +1385,51 @@ class Json
      */
     friend bool operator!=(const_reference json1, const_reference json2);
 
+    /**
+     * Less-than operator. The first Json instance is less-than the second Json instance if one of
+     * the following is true:
+     *
+     * 1. One of the two JSON types are floating-point, the other is a numeric type (signed,
+     *    unsigned, or float) and the default less-than operator returns true after converting both
+     *    types to floating-point.
+     * 2. The two Json instances are an integer type (signed or unsigned) and have the default
+     *    less-than operator returns true after converting the second Json value to the same type as
+     *    the first Json value.
+     * 3. The two Json instances are of the same type and the default less-than operator returns
+     *    true.
+     * 4. The two Json instances are of incompatible types and the type of the first Json instance
+     *    is considered less-than the type of the second Json instance. The ordering is determined
+     *    by the order of the types listed in the fly::json_type alias.
+     *
+     * @return True if the first Json instance is less-than the second Json instance.
+     */
+    friend bool operator<(const_reference json1, const_reference json2);
+
+    /**
+     * Less-than-or-equal-to operator. The first Json instance is less-than-or-equal-to the second
+     * Json instance if any of the conditions of the equality or less-than operators are met.
+     *
+     * @return True if the first Json instance is less-than-or-equal-to the second Json instance.
+     */
+    friend bool operator<=(const_reference json1, const_reference json2);
+
+    /**
+     * Greater-than operator. The first Json instance is greater-than the second Json instance if
+     * none of the conditions of the less-than-or-equal-to operator are met.
+     *
+     * @return True if the first Json instance is greater-than the second Json instance.
+     */
+    friend bool operator>(const_reference json1, const_reference json2);
+
+    /**
+     * Greater-than-or-equal-to operator. The first Json instance is greater-than-or-equal-to the
+     * second Json instance if any of the conditions of the equality or greater-than operators are
+     * met.
+     *
+     * @return True if the first Json instance is greater-than-or-equal-to the second Json instance.
+     */
+    friend bool operator>=(const_reference json1, const_reference json2);
+
 private:
     friend iterator;
     friend const_iterator;