Move overloaded symmetric operators out of classes

This follows the advice of C++ Core Guidelines C.161 and C.168.

The File equality and less than comparison operators have not been
moved because File::GetDisplayName() is not a trivial accessor.

include/loot/metadata/group.h

@@ -62,20 +62,6 @@ public:
                           const std::vector<std::string>& afterGroups = {},
                           const std::string& description = "");
 
-  /**
-   * Check if two Group objects are equal by comparing their names.
-   * @returns True if the objects' fields are equal, false otherwise.
-   */
-  LOOT_API bool operator==(const Group& rhs) const;
-
-  /**
-   * A less-than operator implemented with no semantics so that Group objects
-   * can be stored in sets.
-   * @returns True if this Group is less than the given Group, false
-   *          otherwise.
-   */
-  LOOT_API bool operator<(const Group& rhs) const;
-
   /**
    * Get the name of the group.
    * @return The group's name.
@@ -100,12 +86,26 @@ private:
   std::vector<std::string> afterGroups_;
 };
 
+/**
+ * Check if two Group objects are equal by comparing their names.
+ * @returns True if the objects' fields are equal, false otherwise.
+ */
+LOOT_API bool operator==(const Group& lhs, const Group& rhs);
+
 /**
  * Check if two Group objects are not equal.
  * @returns True if the Group objects are not equal, false otherwise.
  */
 LOOT_API bool operator!=(const Group& lhs, const Group& rhs);
 
+/**
+ * A less-than operator implemented with no semantics so that Group objects
+ * can be stored in sets.
+ * @returns True if the first Group is less than the second Group, false
+ *          otherwise.
+ */
+LOOT_API bool operator<(const Group& lhs, const Group& rhs);
+
 /**
  * Check if the first Group object is greater than the second Group
  * object.

include/loot/metadata/location.h

@@ -52,20 +52,6 @@ public:
   LOOT_API explicit Location(const std::string& url,
                              const std::string& name = "");
 
-  /**
-   * A less-than operator implemented with no semantics so that Location objects
-   * can be stored in sets.
-   * @returns True if this Location is less than the given Location, false
-   *          otherwise.
-   */
-  LOOT_API bool operator<(const Location& rhs) const;
-
-  /**
-   * Check if two Location objects are equal by comparing their fields.
-   * @returns True if the objects' fields are equal, false otherwise.
-   */
-  LOOT_API bool operator==(const Location& rhs) const;
-
   /**
    * Get the object's URL.
    * @return A URL string.
@@ -83,12 +69,26 @@ private:
   std::string name_;
 };
 
+/**
+ * Check if two Location objects are equal by comparing their fields.
+ * @returns True if the objects' fields are equal, false otherwise.
+ */
+LOOT_API bool operator==(const Location& lhs, const Location& rhs);
+
 /**
  * Check if two Location objects are not equal.
  * @returns True if the Location objects are not equal, false otherwise.
  */
 LOOT_API bool operator!=(const Location& lhs, const Location& rhs);
 
+/**
+ * A less-than operator implemented with no semantics so that Location objects
+ * can be stored in sets.
+ * @returns True if the first Location is less than the second Location, false
+ *          otherwise.
+ */
+LOOT_API bool operator<(const Location& lhs, const Location& rhs);
+
 /**
  * Check if the first Location object is greater than the second Location
  * object.

include/loot/metadata/message.h

@@ -84,20 +84,6 @@ public:
    */
   LOOT_API explicit Message(const SimpleMessage& message);
 
-  /**
-   * A less-than operator implemented with no semantics so that Message objects
-   * can be stored in sets.
-   * @returns Returns true if this Message is less than the given Message, and
-   *          false otherwise.
-   */
-  LOOT_API bool operator<(const Message& rhs) const;
-
-  /**
-   * Check if two Message objects are equal by comparing their fields.
-   * @returns True if the objects' fields are equal, false otherwise.
-   */
-  LOOT_API bool operator==(const Message& rhs) const;
-
   /**
    * Get the message type.
    * @return The message type.
@@ -125,12 +111,26 @@ private:
   std::vector<MessageContent> content_;
 };
 
+/**
+ * Check if two Message objects are equal by comparing their fields.
+ * @returns True if the objects' fields are equal, false otherwise.
+ */
+LOOT_API bool operator==(const Message& lhs, const Message& rhs);
+
 /**
  * Check if two Message objects are not equal.
  * @returns True if the Message objects are not equal, false otherwise.
  */
 LOOT_API bool operator!=(const Message& lhs, const Message& rhs);
 
+/**
+ * A less-than operator implemented with no semantics so that Message objects
+ * can be stored in sets.
+ * @returns Returns true if the first Message is less than the second Message,
+ *          and false otherwise.
+ */
+LOOT_API bool operator<(const Message& lhs, const Message& rhs);
+
 /**
  * Check if the first Message object is greater than the second Message object.
  * @returns True if the second Message object is less than the first Message

include/loot/metadata/message_content.h

@@ -72,31 +72,31 @@ public:
    */
   LOOT_API std::string GetLanguage() const;
 
-  /**
-   * A less-than operator implemented with no semantics so that MessageContent
-   * objects can be stored in sets.
-   * @returns True if this MessageContent is less than the given
-   *          MessageContent, false otherwise.
-   */
-  LOOT_API bool operator<(const MessageContent& rhs) const;
-
-  /**
-   * Check if two MessageContent objects are equal by comparing their fields.
-   * @returns True if the objects' fields are equal, false otherwise.
-   */
-  LOOT_API bool operator==(const MessageContent& rhs) const;
-
 private:
   std::string text_;
   std::string language_{DEFAULT_LANGUAGE};
 };
 
+/**
+ * Check if two MessageContent objects are equal by comparing their fields.
+ * @returns True if the objects' fields are equal, false otherwise.
+ */
+LOOT_API bool operator==(const MessageContent& lhs, const MessageContent& rhs);
+
 /**
  * Check if two MessageContent objects are not equal.
  * @returns True if the MessageContent objects are not equal, false otherwise.
  */
 LOOT_API bool operator!=(const MessageContent& lhs, const MessageContent& rhs);
 
+/**
+ * A less-than operator implemented with no semantics so that MessageContent
+ * objects can be stored in sets.
+ * @returns True if the first MessageContent is less than the second
+ *          MessageContent, false otherwise.
+ */
+LOOT_API bool operator<(const MessageContent& lhs, const MessageContent& rhs);
+
 /**
  * Check if the first MessageContent object is greater than the second
  * MessageContent object.

include/loot/metadata/plugin_cleaning_data.h

@@ -83,20 +83,6 @@ public:
       unsigned int itm,
       unsigned int ref,
       unsigned int nav);
-  /**
-   * A less-than operator implemented with no semantics so that
-   * PluginCleaningData objects can be stored in sets.
-   * @returns True if this PluginCleaningData is less than the given
-   *          PluginCleaningData, false otherwise.
-   */
-  LOOT_API bool operator<(const PluginCleaningData& rhs) const;
-
-  /**
-   * Check if two PluginCleaningData objects are equal by comparing their
-   * fields.
-   * @returns True if the objects' fields are equal, false otherwise.
-   */
-  LOOT_API bool operator==(const PluginCleaningData& rhs) const;
 
   /**
    * Get the CRC that identifies the plugin that the cleaning data is for.
@@ -147,6 +133,14 @@ private:
   std::vector<MessageContent> detail_;
 };
 
+/**
+ * Check if two PluginCleaningData objects are equal by comparing their
+ * fields.
+ * @returns True if the objects' fields are equal, false otherwise.
+ */
+LOOT_API bool operator==(const PluginCleaningData& lhs,
+                         const PluginCleaningData& rhs);
+
 /**
  * Check if two MessageContent objects are not equal.
  * @returns True if the MessageContent objects are not equal, false otherwise.
@@ -154,6 +148,15 @@ private:
 LOOT_API bool operator!=(const PluginCleaningData& lhs,
                          const PluginCleaningData& rhs);
 
+/**
+ * A less-than operator implemented with no semantics so that
+ * PluginCleaningData objects can be stored in sets.
+ * @returns True if the first PluginCleaningData is less than the second
+ *          PluginCleaningData, false otherwise.
+ */
+LOOT_API bool operator<(const PluginCleaningData& lhs,
+                        const PluginCleaningData& rhs);
+
 /**
  * Check if the first PluginCleaningData object is greater than the second
  * PluginCleaningData object.

include/loot/metadata/tag.h

@@ -57,19 +57,6 @@ public:
                         const bool isAddition = true,
                         const std::string& condition = "");
 
-  /**
-   * A less-than operator implemented with no semantics so that Tag objects
-   * can be stored in sets.
-   * @returns True if this Tag is less than the given Tag, false otherwise.
-   */
-  LOOT_API bool operator<(const Tag& rhs) const;
-
-  /**
-   * Check if two Tag objects are equal.
-   * @returns True if the objects' fields are equal, false otherwise.
-   */
-  LOOT_API bool operator==(const Tag& rhs) const;
-
   /**
    * Check if the tag should be added.
    * @return True if the tag should be added, false if it should be removed.
@@ -87,12 +74,25 @@ private:
   bool addTag_{true};
 };
 
+/**
+ * Check if two Tag objects are equal.
+ * @returns True if the objects' fields are equal, false otherwise.
+ */
+LOOT_API bool operator==(const Tag& lhs, const Tag& rhs);
+
 /**
  * Check if two Tag objects are not equal.
  * @returns True if the Tag objects are not equal, false otherwise.
  */
 LOOT_API bool operator!=(const Tag& lhs, const Tag& rhs);
 
+/**
+ * A less-than operator implemented with no semantics so that Tag objects
+ * can be stored in sets.
+ * @returns True if the first Tag is less than the second Tag, false otherwise.
+ */
+LOOT_API bool operator<(const Tag& lhs, const Tag& rhs);
+
 /**
  * Check if the first Tag object is greater than the second Tag object.
  * @returns True if the second Tag object is less than the first Tag object,