savegame ex.: revamp the way we (de)serialize JSON

JSON, unlike, say, QDataStream, allows building up objects independent
of some central object, and combining them into a QJsonDocument
later. This suggests returning QJsonObjects from a toJson() const
method instead of having the caller supply a QJsonObject. Doing it
this way enables transparent move semantics to kick in, too.

For deserialization, use a fromJson() named constructor for value-like
classes (where identity doesn't matter, only equality). Keep using
read(), too, and add a note to explain when to use which form.

Also, avoid the triple lookup from

   if (json.contains("key") && json["key"].isSoughtType())
      mFoo = json["key"].toSoughtType();

by using C++17 if-with-initializer and showing the trick with
Undefined never being of isSoughtType():

   if (const QJsonValue v = json["key"]; v.isSoughtType())
      mFoo = v.toSoughtType();

Adjust the discussion to match the new code, up the copyright years
and rename some qdoc snippet markers from nondescript [0]/[1] to
[toJson]/[fromJson].

Task-number: QTBUG-108857
Pick-to: 6.5 6.4 6.2
Change-Id: Icaa14acc7464fef00a59534679d710252e921383
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

7 files changed, 166 insertions, 113 deletions

diff --git a/examples/corelib/serialization/savegame/character.cpp b/examples/corelib/serialization/savegame/character.cpp
index 7be737f308..910e7e9e7a 100644
--- a/examples/corelib/serialization/savegame/character.cpp
+++ b/examples/corelib/serialization/savegame/character.cpp
@@ -48,28 +48,34 @@ void Character::setClassType(Character::ClassType classType)
     mClassType = classType;
 }
 
-//! [0]
-void Character::read(const QJsonObject &json)
+//! [fromJson]
+Character Character::fromJson(const QJsonObject &json)
 {
-    if (json.contains("name") && json["name"].isString())
-        mName = json["name"].toString();
+    Character result;
 
-    if (json.contains("level") && json["level"].isDouble())
-        mLevel = json["level"].toInt();
+    if (const QJsonValue v = json["name"]; v.isString())
+        result.mName = v.toString();
 
-    if (json.contains("classType") && json["classType"].isDouble())
-        mClassType = ClassType(json["classType"].toInt());
+    if (const QJsonValue v = json["level"]; v.isDouble())
+        result.mLevel = v.toInt();
+
+    if (const QJsonValue v = json["classType"]; v.isDouble())
+        result.mClassType = ClassType(v.toInt());
+
+    return result;
 }
-//! [0]
+//! [fromJson]
 
-//! [1]
-void Character::write(QJsonObject &json) const
+//! [toJson]
+QJsonObject Character::toJson() const
 {
+    QJsonObject json;
     json["name"] = mName;
     json["level"] = mLevel;
     json["classType"] = mClassType;
+    return json;
 }
-//! [1]
+//! [toJson]
 
 void Character::print(int indentation) const
 {
diff --git a/examples/corelib/serialization/savegame/character.h b/examples/corelib/serialization/savegame/character.h
index 4dc25139a6..d91bc8519b 100644
--- a/examples/corelib/serialization/savegame/character.h
+++ b/examples/corelib/serialization/savegame/character.h
@@ -31,8 +31,8 @@ public:
     ClassType classType() const;
     void setClassType(ClassType classType);
 
-    void read(const QJsonObject &json);
-    void write(QJsonObject &json) const;
+    static Character fromJson(const QJsonObject &json);
+    QJsonObject toJson() const;
 
     void print(int indentation = 0) const;
 private:
diff --git a/examples/corelib/serialization/savegame/doc/src/savegame.qdoc b/examples/corelib/serialization/savegame/doc/src/savegame.qdoc
index 5302582fcc..b1aa599e79 100644
--- a/examples/corelib/serialization/savegame/doc/src/savegame.qdoc
+++ b/examples/corelib/serialization/savegame/doc/src/savegame.qdoc
@@ -24,45 +24,81 @@
     The Character class represents a non-player character (NPC) in our game, and
     stores the player's name, level, and class type.
 
-    It provides read() and write() functions to serialise its member variables.
+    It provides static fromJson() and non-static toJson() functions to
+    serialise itself.
+
+    \note This pattern (fromJson()/toJson()) works because QJsonObjects can be
+    constructed independent of an owning QJsonDocument, and because the data
+    types being (de)serialized here are value types, so can be copied. When
+    serializing to another format — for example XML or QDataStream, which require passing
+    a document-like object — or when the object identity is important (QObject
+    subclasses, for example), other patterns may be more suitable. See the
+    \l{xml/dombookmarks} and \l{xml/streambookmarks} examples for XML, and the
+    implementation of \l QListWidgetItem::read() and \l QListWidgetItem::write()
+    for idiomatic QDataStream serialization.
 
     \snippet serialization/savegame/character.h 0
 
-    Of particular interest to us are the read and write function
+    Of particular interest to us are the fromJson() and toJson() function
     implementations:
 
-    \snippet serialization/savegame/character.cpp 0
+    \snippet serialization/savegame/character.cpp fromJson
 
-    In the read() function, we assign Character's members values from the
-    QJsonObject argument. You can use either \l QJsonObject::operator[]() or
-    QJsonObject::value() to access values within the JSON object; both are
-    const functions and return QJsonValue::Undefined if the key is invalid. We
-    check if the keys are valid before attempting to read them with
-    QJsonObject::contains().
+    In the fromJson() function, we construct a local \c result Character object
+    and assign \c{result}'s members values from the QJsonObject argument. You
+    can use either \l QJsonObject::operator[]() or QJsonObject::value() to
+    access values within the JSON object; both are const functions and return
+    QJsonValue::Undefined if the key is invalid. In particular, the \c{is...}
+    functions (for example \l QJsonValue::isString(), \l
+    QJsonValue::isDouble()) return \c false for QJsonValue::Undefined, so we
+    can check for existence as well as the correct type in a single lookup.
 
-    \snippet serialization/savegame/character.cpp 1
+    If a value does not exist in the JSON object, or has the wrong type, we
+    don't write to the corresponding \c result member, either, thereby
+    preserving any values the default constructor may have set. This means
+    default values are centrally defined in one location (the default
+    constructor) and need not be repeated in serialisation code
+    (\l{https://en.wikipedia.org/wiki/Don%27t_repeat_yourself}{DRY}).
 
-    In the write() function, we do the reverse of the read() function; assign
-    values from the Character object to the JSON object. As with accessing
-    values, there are two ways to set values on a QJsonObject:
-    \l QJsonObject::operator[]() and QJsonObject::insert(). Both will override
-    any existing value at the given key.
+    Observe the use of
+    \l{https://en.cppreference.com/w/cpp/language/if#If_statements_with_initializer}
+    {C++17 if-with-initializer} to separate scoping and checking of the variable \c v.
+    This means we can keep the variable name short, because its scope is limited.
 
-    Next up is the Level class:
+    Compare that to the naïve approach using \c QJsonObject::contains():
+
+    \badcode
+    if (json.contains("name") && json["name"].isString())
+        result.mName = json["name"].toString();
+    \endcode
+
+    which, beside being less readable, requires a total of three lookups (no,
+    the compiler will \e not optimize these into one), so is three times
+    slower and repeats \c{"name"} three times (violating the DRY principle).
+
+    \snippet serialization/savegame/character.cpp toJson
+
+    In the toJson() function, we do the reverse of the fromJson() function;
+    assign values from the Character object to a new JSON object we then
+    return. As with accessing values, there are two ways to set values on a
+    QJsonObject: \l QJsonObject::operator[]() and \l QJsonObject::insert().
+    Both will override any existing value at the given key.
+
+    \section1 The Level Class
 
     \snippet serialization/savegame/level.h 0
 
-    We want to have several levels in our game, each with several NPCs, so we
-    keep a QList of Character objects. We also provide the familiar read() and
-    write() functions.
+    We want the levels in our game to each each have several NPCs, so we keep a QList
+    of Character objects. We also provide the familiar fromJson() and toJson()
+    functions.
 
-    \snippet serialization/savegame/level.cpp 0
+    \snippet serialization/savegame/level.cpp fromJson
 
-    Containers can be written and read to and from JSON using QJsonArray. In our
+    Containers can be written to and read from JSON using QJsonArray. In our
     case, we construct a QJsonArray from the value associated with the key
     \c "npcs". Then, for each QJsonValue element in the array, we call
-    toObject() to get the Character's JSON object. The Character object can then
-    read their JSON and be appended to our NPC array.
+    toObject() to get the Character's JSON object. Character::fromJson() can
+    then turn that QJSonObject into a Character object to append to our NPC array.
 
     \note \l{Container Classes}{Associate containers} can be written by storing
     the key in each value object (if it's not already). With this approach, the
@@ -70,11 +106,13 @@
     element is used as the key to construct the container when reading it back
     in.
 
-    \snippet serialization/savegame/level.cpp 1
+    \snippet serialization/savegame/level.cpp toJson
 
-    Again, the write() function is similar to the read() function, except
+    Again, the toJson() function is similar to the fromJson() function, except
     reversed.
 
+    \section1 The Game Class
+
     Having established the Character and Level classes, we can move on to
     the Game class:
 
@@ -86,26 +124,43 @@
     Next, we provide accessors for the player and levels. We then expose three
     functions: newGame(), saveGame() and loadGame().
 
-    The read() and write() functions are used by saveGame() and loadGame().
+    The read() and toJson() functions are used by saveGame() and loadGame().
+
+    \div{class="admonition note"}\b{Note:}
+        Despite \c Game being a value class, we assume that the author wants a game to have
+        identity, much like your main window would have. We therefore don't use a
+        static fromJson() function, which would create a new object, but a read()
+        function we can call on existing objects. There's a 1:1 correspondence
+        between read() and fromJson(), in that one can be implemented in terms of
+        the other:
+
+        \code
+        void read(const QJsonObject &json) { *this = fromJson(json); }
+        static Game fromObject(const QJsonObject &json) { Game g; g.read(json); return g; }
+        \endcode
 
-    \snippet serialization/savegame/game.cpp 0
+        We just use what's more convenient for callers of the functions.
+    \enddiv
+
+    \snippet serialization/savegame/game.cpp newGame
 
     To setup a new game, we create the player and populate the levels and their
     NPCs.
 
-    \snippet serialization/savegame/game.cpp 1
+    \snippet serialization/savegame/game.cpp read
 
-    The first thing we do in the read() function is tell the player to read
-    itself. We then clear the level array so that calling loadGame() on the
-    same Game object twice doesn't result in old levels hanging around.
+    The read() function starts by replacing the player with the
+    one read from JSON. We then clear() the level array so that calling
+    loadGame() on the same Game object twice doesn't result in old levels
+    hanging around.
 
     We then populate the level array by reading each Level from a QJsonArray.
 
-    \snippet serialization/savegame/game.cpp 2
+    \snippet serialization/savegame/game.cpp toJson
 
-    We write the game to JSON similarly to how we write Level.
+    Writing the game to JSON is similar to writing a level.
 
-    \snippet serialization/savegame/game.cpp 3
+    \snippet serialization/savegame/game.cpp loadGame
 
     When loading a saved game in loadGame(), the first thing we do is open the
     save file based on which format it was saved to; \c "save.json" for JSON,
@@ -119,14 +174,16 @@
     After constructing the QJsonDocument, we instruct the Game object to read
     itself and then return \c true to indicate success.
 
-    \snippet serialization/savegame/game.cpp 4
+    \snippet serialization/savegame/game.cpp saveGame
 
     Not surprisingly, saveGame() looks very much like loadGame(). We determine
     the file extension based on the format, print a warning and return \c false
     if the opening of the file fails. We then write the Game object to a
-    QJsonDocument, and call either QJsonDocument::toJson() or to
-    QJsonDocument::toBinaryData() to save the game, depending on which format
-    was specified.
+    QJsonObject. To save the game in the format that was specified, we
+    convert the JSON object into either a QJsonDocument for a subsequent
+    QJsonDocument::toJson() call, or a QCborValue for QCborValue::toCbor().
+
+    \section1 Tying It All Together
 
     We are now ready to enter main():
 
diff --git a/examples/corelib/serialization/savegame/game.cpp b/examples/corelib/serialization/savegame/game.cpp
index 21dedf9482..3140178f88 100644
--- a/examples/corelib/serialization/savegame/game.cpp
+++ b/examples/corelib/serialization/savegame/game.cpp
@@ -21,7 +21,7 @@ QList<Level> Game::levels() const
     return mLevels;
 }
 
-//! [0]
+//! [newGame]
 void Game::newGame()
 {
     mPlayer = Character();
@@ -59,9 +59,9 @@ void Game::newGame()
     dungeon.setNpcs(dungeonNpcs);
     mLevels.append(dungeon);
 }
-//! [0]
+//! [newGame]
 
-//! [3]
+//! [loadGame]
 bool Game::loadGame(Game::SaveFormat saveFormat)
 {
     QFile loadFile(saveFormat == Json
@@ -87,9 +87,9 @@ bool Game::loadGame(Game::SaveFormat saveFormat)
                         << (saveFormat != Json ? "CBOR" : "JSON") << "...\n";
     return true;
 }
-//! [3]
+//! [loadGame]
 
-//! [4]
+//! [saveGame]
 bool Game::saveGame(Game::SaveFormat saveFormat) const
 {
     QFile saveFile(saveFormat == Json
@@ -101,52 +101,44 @@ bool Game::saveGame(Game::SaveFormat saveFormat) const
         return false;
     }
 
-    QJsonObject gameObject;
-    write(gameObject);
+    QJsonObject gameObject = toJson();
     saveFile.write(saveFormat == Json
         ? QJsonDocument(gameObject).toJson()
         : QCborValue::fromJsonValue(gameObject).toCbor());
 
     return true;
 }
-//! [4]
+//! [saveGame]
 
-//! [1]
+//! [read]
 void Game::read(const QJsonObject &json)
 {
-    if (json.contains("player") && json["player"].isObject())
-        mPlayer.read(json["player"].toObject());
+    if (const QJsonValue v = json["player"]; v.isObject())
+        mPlayer = Character::fromJson(v.toObject());
 
-    if (json.contains("levels") && json["levels"].isArray()) {
-        QJsonArray levelArray = json["levels"].toArray();
+    if (const QJsonValue v = json["levels"]; v.isArray()) {
+        const QJsonArray levels = v.toArray();
         mLevels.clear();
-        mLevels.reserve(levelArray.size());
-        for (const QJsonValue &v : levelArray) {
-            QJsonObject levelObject = v.toObject();
-            Level level;
-            level.read(levelObject);
-            mLevels.append(level);
-        }
+        mLevels.reserve(levels.size());
+        for (const QJsonValue &level : levels)
+            mLevels.append(Level::fromJson(level.toObject()));
     }
 }
-//! [1]
+//! [read]
 
-//! [2]
-void Game::write(QJsonObject &json) const
+//! [toJson]
+QJsonObject Game::toJson() const
 {
-    QJsonObject playerObject;
-    mPlayer.write(playerObject);
-    json["player"] = playerObject;
-
-    QJsonArray levelArray;
-    for (const Level &level : mLevels) {
-        QJsonObject levelObject;
-        level.write(levelObject);
-        levelArray.append(levelObject);
-    }
-    json["levels"] = levelArray;
+    QJsonObject json;
+    json["player"] = mPlayer.toJson();
+
+    QJsonArray levels;
+    for (const Level &level : mLevels)
+        levels.append(level.toJson());
+    json["levels"] = levels;
+    return json;
 }
-//! [2]
+//! [toJson]
 
 void Game::print(int indentation) const
 {
diff --git a/examples/corelib/serialization/savegame/game.h b/examples/corelib/serialization/savegame/game.h
index 0e91343d27..4c72ea426e 100644
--- a/examples/corelib/serialization/savegame/game.h
+++ b/examples/corelib/serialization/savegame/game.h
@@ -26,7 +26,7 @@ public:
     bool saveGame(SaveFormat saveFormat) const;
 
     void read(const QJsonObject &json);
-    void write(QJsonObject &json) const;
+    QJsonObject toJson() const;
 
     void print(int indentation = 0) const;
 private:
diff --git a/examples/corelib/serialization/savegame/level.cpp b/examples/corelib/serialization/savegame/level.cpp
index c2f88c3434..5a6137715a 100644
--- a/examples/corelib/serialization/savegame/level.cpp
+++ b/examples/corelib/serialization/savegame/level.cpp
@@ -25,39 +25,37 @@ void Level::setNpcs(const QList<Character> &npcs)
     mNpcs = npcs;
 }
 
-//! [0]
-void Level::read(const QJsonObject &json)
+//! [fromJson]
+Level Level::fromJson(const QJsonObject &json)
 {
-    if (json.contains("name") && json["name"].isString())
-        mName = json["name"].toString();
+    Level result;
 
-    if (json.contains("npcs") && json["npcs"].isArray()) {
-        QJsonArray npcArray = json["npcs"].toArray();
-        mNpcs.clear();
-        mNpcs.reserve(npcArray.size());
-        for (const QJsonValue &v : npcArray) {
-            QJsonObject npcObject = v.toObject();
-            Character npc;
-            npc.read(npcObject);
-            mNpcs.append(npc);
-        }
+    if (const QJsonValue v = json["name"]; v.isString())
+        result.mName = v.toString();
+
+    if (const QJsonValue v = json["npcs"]; v.isArray()) {
+        const QJsonArray npcs = v.toArray();
+        result.mNpcs.reserve(npcs.size());
+        for (const QJsonValue &npc : npcs)
+            result.mNpcs.append(Character::fromJson(npc.toObject()));
     }
+
+    return result;
 }
-//! [0]
+//! [fromJson]
 
-//! [1]
-void Level::write(QJsonObject &json) const
+//! [toJson]
+QJsonObject Level::toJson() const
 {
+    QJsonObject json;
     json["name"] = mName;
     QJsonArray npcArray;
-    for (const Character &npc : mNpcs) {
-        QJsonObject npcObject;
-        npc.write(npcObject);
-        npcArray.append(npcObject);
-    }
+    for (const Character &npc : mNpcs)
+        npcArray.append(npc.toJson());
     json["npcs"] = npcArray;
+    return json;
 }
-//! [1]
+//! [toJson]
 
 void Level::print(int indentation) const
 {
diff --git a/examples/corelib/serialization/savegame/level.h b/examples/corelib/serialization/savegame/level.h
index e09e2c9f3c..ce39a611b4 100644
--- a/examples/corelib/serialization/savegame/level.h
+++ b/examples/corelib/serialization/savegame/level.h
@@ -21,8 +21,8 @@ public:
     QList<Character> npcs() const;
     void setNpcs(const QList<Character> &npcs);
 
-    void read(const QJsonObject &json);
-    void write(QJsonObject &json) const;
+    static Level fromJson(const QJsonObject &json);
+    QJsonObject toJson() const;
 
     void print(int indentation = 0) const;
 private: