json: Add operator[] to QJsonDocument for implicit object and array access

Makes it easier to pull out data from a document when the structure is
known up front, while still supporting default values if the structure
does not match the expectation, eg:

 int age = QJsonDocument::fromJson(ba)["users"][0]["age"].toInt(-1);

Change-Id: Ief0899bbb81610f6f22a56e2ac846121bffe77a0
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/json/qjsondocument.cpp

@@ -550,6 +550,58 @@ void QJsonDocument::setArray(const QJsonArray &array)
     d->ref.ref();
 }
 
+/*!
+    Returns a QJsonValue representing the value for the key \a key.
+
+    Equivalent to calling object().value(key).
+
+    The returned QJsonValue is QJsonValue::Undefined if the key does not exist,
+    or if isObject() is false.
+
+    \since 5.10
+
+    \sa QJsonValue, QJsonValue::isUndefined(), QJsonObject
+ */
+const QJsonValue QJsonDocument::operator[](const QString &key) const
+{
+    if (!isObject())
+        return QJsonValue(QJsonValue::Undefined);
+
+    return object().value(key);
+}
+
+/*!
+    \overload
+    \since 5.10
+*/
+const QJsonValue QJsonDocument::operator[](QLatin1String key) const
+{
+    if (!isObject())
+        return QJsonValue(QJsonValue::Undefined);
+
+    return object().value(key);
+}
+
+/*!
+    Returns a QJsonValue representing the value for index \a i.
+
+    Equivalent to calling array().at(i).
+
+    The returned QJsonValue is QJsonValue::Undefined, if \a i is out of bounds,
+    or if isArray() is false.
+
+    \since 5.10
+
+    \sa QJsonValue, QJsonValue::isUndefined(), QJsonArray
+ */
+const QJsonValue QJsonDocument::operator[](int i) const
+{
+    if (!isArray())
+        return QJsonValue(QJsonValue::Undefined);
+
+    return array().at(i);
+}
+
 /*!
     Returns \c true if the \a other document is equal to this document.
  */

src/corelib/json/qjsondocument.h

@@ -148,6 +148,10 @@ public:
     void setObject(const QJsonObject &object);
     void setArray(const QJsonArray &array);
 
+    const QJsonValue operator[](const QString &key) const;
+    const QJsonValue operator[](QLatin1String key) const;
+    const QJsonValue operator[](int i) const;
+
     bool operator==(const QJsonDocument &other) const;
     bool operator!=(const QJsonDocument &other) const { return !(*this == other); }
 

tests/auto/corelib/json/tst_qtjson.cpp

@@ -146,6 +146,7 @@ private Q_SLOTS:
     void parseErrorOffset();
 
     void implicitValueType();
+    void implicitDocumentType();
 
 private:
     QString testDataDir;
@@ -2938,5 +2939,24 @@ void tst_QtJson::implicitValueType()
     QCOMPARE(objectAsValue["array"][1].toInt(), 666);
 }
 
+void tst_QtJson::implicitDocumentType()
+{
+    QJsonDocument emptyDocument;
+    QCOMPARE(emptyDocument["asObject"], QJsonValue(QJsonValue::Undefined));
+    QCOMPARE(emptyDocument[123], QJsonValue(QJsonValue::Undefined));
+
+    QJsonDocument objectDocument(QJsonObject{{"value", 42}});
+    QCOMPARE(objectDocument["value"].toInt(), 42);
+    QCOMPARE(objectDocument["missingValue"], QJsonValue(QJsonValue::Undefined));
+    QCOMPARE(objectDocument[123], QJsonValue(QJsonValue::Undefined));
+    QCOMPARE(objectDocument["missingValue"].toInt(123), 123);
+
+    QJsonDocument arrayDocument(QJsonArray{665, 666, 667});
+    QCOMPARE(arrayDocument[1].toInt(), 666);
+    QCOMPARE(arrayDocument[-1], QJsonValue(QJsonValue::Undefined));
+    QCOMPARE(arrayDocument["asObject"], QJsonValue(QJsonValue::Undefined));
+    QCOMPARE(arrayDocument[-1].toInt(123), 123);
+}
+
 QTEST_MAIN(tst_QtJson)
 #include "tst_qtjson.moc"