Deprecate QString::null

It's a Qt 3 compatibility vehicle, and as such inherits the now-alien
property to distinguish empty and null strings. Particularly worrisome
is the following asymmetry:

   QString("") == QString::null          // false
   QString("") == QString(QString::null) // true

Instead of fixing this behavior, recognize that people might use it as
a weird way to call isNull(), albeit one that once was idiomatic, and
simply deprecate everything that deals with QString::null.

[ChangeLog][QtCore][QString] QString::null is now deprecated. When
used to construct a QString, use QString() instead. When used to
compare to a QString, replace with QString::isNull().

Change-Id: I9f7e84a92522c75666da15f49324c500ae93af42
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Anton Kudryavtsev <antkudr@mail.ru>

src/corelib/tools/qstring.cpp

@@ -779,7 +779,9 @@ inline char qToLower(char ch)
 }
 
 
+#if QT_DEPRECATED_SINCE(5, 9)
 const QString::Null QString::null = { };
+#endif
 
 /*!
   \macro QT_RESTRICTED_CAST_FROM_ASCII

src/corelib/tools/qstring.h

@@ -770,10 +770,13 @@ public:
     NSString *toNSString() const Q_DECL_NS_RETURNS_AUTORELEASED;
 #endif
     // compatibility
+#if QT_DEPRECATED_SINCE(5, 9)
     struct Null { };
+    QT_DEPRECATED_X("use QString()")
     static const Null null;
     inline QString(const Null &): d(Data::sharedNull()) {}
     inline QString &operator=(const Null &) { *this = QString(); return *this; }
+#endif
     inline bool isNull() const { return d == Data::sharedNull(); }
 
 
@@ -1134,13 +1137,18 @@ inline bool QString::contains(QLatin1String s, Qt::CaseSensitivity cs) const
 inline bool QString::contains(QChar c, Qt::CaseSensitivity cs) const
 { return indexOf(c, 0, cs) != -1; }
 
-
+#if QT_DEPRECATED_SINCE(5, 9)
 inline bool operator==(QString::Null, QString::Null) { return true; }
+QT_DEPRECATED_X("use QString::isNull()")
 inline bool operator==(QString::Null, const QString &s) { return s.isNull(); }
+QT_DEPRECATED_X("use QString::isNull()")
 inline bool operator==(const QString &s, QString::Null) { return s.isNull(); }
 inline bool operator!=(QString::Null, QString::Null) { return false; }
+QT_DEPRECATED_X("use !QString::isNull()")
 inline bool operator!=(QString::Null, const QString &s) { return !s.isNull(); }
+QT_DEPRECATED_X("use !QString::isNull()")
 inline bool operator!=(const QString &s, QString::Null) { return !s.isNull(); }
+#endif
 
 inline bool operator==(QLatin1String s1, QLatin1String s2) Q_DECL_NOTHROW
 { return s1.size() == s2.size() && (!s1.size() || !memcmp(s1.latin1(), s2.latin1(), s1.size())); }

src/widgets/accessible/simplewidgets.cpp

@@ -519,7 +519,7 @@ QString QAccessibleDisplay::imageDescription() const
 #ifndef QT_NO_TOOLTIP
     return widget()->toolTip();
 #else
-    return QString::null;
+    return QString();
 #endif
 }
 

src/widgets/widgets/qmdisubwindow.cpp

@@ -1863,7 +1863,7 @@ void QMdiSubWindowPrivate::removeButtonsFromMenuBar()
     topLevelWindow->removeEventFilter(q);
     if (baseWidget && !drawTitleBarWhenMaximized())
         topLevelWindow->setWindowModified(false);
-    originalTitle = QString::null;
+    originalTitle.clear();
 }
 
 #endif // QT_NO_MENUBAR
@@ -2769,7 +2769,7 @@ bool QMdiSubWindow::eventFilter(QObject *object, QEvent *event)
 #ifndef QT_NO_MENUBAR
         } else if (maximizedButtonsWidget() && d->controlContainer->menuBar() && d->controlContainer->menuBar()
                    ->cornerWidget(Qt::TopRightCorner) == maximizedButtonsWidget()) {
-            d->originalTitle = QString::null;
+            d->originalTitle.clear();
             if (d->baseWidget && d->baseWidget->windowTitle() == windowTitle())
                 d->updateWindowTitle(true);
             else

tests/auto/corelib/io/qfile/tst_qfile.cpp

@@ -1684,7 +1684,7 @@ void tst_QFile::isSequential()
 
 void tst_QFile::encodeName()
 {
-    QCOMPARE(QFile::encodeName(QString::null), QByteArray());
+    QCOMPARE(QFile::encodeName(QString()), QByteArray());
 }
 
 void tst_QFile::truncate()

tests/auto/corelib/tools/qstring/tst_qstring.cpp

@@ -3818,7 +3818,7 @@ void tst_QString::startsWith()
     QVERIFY( !a.startsWith(QLatin1Char('x')) );
     QVERIFY( !a.startsWith(QChar()) );
 
-    a = QString::null;
+    a = QString();
     QVERIFY( !a.startsWith("") );
     QVERIFY( a.startsWith(QString::null) );
     QVERIFY( !a.startsWith("ABC") );
@@ -3928,7 +3928,7 @@ void tst_QString::endsWith()
     QVERIFY( a.endsWith(QLatin1String(0)) );
     QVERIFY( !a.endsWith(QLatin1String("ABC")) );
 
-    a = QString::null;
+    a = QString();
     QVERIFY( !a.endsWith("") );
     QVERIFY( a.endsWith(QString::null) );
     QVERIFY( !a.endsWith("ABC") );

tests/auto/network/access/qftp/tst_qftp.cpp

@@ -124,7 +124,7 @@ protected slots:
 private:
     QFtp *newFtp();
     void addCommand( QFtp::Command, int );
-    bool fileExists( const QString &host, quint16 port, const QString &user, const QString &password, const QString &file, const QString &cdDir = QString::null );
+    bool fileExists( const QString &host, quint16 port, const QString &user, const QString &password, const QString &file, const QString &cdDir = QString() );
     bool dirExists( const QString &host, quint16 port, const QString &user, const QString &password, const QString &cdDir, const QString &dirToCreate );
 
     void renameInit( const QString &host, const QString &user, const QString &password, const QString &createFile );

tests/auto/sql/kernel/qsqldatabase/tst_qsqldatabase.cpp

@@ -2191,7 +2191,7 @@ void tst_QSqlDatabase::sqlStatementUseIsNull_189093()
     CHECK_DATABASE(db);
 
     // select a record with NULL value
-    QSqlQuery q(QString::null, db);
+    QSqlQuery q(QString(), db);
     QVERIFY_SQL(q, exec("select * from " + qTableName("qtest", __FILE__, db) + " where id = 4"));
     QVERIFY_SQL(q, next());
 

tests/auto/widgets/kernel/qshortcut/tst_qshortcut.cpp

@@ -701,14 +701,14 @@ void tst_QShortcut::disabledItems()
     sendKeyEvents( Qt::CTRL+Qt::Key_M, 0 );
     QCOMPARE( currentResult, NoResult );
     if (over_330)
-        QCOMPARE( sbText, QString::null );
+        QCOMPARE( sbText, QString() );
 
     currentResult = NoResult;
     sendKeyEvents( Qt::CTRL+Qt::Key_K, 0 );
     sendKeyEvents( Qt::CTRL+Qt::Key_L, 0 );
     QCOMPARE( currentResult, Slot1Triggered );
     if (over_330)
-        QCOMPARE( sbText, QString::null );
+        QCOMPARE( sbText, QString() );
 #endif
     clearAllShortcuts();
     cut1 = 0;

tests/auto/widgets/widgets/qlineedit/tst_qlineedit.cpp

@@ -713,8 +713,8 @@ void tst_QLineEdit::clearInputMask()
 {
     QLineEdit *testWidget = ensureTestWidget();
     testWidget->setInputMask("000.000.000.000");
-    QVERIFY(testWidget->inputMask() != QString::null);
+    QVERIFY(!testWidget->inputMask().isNull());
-    testWidget->setInputMask(QString::null);
+    testWidget->setInputMask(QString());
     QCOMPARE(testWidget->inputMask(), QString());
 }
 
@@ -2277,7 +2277,7 @@ void tst_QLineEdit::textChangedAndTextEdited()
 
     changed_count = 0;
     edited_count = 0;
-    changed_string = QString::null;
+    changed_string.clear();
 
     testWidget->setText("foo");
     QCOMPARE(changed_count, 1);
@@ -2286,7 +2286,7 @@ void tst_QLineEdit::textChangedAndTextEdited()
 
     changed_count = 0;
     edited_count = 0;
-    changed_string = QString::null;
+    changed_string.clear();
 
     testWidget->setText("");
     QCOMPARE(changed_count, 1);
@@ -3108,7 +3108,7 @@ void tst_QLineEdit::maxLengthAndInputMask()
     QVERIFY(testWidget->inputMask().isNull());
     testWidget->setMaxLength(10);
     QCOMPARE(testWidget->maxLength(), 10);
-    testWidget->setInputMask(QString::null);
+    testWidget->setInputMask(QString());
     QVERIFY(testWidget->inputMask().isNull());
     QCOMPARE(testWidget->maxLength(), 10);
 }

tests/auto/widgets/widgets/qplaintextedit/tst_qplaintextedit.cpp

@@ -362,7 +362,7 @@ void tst_QPlainTextEdit::emptyAppend()
 {
     ed->appendPlainText("Blah");
     QCOMPARE(blockCount(), 1);
-    ed->appendPlainText(QString::null);
+    ed->appendPlainText(QString());
     QCOMPARE(blockCount(), 2);
     ed->appendPlainText(QString("   "));
     QCOMPARE(blockCount(), 3);

tests/auto/widgets/widgets/qtextedit/tst_qtextedit.cpp

@@ -676,7 +676,7 @@ void tst_QTextEdit::emptyAppend()
 {
     ed->append("Blah");
     QCOMPARE(blockCount(), 1);
-    ed->append(QString::null);
+    ed->append(QString());
     QCOMPARE(blockCount(), 2);
     ed->append(QString("   "));
     QCOMPARE(blockCount(), 3);