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>

4 files changed, 14 insertions, 4 deletions

diff --git a/src/corelib/tools/qstring.cpp b/src/corelib/tools/qstring.cpp
index d76eb11073..50893536af 100644
--- a/src/corelib/tools/qstring.cpp
+++ b/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
diff --git a/src/corelib/tools/qstring.h b/src/corelib/tools/qstring.h
index e06927d434..1bd436c387 100644
--- a/src/corelib/tools/qstring.h
+++ b/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())); }
diff --git a/src/widgets/accessible/simplewidgets.cpp b/src/widgets/accessible/simplewidgets.cpp
index 113172ff5a..600754f90c 100644
--- a/src/widgets/accessible/simplewidgets.cpp
+++ b/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
 }
 
diff --git a/src/widgets/widgets/qmdisubwindow.cpp b/src/widgets/widgets/qmdisubwindow.cpp
index a627c86871..a8cdca1719 100644
--- a/src/widgets/widgets/qmdisubwindow.cpp
+++ b/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