Add setFlag method to QFlags

This makes implementing class methods that enable or disable a feature
stored in a QFlags<> member easier.

[ChangeLog][QtCore][QFlags] Added setFlag method to set or unset a flag

Task-number: QTBUG-27100
Change-Id: Ic602abbbf3100df78f21b7918573744d1bbd18c1
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/global/qflags.h

@@ -145,6 +145,11 @@ public:
     Q_DECL_CONSTEXPR inline bool operator!() const Q_DECL_NOTHROW { return !i; }
 
     Q_DECL_CONSTEXPR inline bool testFlag(Enum f) const Q_DECL_NOTHROW { return (i & Int(f)) == Int(f) && (Int(f) != 0 || i == Int(f) ); }
+    Q_DECL_RELAXED_CONSTEXPR inline QFlags &setFlag(Enum f, bool on = true) Q_DECL_NOTHROW
+    {
+        return on ? (*this |= f) : (*this &= ~f);
+    }
+
 private:
 #ifdef Q_COMPILER_INITIALIZER_LISTS
     Q_DECL_CONSTEXPR static inline Int initializer_list_helper(typename std::initializer_list<Enum>::const_iterator it,

src/corelib/global/qglobal.cpp

@@ -425,6 +425,14 @@ Q_STATIC_ASSERT_X(QT_POINTER_SIZE == sizeof(void *), "QT_POINTER_SIZE defined in
     Returns \c true if the \a flag is set, otherwise \c false.
 */
 
+/*!
+    \fn QFlags QFlags::setFlag(Enum flag, bool on) const
+    \since 5.7
+
+    Sets the indicated \a flag if \a on is \c true or unsets it if
+    it if \a on is \c false. Returns a reference to this object.
+*/
+
 /*!
   \macro Q_DISABLE_COPY(Class)
   \relates QObject

tests/auto/corelib/global/qflags/tst_qflags.cpp

@@ -38,6 +38,7 @@ private slots:
     void signedness();
     void classEnum();
     void initializerLists();
+    void testSetFlags();
 };
 
 void tst_QFlags::testFlag() const
@@ -278,6 +279,19 @@ void tst_QFlags::initializerLists()
 #endif // Q_COMPILER_INITIALIZER_LISTS
 }
 
+void tst_QFlags::testSetFlags()
+{
+    Qt::MouseButtons btn = Qt::NoButton;
+
+    btn.setFlag(Qt::LeftButton);
+    QVERIFY(btn.testFlag(Qt::LeftButton));
+    QVERIFY(!btn.testFlag(Qt::MidButton));
+
+    btn.setFlag(Qt::LeftButton, false);
+    QVERIFY(!btn.testFlag(Qt::LeftButton));
+    QVERIFY(!btn.testFlag(Qt::MidButton));
+}
+
 // (statically) check QTypeInfo for QFlags instantiations:
 enum MyEnum { Zero, One, Two, Four=4 };
 Q_DECLARE_FLAGS( MyFlags, MyEnum )