QSet: de-pessimize binary operators

Overload the binary QSet operators |, &, + and - for rvalue LHSs, so
chained operations like e.g. in qgesturemanager.cpp:

        QSet<QGesture *> endedGestures =
                finishedGestures + canceledGestures + undeliveredGestures + maybeToCanceledGestures;

become as efficient as chained op+= calls.

Make the operators hidden friends as a drive-by.

[ChangeLog][QtCore][QSet] The binary operators &, |, + and - are now
hidden friends, and chaining them has been made a lot more efficient.

Change-Id: I55d2247b11d088eb1ef88608f89d2bf9e1daeb58
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qset.h

@@ -189,14 +189,18 @@ public:
     inline QSet<T> &operator+=(const T &value) { insert(value); return *this; }
     inline QSet<T> &operator-=(const QSet<T> &other) { subtract(other); return *this; }
     inline QSet<T> &operator-=(const T &value) { remove(value); return *this; }
-    inline QSet<T> operator|(const QSet<T> &other) const
-        { QSet<T> result = *this; result |= other; return result; }
-    inline QSet<T> operator&(const QSet<T> &other) const
-        { QSet<T> result = *this; result &= other; return result; }
-    inline QSet<T> operator+(const QSet<T> &other) const
-        { QSet<T> result = *this; result += other; return result; }
-    inline QSet<T> operator-(const QSet<T> &other) const
-        { QSet<T> result = *this; result -= other; return result; }
+
+    friend QSet operator|(const QSet &lhs, const QSet &rhs) { return QSet(lhs) |= rhs; }
+    friend QSet operator|(QSet &&lhs, const QSet &rhs) { lhs |= rhs; return std::move(lhs); }
+
+    friend QSet operator&(const QSet &lhs, const QSet &rhs) { return QSet(lhs) &= rhs; }
+    friend QSet operator&(QSet &&lhs, const QSet &rhs) { lhs &= rhs; return std::move(lhs); }
+
+    friend QSet operator+(const QSet &lhs, const QSet &rhs) { return QSet(lhs) += rhs; }
+    friend QSet operator+(QSet &&lhs, const QSet &rhs) { lhs += rhs; return std::move(lhs); }
+
+    friend QSet operator-(const QSet &lhs, const QSet &rhs) { return QSet(lhs) -= rhs; }
+    friend QSet operator-(QSet &&lhs, const QSet &rhs) { lhs -= rhs; return std::move(lhs); }
 
     QList<T> values() const;
 

src/corelib/tools/qset.qdoc

@@ -567,29 +567,30 @@
 */
 
 /*!
-    \fn template <class T> QSet<T> QSet<T>::operator|(const QSet<T> &other) const
-    \fn template <class T> QSet<T> QSet<T>::operator+(const QSet<T> &other) const
+    \fn template <class T> QSet<T> QSet<T>::operator|(const QSet &lhs, const QSet &rhs)
+    \fn template <class T> QSet<T> QSet<T>::operator|(QSet &&lhs, const QSet &rhs)
+    \fn template <class T> QSet<T> QSet<T>::operator+(const QSet &lhs, const QSet &rhs)
+    \fn template <class T> QSet<T> QSet<T>::operator+(QSet &&lhs, const QSet &rhs)
 
-    Returns a new QSet that is the union of this set and the
-    \a other set.
+    Returns a new QSet that is the union of sets \a lhs and \a rhs.
 
     \sa unite(), operator|=(), operator&(), operator-()
 */
 
 /*!
-    \fn template <class T> QSet<T> QSet<T>::operator&(const QSet<T> &other) const
+    \fn template <class T> QSet<T> QSet<T>::operator&(const QSet &lhs, const QSet &rhs)
+    \fn template <class T> QSet<T> QSet<T>::operator&(QSet &&lhs, const QSet &rhs)
 
-    Returns a new QSet that is the intersection of this set and the
-    \a other set.
+    Returns a new QSet that is the intersection of sets \a lhs and \a rhs.
 
     \sa intersect(), operator&=(), operator|(), operator-()
 */
 
 /*!
-    \fn template <class T> QSet<T> QSet<T>::operator-(const QSet<T> &other) const
+    \fn template <class T> QSet<T> QSet<T>::operator-(const QSet &lhs, const QSet &rhs)
+    \fn template <class T> QSet<T> QSet<T>::operator-(QSet &&lhs, const QSet &rhs)
 
-    Returns a new QSet that is the set difference of this set and
-    the \a other set, i.e., this set - \a other set.
+    Returns a new QSet that is the set difference of sets \a lhs and \a rhs.
 
     \sa subtract(), operator-=(), operator|(), operator&()
 */