QList: Introduce constFirst() and constEnd()

Allows to easily prevent detaching in common code like: getList().first()

[ChangeLog][QtCore][QList] Added the convenience constFirst and constLast functions.

Task-number: QTBUG-46026
Change-Id: I51ecb51fe91fc7d993ad35b5c7392f4da88e5f7b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

3 files changed, 160 insertions, 2 deletions

diff --git a/src/corelib/tools/qlist.cpp b/src/corelib/tools/qlist.cpp
index f4901d336e..d774548d3a 100644
--- a/src/corelib/tools/qlist.cpp
+++ b/src/corelib/tools/qlist.cpp
@@ -1227,7 +1227,7 @@ void **QListData::erase(void **xi)
     not be empty. If the list can be empty, call isEmpty() before
     calling this function.
 
-    \sa last(), isEmpty()
+    \sa constFirst(), last(), isEmpty()
 */
 
 /*! \fn const T& QList::first() const
@@ -1235,13 +1235,23 @@ void **QListData::erase(void **xi)
     \overload
 */
 
+/*! \fn const T& QList::constFirst() const
+    \since 5.6
+
+    Returns a const reference to the first item in the list. The list must
+    not be empty. If the list can be empty, call isEmpty() before
+    calling this function.
+
+    \sa constLast(), isEmpty(), first()
+*/
+
 /*! \fn T& QList::last()
 
     Returns a reference to the last item in the list.  The list must
     not be empty. If the list can be empty, call isEmpty() before
     calling this function.
 
-    \sa first(), isEmpty()
+    \sa constLast(), first(), isEmpty()
 */
 
 /*! \fn const T& QList::last() const
@@ -1249,6 +1259,16 @@ void **QListData::erase(void **xi)
     \overload
 */
 
+/*! \fn const T& QList::constLast() const
+    \since 5.6
+
+    Returns a reference to the last item in the list. The list must
+    not be empty. If the list can be empty, call isEmpty() before
+    calling this function.
+
+    \sa constFirst(), isEmpty(), last()
+*/
+
 /*! \fn void QList::removeFirst()
 
     Removes the first item in the list. Calling this function is
diff --git a/src/corelib/tools/qlist.h b/src/corelib/tools/qlist.h
index e446a6625b..ac46dc3244 100644
--- a/src/corelib/tools/qlist.h
+++ b/src/corelib/tools/qlist.h
@@ -331,9 +331,11 @@ public:
     inline int count() const { return p.size(); }
     inline int length() const { return p.size(); } // Same as count()
     inline T& first() { Q_ASSERT(!isEmpty()); return *begin(); }
+    inline const T& constFirst() const { return first(); }
     inline const T& first() const { Q_ASSERT(!isEmpty()); return at(0); }
     T& last() { Q_ASSERT(!isEmpty()); return *(--end()); }
     const T& last() const { Q_ASSERT(!isEmpty()); return at(count() - 1); }
+    inline const T& constLast() const { return last(); }
     inline void removeFirst() { Q_ASSERT(!isEmpty()); erase(begin()); }
     inline void removeLast() { Q_ASSERT(!isEmpty()); erase(--end()); }
     inline bool startsWith(const T &t) const { return !isEmpty() && first() == t; }
diff --git a/tests/auto/corelib/tools/qlist/tst_qlist.cpp b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
index dd9371fbe1..9ae69c2f61 100644
--- a/tests/auto/corelib/tools/qlist/tst_qlist.cpp
+++ b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
@@ -313,6 +313,8 @@ private slots:
     void lastOptimal() const;
     void lastMovable() const;
     void lastComplex() const;
+    void constFirst() const;
+    void constLast() const;
     void beginOptimal() const;
     void beginMovable() const;
     void beginComplex() const;
@@ -729,6 +731,140 @@ void tst_QList::firstComplex() const
     QCOMPARE(liveCount, Complex::getLiveCount());
 }
 
+void tst_QList::constFirst() const
+{
+    // Based on tst_QVector::constFirst()
+    QList<int> list;
+    list << 69 << 42 << 3;
+
+    // test it starts ok
+    QCOMPARE(list.constFirst(), 69);
+    QVERIFY(list.isDetached());
+
+    QList<int> listCopy = list;
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    QCOMPARE(list.constFirst(), 69);
+    QCOMPARE(listCopy.constFirst(), 69);
+
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    // test removal changes
+    list.removeAt(0);
+    QVERIFY(list.isDetached());
+    QVERIFY(!list.isSharedWith(listCopy));
+    QCOMPARE(list.constFirst(), 42);
+    QCOMPARE(listCopy.constFirst(), 69);
+
+    listCopy = list;
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    QCOMPARE(list.constFirst(), 42);
+    QCOMPARE(listCopy.constFirst(), 42);
+
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    // test prepend changes
+    list.prepend(23);
+    QVERIFY(list.isDetached());
+    QVERIFY(!list.isSharedWith(listCopy));
+    QCOMPARE(list.constFirst(), 23);
+    QCOMPARE(listCopy.constFirst(), 42);
+
+    listCopy = list;
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    QCOMPARE(list.constFirst(), 23);
+    QCOMPARE(listCopy.constFirst(), 23);
+
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+}
+
+void tst_QList::constLast() const
+{
+    // Based on tst_QVector::constLast()
+    QList<int> list;
+    list << 69 << 42 << 3;
+
+    // test it starts ok
+    QCOMPARE(list.constLast(), 3);
+    QVERIFY(list.isDetached());
+
+    QList<int> listCopy = list;
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    QCOMPARE(list.constLast(), 3);
+    QCOMPARE(listCopy.constLast(), 3);
+
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    // test removal changes
+    list.removeLast();
+    QVERIFY(list.isDetached());
+    QVERIFY(!list.isSharedWith(listCopy));
+    QCOMPARE(list.constLast(), 42);
+    QCOMPARE(listCopy.constLast(), 3);
+
+    listCopy = list;
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    QCOMPARE(list.constLast(), 42);
+    QCOMPARE(listCopy.constLast(), 42);
+
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    // test prepend changes
+    list.append(23);
+    QVERIFY(list.isDetached());
+    QVERIFY(!list.isSharedWith(listCopy));
+    QCOMPARE(list.constLast(), 23);
+    QCOMPARE(listCopy.constLast(), 42);
+
+    listCopy = list;
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+
+    QCOMPARE(list.constLast(), 23);
+    QCOMPARE(listCopy.constLast(), 23);
+
+    QVERIFY(!list.isDetached());
+    QVERIFY(!listCopy.isDetached());
+    QVERIFY(list.isSharedWith(listCopy));
+    QVERIFY(listCopy.isSharedWith(list));
+}
+
 template<typename T>
 void tst_QList::last() const
 {