Introduce QHash key iterators

So we can have interoperability with algorithms.
Motivated by inefficient code like qDeleteAll(hash.keys())

[ChangeLog][QtCore][QHash] Added key iterators, accessible through
keyBegin() and keyEnd().

Change-Id: I1f9db8a7a4294e1556cbb50b8fe5ebdcf0dc29a2
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/doc/snippets/code/src_corelib_tools_qhash.cpp

@@ -298,6 +298,25 @@ while (i != hash.end() && i.key() == "plenty") {
 }
 //! [26]
 
+//! [27]
+for (QHash<int, QString>::const_iterator it = hash.cbegin(), end = hash.cend(); it != end; ++it) {
+    cout << "The key: " << it.key() << endl
+    cout << "The value: " << it.value() << endl;
+    cout << "Also the value: " << (*it) << endl;
+}
+//! [27]
+
+//! [28]
+// Inefficient, keys() is expensive
+QList<int> keys = hash.keys();
+int numPrimes = std::count_if(keys.cbegin(), keys.cend(), isPrimeNumber);
+qDeleteAll(hash2.keys());
+
+// Efficient, no memory allocation needed
+int numPrimes = std::count_if(hash.keyBegin(), hash.keyEnd(), isPrimeNumber);
+qDeleteAll(hash2.keyBegin(), hash2.keyEnd());
+//! [28]
+
 //! [qhashbits]
 inline uint qHash(const std::vector<int> &key, uint seed = 0)
 {

src/corelib/tools/qhash.cpp

@@ -1536,6 +1536,15 @@ uint qHash(long double key, uint seed) Q_DECL_NOTHROW
     \sa begin(), constEnd()
 */
 
+/*! \fn QHash::key_iterator QHash::keyBegin() const
+    \since 5.6
+
+    Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first key
+    in the hash.
+
+    \sa keyEnd()
+*/
+
 /*! \fn QHash::iterator QHash::end()
 
     Returns an \l{STL-style iterators}{STL-style iterator} pointing to the imaginary item
@@ -1566,6 +1575,15 @@ uint qHash(long double key, uint seed) Q_DECL_NOTHROW
     \sa cbegin(), end()
 */
 
+/*! \fn QHash::key_iterator QHash::keyEnd() const
+    \since 5.6
+
+    Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary
+    item after the last key in the hash.
+
+    \sa keyBegin()
+*/
+
 /*! \fn QHash::iterator QHash::erase(iterator pos)
 
     Removes the (key, value) pair associated with the iterator \a pos
@@ -1729,6 +1747,26 @@ uint qHash(long double key, uint seed) Q_DECL_NOTHROW
     \internal
 */
 
+/*! \typedef QHash::key_iterator::difference_type
+    \internal
+*/
+
+/*! \typedef QHash::key_iterator::iterator_category
+    \internal
+*/
+
+/*! \typedef QHash::key_iterator::pointer
+    \internal
+*/
+
+/*! \typedef QHash::key_iterator::reference
+    \internal
+*/
+
+/*! \typedef QHash::key_iterator::value_type
+    \internal
+*/
+
 /*! \class QHash::iterator
     \inmodule QtCore
     \brief The QHash::iterator class provides an STL-style non-const iterator for QHash and QMultiHash.
@@ -1802,7 +1840,7 @@ uint qHash(long double key, uint seed) Q_DECL_NOTHROW
     while iterators are active on that container. For more information,
     read \l{Implicit sharing iterator problem}.
 
-    \sa QHash::const_iterator, QMutableHashIterator
+    \sa QHash::const_iterator, QHash::key_iterator, QMutableHashIterator
 */
 
 /*! \fn QHash::iterator::iterator()
@@ -2155,6 +2193,114 @@ uint qHash(long double key, uint seed) Q_DECL_NOTHROW
     \sa operator+=(), operator-()
 */
 
+/*! \class QHash::key_iterator
+    \inmodule QtCore
+    \since 5.6
+    \brief The QHash::key_iterator class provides an STL-style const iterator for QHash and QMultiHash keys.
+
+    QHash::key_iterator is essentially the same as QHash::const_iterator
+    with the difference that operator*() and operator->() return a key
+    instead of a value.
+
+    For most uses QHash::iterator and QHash::const_iterator should be used,
+    you can easily access the key by calling QHash::iterator::key():
+
+    \snippet code/src_corelib_tools_qhash.cpp 27
+
+    However, to have interoperability between QHash's keys and STL-style
+    algorithms we need an iterator that dereferences to a key instead
+    of a value. With QHash::key_iterator we can apply an algorithm to a
+    range of keys without having to call QHash::keys(), which is inefficient
+    as it costs one QHash iteration and memory allocation to create a temporary
+    QList.
+
+    \snippet code/src_corelib_tools_qhash.cpp 28
+
+    QHash::key_iterator is const, it's not possible to modify the key.
+
+    The default QHash::key_iterator constructor creates an uninitialized
+    iterator. You must initialize it using a QHash function like
+    QHash::keyBegin() or QHash::keyEnd().
+
+    \warning Iterators on implicitly shared containers do not work
+    exactly like STL-iterators. You should avoid copying a container
+    while iterators are active on that container. For more information,
+    read \l{Implicit sharing iterator problem}.
+
+    \sa QHash::const_iterator, QHash::iterator
+*/
+
+/*! \fn const T &QHash::key_iterator::operator*() const
+
+    Returns the current item's key.
+*/
+
+/*! \fn const T *QHash::key_iterator::operator->() const
+
+    Returns a pointer to the current item's key.
+*/
+
+/*! \fn bool QHash::key_iterator::operator==(key_iterator other)
+
+    Returns \c true if \a other points to the same item as this
+    iterator; otherwise returns \c false.
+
+    \sa operator!=()
+*/
+
+/*! \fn bool QHash::key_iterator::operator!=(key_iterator other)
+
+    Returns \c true if \a other points to a different item than this
+    iterator; otherwise returns \c false.
+
+    \sa operator==()
+*/
+
+/*!
+    \fn QHash::key_iterator &QHash::key_iterator::operator++()
+
+    The prefix ++ operator (\c{++i}) advances the iterator to the
+    next item in the hash and returns an iterator to the new current
+    item.
+
+    Calling this function on QHash::keyEnd() leads to undefined results.
+
+    \sa operator--()
+*/
+
+/*! \fn QHash::key_iterator QHash::key_iterator::operator++(int)
+
+    \overload
+
+    The postfix ++ operator (\c{i++}) advances the iterator to the
+    next item in the hash and returns an iterator to the previous
+    item.
+*/
+
+/*! \fn QHash::key_iterator &QHash::key_iterator::operator--()
+
+    The prefix -- operator (\c{--i}) makes the preceding item
+    current and returns an iterator pointing to the new current item.
+
+    Calling this function on QHash::keyBegin() leads to undefined
+    results.
+
+    \sa operator++()
+*/
+
+/*! \fn QHash::key_iterator QHash::key_iterator::operator--(int)
+
+    \overload
+
+    The postfix -- operator (\c{i--}) makes the preceding item
+    current and returns an iterator pointing to the previous
+    item.
+*/
+
+/*! \fn const_iterator QHash::key_iterator::base() const
+    Returns the underlying const_iterator this key_iterator is based on.
+*/
+
 /*! \fn QDataStream &operator<<(QDataStream &out, const QHash<Key, T>& hash)
     \relates QHash
 

src/corelib/tools/qhash.h

@@ -413,6 +413,31 @@ public:
     };
     friend class const_iterator;
 
+    class key_iterator
+    {
+        const_iterator i;
+
+    public:
+        typedef typename const_iterator::iterator_category iterator_category;
+        typedef typename const_iterator::difference_type difference_type;
+        typedef Key value_type;
+        typedef const Key *pointer;
+        typedef const Key &reference;
+
+        explicit key_iterator(const_iterator o) : i(o) { }
+
+        const Key &operator*() const { return i.key(); }
+        const Key *operator->() const { return &i.key(); }
+        bool operator==(key_iterator o) const { return i == o.i; }
+        bool operator!=(key_iterator o) const { return i != o.i; }
+
+        inline key_iterator &operator++() { ++i; return *this; }
+        inline key_iterator operator++(int) { return key_iterator(i++);}
+        inline key_iterator &operator--() { --i; return *this; }
+        inline key_iterator operator--(int) { return key_iterator(i--); }
+        const_iterator base() const { return i; }
+    };
+
     // STL style
     inline iterator begin() { detach(); return iterator(d->firstNode()); }
     inline const_iterator begin() const { return const_iterator(d->firstNode()); }
@@ -422,6 +447,9 @@ public:
     inline const_iterator end() const { return const_iterator(e); }
     inline const_iterator cend() const { return const_iterator(e); }
     inline const_iterator constEnd() const { return const_iterator(e); }
+    inline key_iterator keyBegin() const { return key_iterator(begin()); }
+    inline key_iterator keyEnd() const { return key_iterator(end()); }
+
     iterator erase(iterator it);
 
     // more Qt

tests/auto/corelib/tools/qhash/tst_qhash.cpp

@@ -62,6 +62,7 @@ private slots:
     void compare();
     void compare2();
     void iterators(); // sligthly modified from tst_QMap
+    void keyIterator();
     void keys_values_uniqueKeys(); // slightly modified from tst_QMap
     void noNeedlessRehashes();
 
@@ -965,6 +966,34 @@ void tst_QHash::iterators()
     }
 }
 
+void tst_QHash::keyIterator()
+{
+    QHash<int, int> hash;
+
+    for (int i = 0; i < 100; ++i)
+        hash.insert(i, i*100);
+
+    QHash<int, int>::key_iterator key_it = hash.keyBegin();
+    QHash<int, int>::const_iterator it = hash.cbegin();
+    for (int i = 0; i < 100; ++i) {
+        QCOMPARE(*key_it, it.key());
+        key_it++;
+        it++;
+    }
+
+    key_it = std::find(hash.keyBegin(), hash.keyEnd(), 50);
+    it = std::find(hash.cbegin(), hash.cend(), 50 * 100);
+
+    QVERIFY(key_it != hash.keyEnd());
+    QCOMPARE(*key_it, it.key());
+    QCOMPARE(*(key_it++), (it++).key());
+    QCOMPARE(*(key_it--), (it--).key());
+    QCOMPARE(*(++key_it), (++it).key());
+    QCOMPARE(*(--key_it), (--it).key());
+
+    QCOMPARE(std::count(hash.keyBegin(), hash.keyEnd(), 99), 1);
+}
+
 void tst_QHash::rehash_isnt_quadratic()
 {
     // this test should be incredibly slow if rehash() is quadratic