Add qHash(QHash) and qHash(QMultiHash)

The hash function is carefully designed to give the same result
as the straight-forward implementation of qHash(unordered_map),
which we'll probably add at some point, namely: std::accumulate
over a container of std::pair.

This is one reason to use std:: and not QPair in the implemen-
tation of qHash(QHash). The other is that qHash(QPair) uses a
bad hash combiner, which may xor out the 'seed' from the result.

We can't fix that until Qt 6, but the qHash(std::pair) overload
uses the well-known boost::hash_combine algorithm (implemented
in Qt as QtPrivate::QHashCombine), so we can use that.

I also trust std::pair to work without problems with reference
template arguments, while QPair only very recently gained a very
basic auto-test for reference parameters.

[ChangeLog][QtCore] Added qHash() overloads for QHash, QMultiHash.

Change-Id: I90879d8a99cf1aadb6e84ecc0c3704f52f3691da
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 41 insertions, 0 deletions

diff --git a/src/corelib/tools/qhash.cpp b/src/corelib/tools/qhash.cpp
index 593a87e65d..abec9ebb79 100644
--- a/src/corelib/tools/qhash.cpp
+++ b/src/corelib/tools/qhash.cpp
@@ -2644,4 +2644,24 @@ uint qHash(long double key, uint seed) Q_DECL_NOTHROW
     \sa QHash::constFind()
 */
 
+/*!
+    \fn uint qHash(const QHash<Key, T> &key, uint seed = 0)
+    \since 5.8
+    \relates QHash
+
+    Returns the hash value for the \a key, using \a seed to seed the calculation.
+
+    Type \c T must be supported by qHash().
+*/
+
+/*!
+    \fn uint qHash(const QMultiHash<Key, T> &key, uint seed = 0)
+    \since 5.8
+    \relates QMultiHash
+
+    Returns the hash value for the \a key, using \a seed to seed the calculation.
+
+    Type \c T must be supported by qHash().
+*/
+
 QT_END_NAMESPACE
diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index 7abbeabeae..6a2d7bdd11 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -1095,6 +1095,27 @@ Q_INLINE_TEMPLATE int QMultiHash<Key, T>::count(const Key &key, const T &value)
 Q_DECLARE_ASSOCIATIVE_ITERATOR(Hash)
 Q_DECLARE_MUTABLE_ASSOCIATIVE_ITERATOR(Hash)
 
+template <class Key, class T>
+uint qHash(const QHash<Key, T> &key, uint seed = 0)
+    Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(std::declval<Key&>())) && noexcept(qHash(std::declval<T&>())))
+{
+    QtPrivate::QHashCombineCommutative hash;
+    for (auto it = key.begin(), end = key.end(); it != end; ++it) {
+        const Key &k = it.key();
+        const T   &v = it.value();
+        seed = hash(seed, std::pair<const Key&, const T&>(k, v));
+    }
+    return seed;
+}
+
+template <class Key, class T>
+inline uint qHash(const QMultiHash<Key, T> &key, uint seed = 0)
+    Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(std::declval<Key&>())) && noexcept(qHash(std::declval<T&>())))
+{
+    const QHash<Key, T> &key2 = key;
+    return qHash(key2, seed);
+}
+
 QT_END_NAMESPACE
 
 #if defined(Q_CC_MSVC)