Add qHashBits(), a hash function for a memory block

The function arguments have been chosen to avoid caller-side casting of
argument types, when passing object addresses and type sizeof()s.

[ChangeLog][QtCore] Added qHashBits() to aid implementing qHash() overloads
for custom types.

Change-Id: I983a8560769bb27e489f23ebb6db51850ddd65f2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

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

@@ -297,3 +297,13 @@ while (i != hash.end() && i.key() == "plenty") {
     ++i;
 }
 //! [26]
+
+//! [qhashbits]
+inline uint qHash(const std::vector<int> &key, uint seed = 0)
+{
+    if (key.empty())
+        return seed;
+    else
+        return qHashBits(&key.front(), key.size() * sizeof(int), seed);
+}
+//! [qhashbits]

src/corelib/tools/qhash.cpp

@@ -163,6 +163,11 @@ static inline uint hash(const uchar *p, int len, uint seed) Q_DECL_NOTHROW
     return h;
 }
 
+uint qHashBits(const void *p, size_t len, uint seed) Q_DECL_NOTHROW
+{
+    return hash(static_cast<const uchar*>(p), int(len), seed);
+}
+
 static inline uint hash(const QChar *p, int len, uint seed) Q_DECL_NOTHROW
 {
     uint h = seed;
@@ -669,6 +674,25 @@ void QHashData::checkSanity()
     Types \c T1 and \c T2 must be supported by qHash().
 */
 
+/*! \fn uint qHashBits(const void *p, size_t len, uint seed = 0)
+    \relates QHash
+    \since 5.4
+
+    Returns the hash value for the memory block of size \a len pointed
+    to by \a p, using \a seed to seed the calculation.
+
+    Use this function only to implement qHash() for your own custom
+    types. E.g., here's how you could implement a qHash() overload for
+    std::vector<int>:
+
+    \snippet code/src_corelib_tools_qhash.cpp qhashbits
+
+    It bears repeating that the implementation of qHashBits() - like
+    the qHash() overloads offered by Qt - may change at any time. You
+    \b{must not} rely on the fact that qHashBits() will give the same
+    results (for the same inputs) across different Qt versions.
+*/
+
 /*! \fn uint qHash(char key, uint seed = 0)
     \relates QHash
     \since 5.0

src/corelib/tools/qhash.h

@@ -60,6 +60,8 @@ class QString;
 class QStringRef;
 class QLatin1String;
 
+Q_CORE_EXPORT uint qHashBits(const void *p, size_t size, uint seed = 0) Q_DECL_NOTHROW;
+
 inline uint qHash(char key, uint seed = 0) Q_DECL_NOTHROW { return uint(key) ^ seed; }
 inline uint qHash(uchar key, uint seed = 0) Q_DECL_NOTHROW { return uint(key) ^ seed; }
 inline uint qHash(signed char key, uint seed = 0) Q_DECL_NOTHROW { return uint(key) ^ seed; }