QMap: use std::less for defining an order between pointers

Reinterpret_cast()ing a pointer to a suitably sized integer is not guaranteed
to always give the same result for the same pointer (!). Therefore the
resulting integers are not comparable in a meaningful way. std::less is
supposed to be used to compare arbitrary pointers, so use it.

(Hopefully and reasonably, under the hood std::less does exactly what we
were doing, so this isn't BiC.)

Change-Id: I9960b3d6e35657fe7a25b842054f5d338280e850
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qmap.cpp

@@ -399,7 +399,9 @@ void QMapDataBase::freeData(QMapDataBase *d)
        With QMap, the items are always sorted by key.
     \li The key type of a QHash must provide operator==() and a global
        qHash(Key) function. The key type of a QMap must provide
-       operator<() specifying a total order.
+       operator<() specifying a total order. Since Qt 5.8.1 it is also safe
+       to use a pointer type as key, even if the underlying operator<()
+       does not provide a total order.
     \endlist
 
     Here's an example QMap with QString keys and \c int values:

src/corelib/tools/qmap.h

@@ -51,6 +51,7 @@
 
 #include <map>
 #include <new>
+#include <functional>
 
 #ifdef Q_COMPILER_INITIALIZER_LISTS
 #include <initializer_list>
@@ -61,11 +62,8 @@ QT_BEGIN_NAMESPACE
 /*
     QMap uses qMapLessThanKey() to compare keys. The default
     implementation uses operator<(). For pointer types,
-    qMapLessThanKey() casts the pointers to integers before it
-    compares them, because operator<() is undefined on pointers
-    that come from different memory blocks. (In practice, this
-    is only a problem when running a program such as
-    BoundsChecker.)
+    qMapLessThanKey() uses std::less (because operator<() on
+    pointers can be used only between pointers in the same array).
 */
 
 template <class Key> inline bool qMapLessThanKey(const Key &key1, const Key &key2)
@@ -75,8 +73,7 @@ template <class Key> inline bool qMapLessThanKey(const Key &key1, const Key &key
 
 template <class Ptr> inline bool qMapLessThanKey(const Ptr *key1, const Ptr *key2)
 {
-    Q_STATIC_ASSERT(sizeof(quintptr) == sizeof(const Ptr *));
-    return quintptr(key1) < quintptr(key2);
+    return std::less<const Ptr *>()(key1, key2);
 }
 
 struct QMapDataBase;