Add a qHashEquals() method and use it to compare keys in QHash

In some cases, the default equality operator for a class is not suitable
for using in hashing (for example because it uses fuzzy comparisons).

Add a qHashEquals() method that by default uses the equality operator, but
allows to tailor the operations that should be used when using the class
as a key in QHash.

Pick-to: 6.0 dev
Task-number: QTBUG-88966
Change-Id: I346cf0e6e923277a8b42a79e50342a1c2511fd80
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

3 files changed, 22 insertions, 1 deletions

diff --git a/src/corelib/tools/qhash.cpp b/src/corelib/tools/qhash.cpp
index 112a492526..1087f21564 100644
--- a/src/corelib/tools/qhash.cpp
+++ b/src/corelib/tools/qhash.cpp
@@ -1074,6 +1074,21 @@ size_t qHash(long double key, size_t seed) noexcept
     Returns the hash value for the \a key, using \a seed to seed the calculation.
 */
 
+/*! \fn template<typename T> bool qHashEquals(const T &a, const T &b)
+    \relates QHash
+    \since 6.0
+    \internal
+
+    This method is being used by QHash to compare two keys. Returns true if the
+    keys \a a and \a b are considered equal for hashing purposes.
+
+    The default implementation returns the result of (a == b). It can be reimplemented
+    for a certain type if the equality operator is not suitable for hashing purposes.
+    This is for example the case if the equality operator uses qFuzzyCompare to compare
+    floating point values.
+*/
+
+
 /*!
     \class QHash
     \inmodule QtCore
diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index 5dc88943fc..70e98bed60 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -560,7 +560,7 @@ struct Data
                 return iterator{ this, bucket };
             } else {
                 Node &n = s.atOffset(offset);
-                if (n.key == key)
+                if (qHashEquals(n.key, key))
                     return iterator{ this, bucket };
             }
             bucket = nextBucket(bucket);
diff --git a/src/corelib/tools/qhashfunctions.h b/src/corelib/tools/qhashfunctions.h
index 9cfa852ba2..a8b5e0f7bc 100644
--- a/src/corelib/tools/qhashfunctions.h
+++ b/src/corelib/tools/qhashfunctions.h
@@ -170,6 +170,12 @@ template<typename T> inline size_t qHash(const T &t, size_t seed)
     noexcept(noexcept(qHash(t)))
 { return qHash(t) ^ seed; }
 
+template<typename T>
+bool qHashEquals(const T &a, const T &b)
+{
+    return a == b;
+}
+
 namespace QtPrivate {
 
 struct QHashCombine