QHash: support std::hash as hashing function

In addition (and as a fallback) from requiring qHash, add support
for std::hash specializations. This catches two birds with one stone:

1) users of Qt can simply specialize std::hash for their datatypes,
and use them in both QHash and stdlib unordered associative containers;

2) we get QHash support for any (stdlib) datatype that is hashable
without having to overload qHash for them.

[ChangeLog][QtCore][QHash] QHash, QMultiHash and QSet now support
for key types anything that can be hashed via std::hash, instead of
always requiring a qHash() overload.

Change-Id: Ib5ecba86e4b376d318389500bd24883ac6534c5f
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/corelib/doc/snippets/code/src_corelib_tools_qhash.cpp b/src/corelib/doc/snippets/code/src_corelib_tools_qhash.cpp
index 77981075a2..ee9f88fc65 100644
--- a/src/corelib/doc/snippets/code/src_corelib_tools_qhash.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_tools_qhash.cpp
@@ -350,3 +350,13 @@ size_t qHash(const K &key);
 size_t qHash(K key, size_t seed);
 size_t qHash(const K &key, size_t seed);
 //! [32]
+
+//! [33]
+namespace std {
+template <> struct hash<K>
+{
+    // seed is optional
+    size_t operator()(const K &key, size_t seed = 0) const;
+};
+}
+//! [33]