Associative containers: add a way to obtain a key/value range

Our associative containers' iterator's value_type isn't a destructurable
type (yielding key/value). This means that something like

  for (auto [k, v] : map)

doesn't even compile -- one can only "directly" iterate on the
values. For quite some time we've had QKeyValueIterator to allow
key/value iteration, but then one had to resort to a "traditional" for
loop:

  for (auto i = map.keyValueBegin(), e = keyValueEnd(); i!=e; ++i)

This can be easily packaged in an adaptor class, which is what this
commmit does, thereby offering a C++17-compatible way to obtain
key/value iteration over associative containers.

Something possibly peculiar is the fact that the range so obtained is
a range of pairs of references -- not a range of references to pairs.
But that's easily explained by the fact that we have no pairs to build
references to; hence,

 for (auto &[k, v] : map.asKeyValueRange())

doesn't compile (lvalue reference doesn't bind to prvalue pair).
Instead, both of these compile:

  for (auto [k, v] : map.asKeyValueRange())
  for (auto &&[k, v] : map.asKeyValueRange())

and in *both* cases one gets references to the keys/values in the map.
If the map is non-const, the reference to the value is mutable.

Last but not least, implement pinning for rvalue containers.

[ChangeLog][QtCore][QMap] Added asKeyValueRange().
[ChangeLog][QtCore][QMultiMap] Added asKeyValueRange().
[ChangeLog][QtCore][QHash] Added asKeyValueRange().
[ChangeLog][QtCore][QMultiHash] Added asKeyValueRange().

Task-number: QTBUG-4615
Change-Id: Ic8506bff38b2f753494b21ab76f52e05c06ffc8b
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 41 insertions, 0 deletions

diff --git a/src/corelib/tools/qiterator.h b/src/corelib/tools/qiterator.h
index 8922f34758..559023d493 100644
--- a/src/corelib/tools/qiterator.h
+++ b/src/corelib/tools/qiterator.h
@@ -301,6 +301,47 @@ private:
     Iterator i;
 };
 
+namespace QtPrivate {
+
+template <typename Map>
+class QKeyValueRangeStorage
+{
+protected:
+    Map m_map;
+public:
+    explicit QKeyValueRangeStorage(const Map &map) : m_map(map) {}
+    explicit QKeyValueRangeStorage(Map &&map) : m_map(std::move(map)) {}
+};
+
+template <typename Map>
+class QKeyValueRangeStorage<Map &>
+{
+protected:
+    Map &m_map;
+public:
+    explicit QKeyValueRangeStorage(Map &map) : m_map(map) {}
+};
+
+template <typename Map>
+class QKeyValueRange : public QKeyValueRangeStorage<Map>
+{
+public:
+    using QKeyValueRangeStorage<Map>::QKeyValueRangeStorage;
+    auto begin() { return this->m_map.keyValueBegin(); }
+    auto begin() const { return this->m_map.keyValueBegin(); }
+    auto end() { return this->m_map.keyValueEnd(); }
+    auto end() const { return this->m_map.keyValueEnd(); }
+};
+
+template <typename Map>
+QKeyValueRange(Map &) -> QKeyValueRange<Map &>;
+
+template <typename Map, std::enable_if_t<!std::is_reference_v<Map>, bool> = false>
+QKeyValueRange(Map &&) -> QKeyValueRange<std::remove_const_t<Map>>;
+
+} // namespace QtPrivate
+
+
 QT_END_NAMESPACE
 
 #endif // QITERATOR_H