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, 19 insertions, 0 deletions

diff --git a/src/corelib/tools/qmap.qdoc b/src/corelib/tools/qmap.qdoc
index 55c2bcb5d9..eeb6b0f939 100644
--- a/src/corelib/tools/qmap.qdoc
+++ b/src/corelib/tools/qmap.qdoc
@@ -663,6 +663,25 @@
     \sa constKeyValueBegin()
 */
 
+/*! \fn template <class Key, class T> auto QMap<Key, T>::asKeyValueRange() &
+    \fn template <class Key, class T> auto QMap<Key, T>::asKeyValueRange() const &
+    \fn template <class Key, class T> auto QMap<Key, T>::asKeyValueRange() &&
+    \fn template <class Key, class T> auto QMap<Key, T>::asKeyValueRange() const &&
+    \since 6.4
+
+    Returns a range object that allows iteration over this map as
+    key/value pairs. For instance, this range object can be used in a
+    range-based for loop, in combination with a structured binding declaration:
+
+    \snippet code/src_corelib_tools_qmap.cpp 28
+
+    Note that both the key and the value obtained this way are
+    references to the ones in the map. Specifically, mutating the value
+    will modify the map itself.
+
+    \sa QKeyValueIterator
+*/
+
 /*! \fn template <class Key, class T> QMap<Key, T>::iterator QMap<Key, T>::erase(const_iterator pos)
 
     Removes the (key, value) pair pointed to by the iterator \a pos