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>

3 files changed, 52 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 ee9f88fc65..844c5f1b7c 100644
--- a/src/corelib/doc/snippets/code/src_corelib_tools_qhash.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_tools_qhash.cpp
@@ -360,3 +360,29 @@ template <> struct hash<K>
 };
 }
 //! [33]
+
+//! [34]
+QHash<QString, int> hash;
+hash.insert("January", 1);
+hash.insert("February", 2);
+// ...
+hash.insert("December", 12);
+
+for (auto [key, value] : hash.asKeyValueRange()) {
+    cout << key << ": " << value << Qt::endl;
+    --value; // convert to JS month indexing
+}
+//! [34]
+
+//! [35]
+QMultiHash<QString, int> hash;
+hash.insert("January", 1);
+hash.insert("February", 2);
+// ...
+hash.insert("December", 12);
+
+for (auto [key, value] : hash.asKeyValueRange()) {
+    cout << key << ": " << value << Qt::endl;
+    --value; // convert to JS month indexing
+}
+//! [35]
diff --git a/src/corelib/doc/snippets/code/src_corelib_tools_qmap.cpp b/src/corelib/doc/snippets/code/src_corelib_tools_qmap.cpp
index 145cd10d84..a0e280e759 100644
--- a/src/corelib/doc/snippets/code/src_corelib_tools_qmap.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_tools_qmap.cpp
@@ -340,3 +340,16 @@ qDeleteAll(map2.keys());
 int numPrimes = std::count_if(map.keyBegin(), map.keyEnd(), isPrimeNumber);
 qDeleteAll(map2.keyBegin(), map2.keyEnd());
 //! [keyiterator2]
+
+//! [28]
+QMap<QString, int> map;
+map.insert("January", 1);
+map.insert("February", 2);
+// ...
+map.insert("December", 12);
+
+for (auto [key, value] : map.asKeyValueRange()) {
+    cout << key << ": " << value << Qt::endl;
+    --value; // convert to JS month indexing
+}
+//! [28]
diff --git a/src/corelib/doc/snippets/code/src_corelib_tools_qmultimap.cpp b/src/corelib/doc/snippets/code/src_corelib_tools_qmultimap.cpp
index 9c2a01834b..bd188c3fb4 100644
--- a/src/corelib/doc/snippets/code/src_corelib_tools_qmultimap.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_tools_qmultimap.cpp
@@ -322,3 +322,16 @@ qDeleteAll(multimap2.keys());
 int numPrimes = std::count_if(multimap.keyBegin(), multimap.keyEnd(), isPrimeNumber);
 qDeleteAll(multimap2.keyBegin(), multimap2.keyEnd());
 //! [keyiterator2]
+
+//! [26]
+QMultiMap<QString, int> map;
+map.insert("January", 1);
+map.insert("February", 2);
+// ...
+map.insert("December", 12);
+
+for (auto [key, value] : map.asKeyValueRange()) {
+    cout << key << ": " << value << Qt::endl;
+    --value; // convert to JS month indexing
+}
+//! [26]