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, 8 insertions, 0 deletions

diff --git a/src/corelib/tools/qmap.h b/src/corelib/tools/qmap.h
index 5daf24189b..672d01791a 100644
--- a/src/corelib/tools/qmap.h
+++ b/src/corelib/tools/qmap.h
@@ -646,6 +646,10 @@ public:
     const_key_value_iterator constKeyValueBegin() const { return const_key_value_iterator(begin()); }
     const_key_value_iterator keyValueEnd() const { return const_key_value_iterator(end()); }
     const_key_value_iterator constKeyValueEnd() const { return const_key_value_iterator(end()); }
+    auto asKeyValueRange() & { return QtPrivate::QKeyValueRange(*this); }
+    auto asKeyValueRange() const & { return QtPrivate::QKeyValueRange(*this); }
+    auto asKeyValueRange() && { return QtPrivate::QKeyValueRange(std::move(*this)); }
+    auto asKeyValueRange() const && { return QtPrivate::QKeyValueRange(std::move(*this)); }
 
     iterator erase(const_iterator it)
     {
@@ -1341,6 +1345,10 @@ public:
     const_key_value_iterator constKeyValueBegin() const { return const_key_value_iterator(begin()); }
     const_key_value_iterator keyValueEnd() const { return const_key_value_iterator(end()); }
     const_key_value_iterator constKeyValueEnd() const { return const_key_value_iterator(end()); }
+    auto asKeyValueRange() & { return QtPrivate::QKeyValueRange(*this); }
+    auto asKeyValueRange() const & { return QtPrivate::QKeyValueRange(*this); }
+    auto asKeyValueRange() && { return QtPrivate::QKeyValueRange(std::move(*this)); }
+    auto asKeyValueRange() const && { return QtPrivate::QKeyValueRange(std::move(*this)); }
 
     iterator erase(const_iterator it)
     {