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>
0deff80eab