QFlatMap: add try_emplace (w/o hint)

QFlatMap, like its public brethren, features the broken Qt-style
insert() behavior (what the STL calls insert_or_assign()), which
makes its insert() unusable for actual STL-style insert() work,
with no replacement except the size-check-and-index-operator trick:

    const auto oldSize = c.size();
    auto &e = c[key];
    if (c.size() != oldSize) {
        // inserted
    }

Even though QFlatMap::insert() appears to return the correct info,
it's useless, because the old value has been assigned over by the
time insert() returns.

Pick-to: 6.3 6.2
Change-Id: If4173c42523a128dfd22ab496dde0089ba73f41c
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

1 files changed, 24 insertions, 0 deletions

diff --git a/src/corelib/tools/qflatmap_p.h b/src/corelib/tools/qflatmap_p.h
index 621eb7adba..d7f8ff48cb 100644
--- a/src/corelib/tools/qflatmap_p.h
+++ b/src/corelib/tools/qflatmap_p.h
@@ -726,6 +726,30 @@ public:
         }
     }
 
+    template <typename...Args>
+    std::pair<iterator, bool> try_emplace(const Key &key, Args&&...args)
+    {
+        auto it = lower_bound(key);
+        if (it == end() || key_compare::operator()(key, it.key())) {
+            c.values.emplace(toValuesIterator(it), std::forward<Args>(args)...);
+            return { fromKeysIterator(c.keys.insert(toKeysIterator(it), key)), true };
+        } else {
+            return {it, false};
+        }
+    }
+
+    template <typename...Args>
+    std::pair<iterator, bool> try_emplace(Key &&key, Args&&...args)
+    {
+        auto it = lower_bound(key);
+        if (it == end() || key_compare::operator()(key, it.key())) {
+            c.values.emplace(toValuesIterator(it), std::forward<Args>(args)...);
+            return { fromKeysIterator(c.keys.insert(toKeysIterator(it), std::move(key))), true };
+        } else {
+            return {it, false};
+        }
+    }
+
     template <class InputIt, is_compatible_iterator<InputIt> = nullptr>
     void insert(InputIt first, InputIt last)
     {