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>

2 files changed, 107 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)
     {
diff --git a/tests/auto/corelib/tools/qflatmap/tst_qflatmap.cpp b/tests/auto/corelib/tools/qflatmap/tst_qflatmap.cpp
index f8f48d7bfb..af616f0a4c 100644
--- a/tests/auto/corelib/tools/qflatmap/tst_qflatmap.cpp
+++ b/tests/auto/corelib/tools/qflatmap/tst_qflatmap.cpp
@@ -26,6 +26,8 @@
 **
 ****************************************************************************/
 
+#define QT_USE_QSTRINGBUILDER
+
 #include <QTest>
 
 #include <private/qflatmap_p.h>
@@ -51,6 +53,7 @@ private slots:
     void iterators();
     void statefulComparator();
     void transparency();
+    void try_emplace();
     void viewIterators();
     void varLengthArray();
 };
@@ -425,6 +428,86 @@ void tst_QFlatMap::transparency()
     QCOMPARE(m.lower_bound(sv3).value(), "dree");
 }
 
+void tst_QFlatMap::try_emplace()
+{
+    using Map = QFlatMap<QByteArray, QByteArray>;
+
+    const QByteArray foo = QByteArrayLiteral("foo");
+    const qsizetype qqq_1 = 3;
+    const char qqq_2 = 'q';
+    const QByteArray qqq = QByteArray(qqq_1, qqq_2);
+
+    auto sb = [] (const auto &str) { return str % ""; };
+    auto rvalue = [](const auto &x) { return x; };
+#define lvalue(x) x
+#define CHECKS() \
+    do { \
+        QVERIFY(!m.try_emplace(rvalue(foo), lvalue(foo)).second); \
+        QCOMPARE(m.value(foo), qqq); \
+        QVERIFY(!m.try_emplace(lvalue(foo), lvalue(foo)).second); \
+        QCOMPARE(m.value(foo), qqq); \
+        QVERIFY(!m.try_emplace(lvalue(foo), sb(foo)).second); \
+        QCOMPARE(m.value(foo), qqq); \
+        QVERIFY(!m.try_emplace(rvalue(foo), sb(foo)).second); \
+        QCOMPARE(m.value(foo), qqq); \
+    } while (0) \
+    /* end */
+
+    {
+        Map m;
+        QVERIFY(m.try_emplace(lvalue(foo), lvalue(qqq)).second);
+        CHECKS();
+    }
+
+    {
+        Map m;
+        QVERIFY(m.try_emplace(lvalue(foo), rvalue(qqq)).second);
+        CHECKS();
+    }
+
+    {
+        Map m;
+        QVERIFY(m.try_emplace(lvalue(foo), qqq_1, qqq_2).second);
+        QCOMPARE(m.value(foo), qqq);
+        CHECKS();
+    }
+
+    {
+        Map m;
+        QVERIFY(m.try_emplace(lvalue(foo), sb(qqq)).second);
+        QCOMPARE(m.value(foo), qqq);
+        CHECKS();
+    }
+
+    {
+        Map m;
+        QVERIFY(m.try_emplace(rvalue(foo), lvalue(qqq)).second);
+        CHECKS();
+    }
+
+    {
+        Map m;
+        QVERIFY(m.try_emplace(rvalue(foo), rvalue(qqq)).second);
+        CHECKS();
+    }
+
+    {
+        Map m;
+        QVERIFY(m.try_emplace(rvalue(foo), qqq_1, qqq_2).second);
+        QCOMPARE(m.value(foo), qqq);
+        CHECKS();
+    }
+
+    {
+        Map m;
+        QVERIFY(m.try_emplace(rvalue(foo), sb(qqq)).second);
+        QCOMPARE(m.value(foo), qqq);
+        CHECKS();
+    }
+#undef CHECKS
+#undef lvalue
+}
+
 void tst_QFlatMap::viewIterators()
 {
     using Map = QFlatMap<QByteArray, QByteArray>;