diff --git a/qmake/library/proitems.cpp b/qmake/library/proitems.cpp
index ba817596c50..a8f4dfe0394 100644
--- a/qmake/library/proitems.cpp
+++ b/qmake/library/proitems.cpp
@@ -404,8 +404,7 @@ void ProStringList::removeEmpty()
 
 void ProStringList::removeDuplicates()
 {
-    QDuplicateTracker<ProString> seen;
-    seen.reserve(size());
+    QDuplicateTracker<ProString> seen(size());
     removeIf([&](const ProString &s) { return seen.hasSeen(s); });
 }
 
diff --git a/src/corelib/io/qresource.cpp b/src/corelib/io/qresource.cpp
index 936218608b4..864fb7a5d99 100644
--- a/src/corelib/io/qresource.cpp
+++ b/src/corelib/io/qresource.cpp
@@ -416,8 +416,7 @@ void QResourcePrivate::ensureChildren() const
     QString path = absoluteFilePath, k;
     if (path.startsWith(QLatin1Char(':')))
         path = path.mid(1);
-    QDuplicateTracker<QString> kids;
-    kids.reserve(related.size());
+    QDuplicateTracker<QString> kids(related.size());
     QString cleaned = cleanPath(path);
     for (int i = 0; i < related.size(); ++i) {
         QResourceRoot *res = related.at(i);
diff --git a/src/corelib/text/qstringlist.cpp b/src/corelib/text/qstringlist.cpp
index 440e26bc894..c2eea609ea4 100644
--- a/src/corelib/text/qstringlist.cpp
+++ b/src/corelib/text/qstringlist.cpp
@@ -648,8 +648,7 @@ qsizetype QtPrivate::QStringList_lastIndexOf(const QStringList *that, const QReg
 */
 qsizetype QtPrivate::QStringList_removeDuplicates(QStringList *that)
 {
-    QDuplicateTracker<QString> seen;
-    seen.reserve(that->size());
+    QDuplicateTracker<QString> seen(that->size());
     return that->removeIf([&](const QString &s) { return seen.hasSeen(s); });
 }
 
diff --git a/src/corelib/tools/qduplicatetracker_p.h b/src/corelib/tools/qduplicatetracker_p.h
index 7bc416140ab..565d592ac2c 100644
--- a/src/corelib/tools/qduplicatetracker_p.h
+++ b/src/corelib/tools/qduplicatetracker_p.h
@@ -106,6 +106,14 @@ class QDuplicateTracker {
     Q_DISABLE_COPY_MOVE(QDuplicateTracker);
 public:
     QDuplicateTracker() = default;
+    explicit QDuplicateTracker(qsizetype n)
+#ifdef __cpp_lib_memory_resource
+        : set{size_t(n), &res}
+#else
+        : set{n}
+#endif
+    {}
+    Q_DECL_DEPRECATED_X("Pass the capacity to reserve() to the ctor instead.")
     void reserve(qsizetype n) { set.reserve(n); }
     [[nodiscard]] bool hasSeen(const T &s)
     {
diff --git a/src/gui/text/unix/qfontconfigdatabase.cpp b/src/gui/text/unix/qfontconfigdatabase.cpp
index c9f3c3cb813..ee9d3237476 100644
--- a/src/gui/text/unix/qfontconfigdatabase.cpp
+++ b/src/gui/text/unix/qfontconfigdatabase.cpp
@@ -810,8 +810,7 @@ QStringList QFontconfigDatabase::fallbacksForFamily(const QString &family, QFont
     FcPatternDestroy(pattern);
 
     if (fontSet) {
-        QDuplicateTracker<QString> duplicates;
-        duplicates.reserve(fontSet->nfont + 1);
+        QDuplicateTracker<QString> duplicates(fontSet->nfont + 1);
         (void)duplicates.hasSeen(family.toCaseFolded());
         for (int i = 0; i < fontSet->nfont; i++) {
             FcChar8 *value = nullptr;
diff --git a/src/tools/moc/moc.cpp b/src/tools/moc/moc.cpp
index bce75644f5c..d57e43b59df 100644
--- a/src/tools/moc/moc.cpp
+++ b/src/tools/moc/moc.cpp
@@ -1833,7 +1833,7 @@ void Moc::checkProperties(ClassDef *cdef)
     // specify get function, for compatibiliy we accept functions
     // returning pointers, or const char * for QByteArray.
     //
-    QDuplicateTracker<QByteArray> definedProperties;
+    QDuplicateTracker<QByteArray> definedProperties(cdef->propertyList.count());
     for (int i = 0; i < cdef->propertyList.count(); ++i) {
         PropertyDef &p = cdef->propertyList[i];
         if (definedProperties.hasSeen(p.name)) {
diff --git a/src/widgets/itemviews/qtreewidget.cpp b/src/widgets/itemviews/qtreewidget.cpp
index b684282e9d9..b9af2ce8af1 100644
--- a/src/widgets/itemviews/qtreewidget.cpp
+++ b/src/widgets/itemviews/qtreewidget.cpp
@@ -3101,8 +3101,7 @@ QList<QTreeWidgetItem*> QTreeWidget::selectedItems() const
     const QModelIndexList indexes = selectionModel()->selectedIndexes();
     QList<QTreeWidgetItem*> items;
     items.reserve(indexes.count());
-    QDuplicateTracker<QTreeWidgetItem *> seen;
-    seen.reserve(indexes.count());
+    QDuplicateTracker<QTreeWidgetItem *> seen(indexes.count());
     for (const auto &index : indexes) {
         QTreeWidgetItem *item = d->item(index);
         if (item->isHidden() || seen.hasSeen(item))
diff --git a/src/widgets/styles/qstylesheetstyle.cpp b/src/widgets/styles/qstylesheetstyle.cpp
index 226c1ecbf94..74037eba195 100644
--- a/src/widgets/styles/qstylesheetstyle.cpp
+++ b/src/widgets/styles/qstylesheetstyle.cpp
@@ -2625,8 +2625,7 @@ void QStyleSheetStyle::setProperties(QWidget *w)
 
     {
         // scan decls for final occurrence of each "qproperty"
-        QDuplicateTracker<QString> propertySet;
-        propertySet.reserve(decls.size());
+        QDuplicateTracker<QString> propertySet(decls.size());
         for (int i = decls.count() - 1; i >= 0; --i) {
             const QString property = decls.at(i).d->property;
             if (!property.startsWith(QLatin1String("qproperty-"), Qt::CaseInsensitive))
diff --git a/tests/auto/corelib/tools/qduplicatetracker/tst_qduplicatetracker.cpp b/tests/auto/corelib/tools/qduplicatetracker/tst_qduplicatetracker.cpp
index f4f038ca940..6462ef0dd77 100644
--- a/tests/auto/corelib/tools/qduplicatetracker/tst_qduplicatetracker.cpp
+++ b/tests/auto/corelib/tools/qduplicatetracker/tst_qduplicatetracker.cpp
@@ -168,8 +168,7 @@ size_t qHash(const ConstructionCounted &c, std::size_t seed = 0)
 
 void tst_QDuplicateTracker::appendTo_special()
 {
-    QDuplicateTracker<ConstructionCounted> tracker;
-    tracker.reserve(3);
+    QDuplicateTracker<ConstructionCounted> tracker(3);
     QVERIFY(!tracker.hasSeen(1));
     QVERIFY(!tracker.hasSeen(2));
     QVERIFY(!tracker.hasSeen(3));