QVector: preserve capacity in clear()

This is what std::vector implementations usually do,
because it minimizes memory fragmentation and useless
allocations since no user will call clear() unless
she intends to append new data afterwards.

Fix calls to resize(0) that show how existing code
tried to work around the issue.

Adjust test. Port from QVERIFY(==) to QCOMPARE as a
drive-by.

[ChangeLog][QtCore][QVector] clear() now preserves
capacity. To shed capacity, call squeeze() or swap
with a default-constructed QVector object, see the
documentation for an example.

Change-Id: I9cebe611a97e027a89e821e64408a4741b31f1f6
Reviewed-by: Lars Knoll <[email protected]>

12 files changed, 31 insertions, 21 deletions

diff --git a/src/corelib/kernel/qeventdispatcher_unix.cpp b/src/corelib/kernel/qeventdispatcher_unix.cpp
index 64b813bb133..9afbb84abfc 100644
--- a/src/corelib/kernel/qeventdispatcher_unix.cpp
+++ b/src/corelib/kernel/qeventdispatcher_unix.cpp
@@ -286,7 +286,7 @@ void QEventDispatcherUNIXPrivate::markPendingSocketNotifiers()
         }
     }
 
-    pollfds.resize(0);
+    pollfds.clear();
 }
 
 int QEventDispatcherUNIXPrivate::activateSocketNotifiers()
@@ -480,8 +480,8 @@ bool QEventDispatcherUNIX::processEvents(QEventLoop::ProcessEventsFlags flags)
     if (!canWait || (include_timers && d->timerList.timerWait(wait_tm)))
         tm = &wait_tm;
 
+    d->pollfds.clear();
     d->pollfds.reserve(1 + (include_notifiers ? d->socketNotifiers.size() : 0));
-    d->pollfds.resize(0);
 
     if (include_notifiers)
         for (auto it = d->socketNotifiers.cbegin(); it != d->socketNotifiers.cend(); ++it)
diff --git a/src/corelib/tools/qregexp.cpp b/src/corelib/tools/qregexp.cpp
index fd671933020..f8f3347786b 100644
--- a/src/corelib/tools/qregexp.cpp
+++ b/src/corelib/tools/qregexp.cpp
@@ -2335,7 +2335,7 @@ QRegExpCharClass::QRegExpCharClass()
 void QRegExpCharClass::clear()
 {
     c = 0;
-    r.resize(0);
+    r.clear();
     n = false;
 }
 
diff --git a/src/corelib/tools/qvector.cpp b/src/corelib/tools/qvector.cpp
index b3247b8af52..ef1c9c17b04 100644
--- a/src/corelib/tools/qvector.cpp
+++ b/src/corelib/tools/qvector.cpp
@@ -502,8 +502,19 @@
 
 /*! \fn void QVector::clear()
 
-    Removes all the elements from the vector and releases the memory used by
-    the vector.
+    Removes all the elements from the vector.
+
+    \note Until Qt 5.6, this also released the memory used by
+    the vector. From Qt 5.7, the capacity is preserved. To shed
+    all capacity, swap with a default-constructed vector:
+    \code
+    QVector<T> v ...;
+    QVector<T>().swap(v);
+    Q_ASSERT(v.capacity() == 0);
+    \endcode
+    or call squeeze().
+
+    \sa squeeze()
 */
 
 /*! \fn const T &QVector::at(int i) const
diff --git a/src/corelib/tools/qvector.h b/src/corelib/tools/qvector.h
index d60be78ade3..13ae121450f 100644
--- a/src/corelib/tools/qvector.h
+++ b/src/corelib/tools/qvector.h
@@ -419,7 +419,7 @@ void QVector<T>::resize(int asize)
 }
 template <typename T>
 inline void QVector<T>::clear()
-{ *this = QVector<T>(); }
+{ resize(0); }
 template <typename T>
 inline const T &QVector<T>::at(int i) const
 { Q_ASSERT_X(i >= 0 && i < d->size, "QVector<T>::at", "index out of range");
diff --git a/src/corelib/xml/qxmlstream_p.h b/src/corelib/xml/qxmlstream_p.h
index 4c6c206bf34..ab0bbba4136 100644
--- a/src/corelib/xml/qxmlstream_p.h
+++ b/src/corelib/xml/qxmlstream_p.h
@@ -1020,10 +1020,8 @@ bool QXmlStreamReaderPrivate::parse()
         prefix.clear();
         qualifiedName.clear();
         namespaceUri.clear();
-        if (publicNamespaceDeclarations.size())
-            publicNamespaceDeclarations.clear();
-        if (attributes.size())
-            attributes.resize(0);
+        publicNamespaceDeclarations.clear();
+        attributes.clear();
         if (isEmptyElement) {
             setType(QXmlStreamReader::EndElement);
             Tag &tag = tagStack_pop();
diff --git a/src/gui/kernel/qshortcutmap.cpp b/src/gui/kernel/qshortcutmap.cpp
index 5eb216d58e8..3b40aba49e9 100644
--- a/src/gui/kernel/qshortcutmap.cpp
+++ b/src/gui/kernel/qshortcutmap.cpp
@@ -375,7 +375,7 @@ QKeySequence::SequenceMatch QShortcutMap::nextState(QKeyEvent *e)
     QKeySequence::SequenceMatch result = QKeySequence::NoMatch;
 
     // We start fresh each time..
-    d->identicals.resize(0);
+    d->identicals.clear();
 
     result = find(e);
     if (result == QKeySequence::NoMatch && (e->modifiers() & Qt::KeypadModifier)) {
@@ -448,7 +448,7 @@ QKeySequence::SequenceMatch QShortcutMap::find(QKeyEvent *e, int ignoredModifier
     }
 
     // Looking for new identicals, scrap old
-    d->identicals.resize(0);
+    d->identicals.clear();
 
     bool partialFound = false;
     bool identicalDisabledFound = false;
diff --git a/src/gui/text/qcssparser.cpp b/src/gui/text/qcssparser.cpp
index fceb11959ab..b9e05e726e0 100644
--- a/src/gui/text/qcssparser.cpp
+++ b/src/gui/text/qcssparser.cpp
@@ -2166,7 +2166,7 @@ void Parser::init(const QString &css, bool isFile)
     }
 
     hasEscapeSequences = false;
-    symbols.resize(0);
+    symbols.clear();
     symbols.reserve(8);
     Scanner::scan(Scanner::preprocess(styleSheet, &hasEscapeSequences), &symbols);
     index = 0;
diff --git a/src/gui/text/qtextdocument_p.cpp b/src/gui/text/qtextdocument_p.cpp
index 348fa837566..93071aaf59a 100644
--- a/src/gui/text/qtextdocument_p.cpp
+++ b/src/gui/text/qtextdocument_p.cpp
@@ -1128,7 +1128,7 @@ void QTextDocumentPrivate::clearUndoRedoStacks(QTextDocument::Stacks stacksToCle
                 delete c.custom;
         }
         undoState = 0;
-        undoStack.resize(0);
+        undoStack.clear();
         if (emitSignals && undoCommandsAvailable)
             emitUndoAvailable(false);
         if (emitSignals && redoCommandsAvailable)
diff --git a/src/widgets/widgets/qmdiarea.cpp b/src/widgets/widgets/qmdiarea.cpp
index 2b734b603f7..b42ebe7e489 100644
--- a/src/widgets/widgets/qmdiarea.cpp
+++ b/src/widgets/widgets/qmdiarea.cpp
@@ -516,7 +516,7 @@ QVector<QRect> MinOverlapPlacer::findMaxOverlappers(const QRect &domain, const Q
         if (overlap >= maxOverlap || maxOverlap == -1) {
             if (overlap > maxOverlap) {
                 maxOverlap = overlap;
-                result.resize(0);
+                result.clear();
             }
             result << srcRect;
         }
diff --git a/src/widgets/widgets/qmenubar.cpp b/src/widgets/widgets/qmenubar.cpp
index f15ce06e23a..261ff0bca3b 100644
--- a/src/widgets/widgets/qmenubar.cpp
+++ b/src/widgets/widgets/qmenubar.cpp
@@ -200,7 +200,7 @@ void QMenuBarPrivate::updateGeometries()
     if(itemsDirty) {
         for(int j = 0; j < shortcutIndexMap.size(); ++j)
             q->releaseShortcut(shortcutIndexMap.value(j));
-        shortcutIndexMap.resize(0); // faster than clear
+        shortcutIndexMap.clear();
         const int actionsCount = actions.count();
         shortcutIndexMap.reserve(actionsCount);
         for (int i = 0; i < actionsCount; i++)
diff --git a/src/widgets/widgets/qtextbrowser.cpp b/src/widgets/widgets/qtextbrowser.cpp
index d8a7b27d603..db66d39089e 100644
--- a/src/widgets/widgets/qtextbrowser.cpp
+++ b/src/widgets/widgets/qtextbrowser.cpp
@@ -1141,7 +1141,7 @@ void QTextBrowser::clearHistory()
     d->forwardStack.clear();
     if (!d->stack.isEmpty()) {
         QTextBrowserPrivate::HistoryEntry historyEntry = d->stack.top();
-        d->stack.resize(0);
+        d->stack.clear();
         d->stack.push(historyEntry);
         d->home = historyEntry.url;
     }
diff --git a/tests/auto/corelib/tools/qvector/tst_qvector.cpp b/tests/auto/corelib/tools/qvector/tst_qvector.cpp
index 8fad672f423..eb37d6b8e64 100644
--- a/tests/auto/corelib/tools/qvector/tst_qvector.cpp
+++ b/tests/auto/corelib/tools/qvector/tst_qvector.cpp
@@ -734,10 +734,11 @@ void tst_QVector::clear() const
     QVector<T> myvec;
     myvec << SimpleValue<T>::at(0) << SimpleValue<T>::at(1) << SimpleValue<T>::at(2);
 
-    QVERIFY(myvec.size() == 3);
+    const auto oldCapacity = myvec.capacity();
+    QCOMPARE(myvec.size(), 3);
     myvec.clear();
-    QVERIFY(myvec.size() == 0);
-    QVERIFY(myvec.capacity() == 0);
+    QCOMPARE(myvec.size(), 0);
+    QCOMPARE(myvec.capacity(), oldCapacity);
 }
 
 void tst_QVector::clearInt() const
@@ -1945,7 +1946,7 @@ void tst_QVector::resizePOD() const
 
     const int capacity = vector.capacity();
 
-    vector.resize(0);
+    vector.clear();
     QCOMPARE(vector.size(), 0);
     QVERIFY(vector.capacity() <= capacity);
 }