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 <lars.knoll@theqtcompany.com>

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)

src/corelib/tools/qregexp.cpp

@@ -2335,7 +2335,7 @@ QRegExpCharClass::QRegExpCharClass()
 void QRegExpCharClass::clear()
 {
     c = 0;
-    r.resize(0);
+    r.clear();
     n = false;
 }
 

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

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");

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();

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;

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;

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)

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;
         }

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++)

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;
     }

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);
 }