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>

12 files changed, 31 insertions, 21 deletions

diff --git a/src/corelib/kernel/qeventdispatcher_unix.cpp b/src/corelib/kernel/qeventdispatcher_unix.cpp
index 64b813bb13..9afbb84abf 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 fd67193302..f8f3347786 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 b3247b8af5..ef1c9c17b0 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 d60be78ade..13ae121450 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 4c6c206bf3..ab0bbba413 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 5eb216d58e..3b40aba49e 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 fceb11959a..b9e05e726e 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 348fa83756..93071aaf59 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 2b734b603f..b42ebe7e48 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 f15ce06e23..261ff0bca3 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 d8a7b27d60..db66d39089 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 8fad672f42..eb37d6b8e6 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);
 }