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>

1 files changed, 13 insertions, 2 deletions

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