QRingBuffer: avoid allocation in c'tor

Reduces creation time and memory consumption when the buffer is unused.
Robin's benchmark:

  QBENCHMARK {
      QFile file(filename);
  }

reports the following results (run with -median 10 -iterations 524288):

before: 0.00028 msecs per iteration (total: 149, iterations: 524288)
after:  0.00017 msecs per iteration (total: 93, iterations: 524288).

Change-Id: Ied4e7caeca794b94260b8fc59b3ba656f4719c30
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

1 files changed, 18 insertions, 8 deletions

diff --git a/tests/auto/corelib/tools/qringbuffer/tst_qringbuffer.cpp b/tests/auto/corelib/tools/qringbuffer/tst_qringbuffer.cpp
index 74b6edf11f..2695e6238c 100644
--- a/tests/auto/corelib/tools/qringbuffer/tst_qringbuffer.cpp
+++ b/tests/auto/corelib/tools/qringbuffer/tst_qringbuffer.cpp
@@ -39,11 +39,11 @@ class tst_QRingBuffer : public QObject
 {
     Q_OBJECT
 private slots:
+    void constructing();
     void readPointerAtPositionWriteRead();
     void readPointerAtPositionEmptyRead();
     void readPointerAtPositionWithHead();
     void readPointerAtPositionReadTooMuch();
-    void sizeWhenEmpty();
     void sizeWhenReservedAndChopped();
     void sizeWhenReserved();
     void free();
@@ -57,6 +57,23 @@ private slots:
     void readLine();
 };
 
+void tst_QRingBuffer::constructing()
+{
+    QRingBuffer ringBuffer;
+
+    QCOMPARE(ringBuffer.size(), Q_INT64_C(0));
+    QVERIFY(ringBuffer.isEmpty());
+    QCOMPARE(ringBuffer.nextDataBlockSize(), Q_INT64_C(0));
+    QVERIFY(ringBuffer.readPointer() == Q_NULLPTR);
+    QCOMPARE(ringBuffer.skip(5), Q_INT64_C(0));
+    QCOMPARE(ringBuffer.read(), QByteArray());
+    QCOMPARE(ringBuffer.getChar(), -1);
+    QVERIFY(!ringBuffer.canReadLine());
+
+    char buf[5];
+    QCOMPARE(ringBuffer.peek(buf, sizeof(buf)), Q_INT64_C(0));
+}
+
 void tst_QRingBuffer::sizeWhenReserved()
 {
     QRingBuffer ringBuffer;
@@ -74,13 +91,6 @@ void tst_QRingBuffer::sizeWhenReservedAndChopped()
     QCOMPARE(ringBuffer.size(), Q_INT64_C(0));
 }
 
-void tst_QRingBuffer::sizeWhenEmpty()
-{
-    QRingBuffer ringBuffer;
-
-    QCOMPARE(ringBuffer.size(), Q_INT64_C(0));
-}
-
 void tst_QRingBuffer::readPointerAtPositionReadTooMuch()
 {
     QRingBuffer ringBuffer;