Make QElapsedTimer default to invalid (and now non-POD).

The practical uses of a POD QElapsedTimer are not really that clear, and the
number of misuses of this API are quite high. Default the state to invalid to
prevent against mistakes.

[ChangeLog][QtCore][QElapsedTimer] Is no longer a POD.

Change-Id: I267292acf2bfca7404e3e449dd04410441d7ce26
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/corelib/tools/qelapsedtimer.cpp

@@ -201,6 +201,17 @@ QT_BEGIN_NAMESPACE
     \sa clockType(), isMonotonic()
 */
 
+/*!
+    \fn QElapsedTimer::QElapsedTimer()
+    \since 5.4
+
+    Constructs an invalid QElapsedTimer. A timer becomes valid once it has been
+    started.
+
+    \sa isValid(), start()
+*/
+
+
 /*!
     \fn bool QElapsedTimer::operator ==(const QElapsedTimer &other) const
 
@@ -230,8 +241,8 @@ void QElapsedTimer::invalidate() Q_DECL_NOTHROW
 }
 
 /*!
-    Returns \c false if this object was invalidated by a call to invalidate() and
-    has not been restarted since.
+    Returns \c false if the timer has never been started or invalidated by a
+    call to invalidate().
 
     \sa invalidate(), start(), restart()
 */

src/corelib/tools/qelapsedtimer.h

@@ -57,6 +57,13 @@ public:
         MachAbsoluteTime,
         PerformanceCounter
     };
+
+    Q_DECL_CONSTEXPR QElapsedTimer()
+        : t1(Q_INT64_C(0x8000000000000000))
+        , t2(Q_INT64_C(0x8000000000000000))
+    {
+    }
+
     static ClockType clockType() Q_DECL_NOTHROW;
     static bool isMonotonic() Q_DECL_NOTHROW;
 

src/corelib/tools/qelapsedtimer_generic.cpp

@@ -87,6 +87,8 @@ void QElapsedTimer::start() Q_DECL_NOTHROW
     and then starting the timer again with start(), but it does so in one
     single operation, avoiding the need to obtain the clock value twice.
 
+    Restarting the timer makes it valid again.
+
     The following example illustrates how to use this function to calibrate a
     parameter to a slow operation (for example, an iteration count) so that
     this operation takes at least 250 milliseconds:

tests/auto/corelib/tools/qelapsedtimer/tst_qelapsedtimer.cpp

@@ -48,12 +48,7 @@ static const int minResolution = 50; // the minimum resolution for the tests
 
 QDebug operator<<(QDebug s, const QElapsedTimer &t)
 {
-    union {
-        QElapsedTimer t;
-        struct { qint64 t1, t2; } i;
-    } copy;
-    copy.t = t;
-    s.nospace() << "(" <<  copy.i.t1 << ", " << copy.i.t2 << ")";
+    s.nospace() << "(" << t.msecsSinceReference() << ")";
     return s.space();
 }
 
@@ -81,8 +76,7 @@ void tst_QElapsedTimer::validity()
 {
     QElapsedTimer t;
 
-    t.invalidate();
-    QVERIFY(!t.isValid());
+    QVERIFY(!t.isValid()); // non-POD now, it should always start invalid
 
     t.start();
     QVERIFY(t.isValid());