QFuture - add ability to move results from QFuture

QFuture's original design pre-dates C++11 and its
introduction of move semantics. QFuture is documented
as requiring copy-constructible classes and uses copy
operations for results (which in Qt's universe in general
is relatively cheap, due to the use of COW/data sharing).
QFuture::result(), QFuture::results(), QFuture::resultAt()
return copies. Now that the year is 2020, it makes some
sense to add support for move semantics and, in particular,
move-only types, like std::unique_ptr (that cannot be
obtained from QFuture using result etc.). Taking a result
or results from a QFuture renders it invalid.  This patch
adds QFuture<T>::takeResults(), takeResult() and isValid().
'Taking' functions are 'enabled_if' for non-void types only
to improve the compiler's diagnostic (which would otherwise
spit some semi-articulate diagnostic).
As a bonus a bug was found in the pre-existing code (after
initially copy and pasted into the new function) - the one
where we incorrectly report ready results in (rather obscure)
filter mode.

Fixes: QTBUG-81941
Fixes: QTBUG-83182
Change-Id: I8ccdfc50aa310a3a79eef2cdc55f5ea210f889c3
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 8 insertions, 1 deletions

diff --git a/src/concurrent/qtconcurrentrunbase.h b/src/concurrent/qtconcurrentrunbase.h
index e84c0cdb67..5e9fa0aa00 100644
--- a/src/concurrent/qtconcurrentrunbase.h
+++ b/src/concurrent/qtconcurrentrunbase.h
@@ -48,6 +48,9 @@
 #include <QtCore/qrunnable.h>
 #include <QtCore/qthreadpool.h>
 
+#include <type_traits>
+#include <utility>
+
 QT_BEGIN_NAMESPACE
 
 
@@ -123,7 +126,11 @@ public:
         }
 #endif
 
-        this->reportResult(result);
+        if constexpr (std::is_move_constructible_v<T>)
+            this->reportAndMoveResult(std::move(result));
+        else if constexpr (std::is_copy_constructible_v<T>)
+            this->reportResult(result);
+
         this->reportFinished();
     }
     T result;