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, 25 insertions, 4 deletions

diff --git a/src/corelib/thread/qresultstore.h b/src/corelib/thread/qresultstore.h
index 7a65089396..c150876e74 100644
--- a/src/corelib/thread/qresultstore.h
+++ b/src/corelib/thread/qresultstore.h
@@ -43,6 +43,8 @@
 #include <QtCore/qmap.h>
 #include <QtCore/qdebug.h>
 
+#include <utility>
+
 QT_REQUIRE_CONFIG(future);
 
 QT_BEGIN_NAMESPACE
@@ -97,6 +99,19 @@ public:
         return *pointer<T>();
     }
 
+    template<typename T>
+    T &value()
+    {
+        return *pointer<T>();
+    }
+
+    template <typename T>
+    T *pointer()
+    {
+        const T *p = qAsConst(*this).pointer<T>();
+        return const_cast<T *>(p);
+    }
+
     template <typename T>
     const T *pointer() const
     {
@@ -144,8 +159,14 @@ public:
     {
         if (result == nullptr)
             return addResult(index, static_cast<void *>(nullptr));
-        else
-            return addResult(index, static_cast<void *>(new T(*result)));
+
+        return addResult(index, static_cast<void *>(new T(*result)));
+    }
+
+    template <typename T>
+    int moveResult(int index, T &&result)
+    {
+        return addResult(index, static_cast<void *>(new T(std::move_if_noexcept(result))));
     }
 
     template <typename T>
@@ -159,8 +180,8 @@ public:
     {
         if (m_filterMode == true && results->count() != totalCount && 0 == results->count())
             return addResults(index, nullptr, 0, totalCount);
-        else
-            return addResults(index, new QVector<T>(*results), results->count(), totalCount);
+
+        return addResults(index, new QVector<T>(*results), results->count(), totalCount);
     }
 
     int addCanceledResult(int index)