Introduce swap functions for QPromise/QFutureInterface

Made QPromise::swap public, added free standing swap() for
QFutureInterface and QPromise. Updated QPromise special member
functions. Extended tests

Task-number: QTBUG-84977
Change-Id: I5daf6876df306d082441dbcdf5ae4dee3bfc0ead
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

4 files changed, 35 insertions, 15 deletions

diff --git a/src/corelib/thread/qfutureinterface.cpp b/src/corelib/thread/qfutureinterface.cpp
index 1785815cf3..a625e05aac 100644
--- a/src/corelib/thread/qfutureinterface.cpp
+++ b/src/corelib/thread/qfutureinterface.cpp
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -545,6 +545,11 @@ QFutureInterfaceBase &QFutureInterfaceBase::operator=(const QFutureInterfaceBase
     return *this;
 }
 
+void QFutureInterfaceBase::swap(QFutureInterfaceBase &other) noexcept
+{
+    qSwap(d, other.d);
+}
+
 bool QFutureInterfaceBase::refT() const
 {
     return d->refCount.refT();
diff --git a/src/corelib/thread/qfutureinterface.h b/src/corelib/thread/qfutureinterface.h
index 2e15cbb013..047d2eb2d3 100644
--- a/src/corelib/thread/qfutureinterface.h
+++ b/src/corelib/thread/qfutureinterface.h
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -163,6 +163,8 @@ public:
     inline bool operator!=(const QFutureInterfaceBase &other) const { return d != other.d; }
     QFutureInterfaceBase &operator=(const QFutureInterfaceBase &other);
 
+    void swap(QFutureInterfaceBase &other) noexcept;
+
 protected:
     bool refT() const;
     bool derefT() const;
@@ -426,6 +428,12 @@ public:
     }
 };
 
+template<typename T>
+inline void swap(QFutureInterface<T> &a, QFutureInterface<T> &b) noexcept
+{
+    a.swap(b);
+}
+
 QT_END_NAMESPACE
 
 #endif // QFUTUREINTERFACE_H
diff --git a/src/corelib/thread/qpromise.h b/src/corelib/thread/qpromise.h
index 9cd9238381..32aaa202f5 100644
--- a/src/corelib/thread/qpromise.h
+++ b/src/corelib/thread/qpromise.h
@@ -43,6 +43,8 @@
 #include <QtCore/qglobal.h>
 #include <QtCore/qfuture.h>
 
+#include <utility>
+
 QT_REQUIRE_CONFIG(future);
 
 QT_BEGIN_NAMESPACE
@@ -59,13 +61,12 @@ public:
     Q_DISABLE_COPY(QPromise)
     QPromise(QPromise<T> &&other) : d(other.d)
     {
-        // In constructor, there's no need to perform swap(). Assign new
-        // QFutureInterface to other.d instead which is slightly cheaper.
         other.d = QFutureInterface<T>();
     }
     QPromise& operator=(QPromise<T> &&other)
     {
-        swap(other);
+        QPromise<T> tmp(std::move(other));
+        tmp.swap(*this);
         return *this;
     }
     ~QPromise()
@@ -110,24 +111,25 @@ public:
         d.setProgressValueAndText(progressValue, progressText);
     }
 
+    void swap(QPromise<T> &other) noexcept
+    {
+        qSwap(this->d, other.d);
+    }
+
 #if defined(Q_CLANG_QDOC)  // documentation-only simplified signatures
     void addResult(const T &result, int index = -1) { }
     void addResult(T &&result, int index = -1) { }
 #endif
-
 private:
     mutable QFutureInterface<T> d = QFutureInterface<T>();
-
-    void swap(QPromise<T> &other)
-    {
-        // Note: copy operations are expensive! They trigger several atomic
-        //       reference counts
-        auto tmp = this->d;
-        this->d = other.d;
-        other.d = tmp;
-    }
 };
 
+template<typename T>
+inline void swap(QPromise<T> &a, QPromise<T> &b) noexcept
+{
+    a.swap(b);
+}
+
 QT_END_NAMESPACE
 
 #endif  // QPROMISE_H
diff --git a/src/corelib/thread/qpromise.qdoc b/src/corelib/thread/qpromise.qdoc
index e78e16bdd6..b404732a9e 100644
--- a/src/corelib/thread/qpromise.qdoc
+++ b/src/corelib/thread/qpromise.qdoc
@@ -229,3 +229,8 @@
     \sa QFuture::progressValue(), QFuture::progressText(), QFuture::cancel(),
     reportFinished()
 */
+
+/*! \fn template<typename T> void QPromise<T>::swap(QPromise<T> &other) noexcept
+
+    Swaps promise \a other with this promise. This operation is very fast and never fails.
+*/