From 9bbef41a5022ab887c501a0ab70c1f19279ffa94 Mon Sep 17 00:00:00 2001
From: Simon Brunel <simonbrunel@users.noreply.github.com>
Date: Sat, 3 Jun 2017 10:04:28 +0200
Subject: [PATCH] C++11 optimizations and (basic) benchmark

Make continuation methods const (then/fail/finally) and ensure that the resolved promise value/error is copied only when required, same for user lambdas (dispatching result is now fully handled by the PromiseData).
---
 src/qtpromise/qpromise.h               |  78 ++++-----
 src/qtpromise/qpromise.inl             | 230 ++++++++++---------------
 src/qtpromise/qpromise_p.h             | 219 ++++++++++++++++++-----
 src/qtpromise/qpromiseerror.h          |  34 +++-
 src/qtpromise/qpromisefuture.h         |   3 +-
 src/qtpromise/qpromisehelpers.h        |   6 +-
 tests/auto/auto.pro                    |   1 +
 tests/auto/benchmark/benchmark.pro     |   5 +
 tests/auto/benchmark/tst_benchmark.cpp | 223 ++++++++++++++++++++++++
 9 files changed, 558 insertions(+), 241 deletions(-)
 create mode 100644 tests/auto/benchmark/benchmark.pro
 create mode 100644 tests/auto/benchmark/tst_benchmark.cpp

diff --git a/src/qtpromise/qpromise.h b/src/qtpromise/qpromise.h
index 80c65fa..e7aa208 100644
--- a/src/qtpromise/qpromise.h
+++ b/src/qtpromise/qpromise.h
@@ -16,33 +16,9 @@ class QPromiseBase
 public:
     using Type = T;
 
-    virtual ~QPromiseBase() { }
-
-    bool isFulfilled() const { return m_d->resolved && !m_d->rejected; }
-    bool isRejected() const { return m_d->resolved && m_d->rejected; }
-    bool isPending() const { return !m_d->resolved; }
-
-    template <typename TFulfilled, typename TRejected = std::nullptr_t>
-    inline typename QtPromisePrivate::PromiseHandler<T, TFulfilled>::Promise
-    then(TFulfilled fulfilled, TRejected rejected = nullptr);
-
-    template <typename TRejected>
-    inline typename QtPromisePrivate::PromiseHandler<T, std::nullptr_t>::Promise
-    fail(TRejected rejected);
-
-    inline QPromise<T> wait() const;
-
-public: // STATIC
-    template <typename E>
-    inline static QPromise<T> reject(const E& error);
-
-protected:
-    friend class QPromiseResolve<T>;
-    friend class QPromiseReject<T>;
-
-    QExplicitlySharedDataPointer<QtPromisePrivate::PromiseData<T> > m_d;
-
-    inline QPromiseBase();
+    QPromiseBase(const QPromiseBase<T>& other): m_d(other.m_d) {}
+    QPromiseBase(const QPromise<T>& other): m_d(other.m_d) {}
+    QPromiseBase(QPromiseBase<T>&& other) { swap(other); }
 
     template <typename F, typename std::enable_if<QtPromisePrivate::ArgsOf<F>::count == 1, int>::type = 0>
     inline QPromiseBase(F resolver);
@@ -50,9 +26,33 @@ protected:
     template <typename F, typename std::enable_if<QtPromisePrivate::ArgsOf<F>::count != 1, int>::type = 0>
     inline QPromiseBase(F resolver);
 
-    virtual void notify(const typename QtPromisePrivate::PromiseData<T>::HandlerList& handlers) const = 0;
+    virtual ~QPromiseBase() { }
 
-    inline void dispatch();
+    bool isFulfilled() const { return m_d->isFulfilled(); }
+    bool isRejected() const { return m_d->isRejected(); }
+    bool isPending() const { return m_d->isPending(); }
+
+    template <typename TFulfilled, typename TRejected = std::nullptr_t>
+    inline typename QtPromisePrivate::PromiseHandler<T, TFulfilled>::Promise
+    then(const TFulfilled& fulfilled, const TRejected& rejected = nullptr) const;
+
+    template <typename TRejected>
+    inline typename QtPromisePrivate::PromiseHandler<T, std::nullptr_t>::Promise
+    fail(TRejected&& rejected) const;
+
+    inline QPromise<T> wait() const;
+
+    void swap(QPromiseBase<T>& other) { qSwap(m_d, other.m_d); }
+
+public: // STATIC
+    template <typename E>
+    inline static QPromise<T> reject(E&& error);
+
+protected:
+    friend class QPromiseResolve<T>;
+    friend class QPromiseReject<T>;
+
+    QExplicitlySharedDataPointer<QtPromisePrivate::PromiseData<T> > m_d;
 };
 
 template <typename T>
@@ -60,22 +60,17 @@ class QPromise: public QPromiseBase<T>
 {
 public:
     template <typename F>
-    QPromise(F resolver): QPromiseBase<T>(resolver) { }
+    QPromise(F&& resolver): QPromiseBase<T>(std::forward<F>(resolver)) { }
 
     template <typename THandler>
-    inline QPromise<T> finally(THandler handler);
+    inline QPromise<T> finally(THandler handler) const;
 
 public: // STATIC
     inline static QPromise<QVector<T> > all(const QVector<QPromise<T> >& promises);
-    inline static QPromise<T> resolve(const T& value);
-
-protected:
-    inline void notify(const typename QtPromisePrivate::PromiseData<T>::HandlerList& handlers) const Q_DECL_OVERRIDE;
+    inline static QPromise<T> resolve(T&& value);
 
 private:
     friend class QPromiseBase<T>;
-
-    QPromise(const QPromiseBase<T>& p) : QPromiseBase<T>(p) { }
 };
 
 template <>
@@ -83,22 +78,17 @@ class QPromise<void>: public QPromiseBase<void>
 {
 public:
     template <typename F>
-    QPromise(F resolver): QPromiseBase<void>(resolver) { }
+    QPromise(F&& resolver): QPromiseBase<void>(std::forward<F>(resolver)) { }
 
     template <typename THandler>
-    inline QPromise<void> finally(THandler handler);
+    inline QPromise<void> finally(THandler handler) const;
 
 public: // STATIC
     inline static QPromise<void> all(const QVector<QPromise<void> >& promises);
     inline static QPromise<void> resolve();
 
-protected:
-    inline void notify(const typename QtPromisePrivate::PromiseData<void>::HandlerList& handlers) const Q_DECL_OVERRIDE;
-
 private:
     friend class QPromiseBase<void>;
-
-    QPromise(const QPromiseBase<void>& p) : QPromiseBase<void>(p) { }
 };
 
 } // namespace QtPromise
diff --git a/src/qtpromise/qpromise.inl b/src/qtpromise/qpromise.inl
index c598c13..6a0ef99 100644
--- a/src/qtpromise/qpromise.inl
+++ b/src/qtpromise/qpromise.inl
@@ -4,141 +4,87 @@
 
 namespace QtPromise {
 
-template <class T = void>
+template <class T>
 class QPromiseResolve
 {
 public:
-    QPromiseResolve(const QPromise<T>& p)
-        : m_promise(p)
+    QPromiseResolve(QPromise<T> p)
+        : m_promise(new QPromise<T>(std::move(p)))
     { }
 
-    void operator()(const T& value) const
+    void operator()(const T& value) const {
+        resolve(value);
+    }
+
+    void operator()(T&& value) const
     {
-        auto p = m_promise;
-        if (p.isPending()) {
-            p.m_d->rejected = false;
-            p.m_d->resolved = true;
-            p.m_d->value = value;
-            p.dispatch();
-        }
+        resolve(std::move(value));
     }
 
 private:
-    QPromise<T> m_promise;
+    QSharedPointer<QPromise<T> > m_promise;
+
+    template <typename U>
+    void resolve(U&& value) const
+    {
+        Q_ASSERT(!m_promise.isNull());
+        if (m_promise->isPending()) {
+            m_promise->m_d->resolve(std::forward<U>(value));
+            m_promise->m_d->dispatch();
+        }
+    }
 };
 
 template <>
 class QPromiseResolve<void>
 {
 public:
-    QPromiseResolve(const QPromise<void>& p)
-        : m_promise(p)
+    QPromiseResolve(QPromise<void> p)
+        : m_promise(new QPromise<void>(std::move(p)))
     { }
 
     void operator()() const
     {
-        auto p = m_promise;
-        if (p.isPending()) {
-            p.m_d->rejected = false;
-            p.m_d->resolved = true;
-            p.dispatch();
+        Q_ASSERT(!m_promise.isNull());
+        if (m_promise->isPending()) {
+            m_promise->m_d->resolve();
+            m_promise->m_d->dispatch();
         }
     }
 
 private:
-    QPromise<void> m_promise;
+    QSharedPointer<QPromise<void> > m_promise;
 };
 
-template <class T = void>
+template <class T>
 class QPromiseReject
 {
 public:
-    QPromiseReject()
+    QPromiseReject(QPromise<T> p)
+        : m_promise(new QPromise<T>(std::move(p)))
     { }
 
-    QPromiseReject(const QPromise<T>& p)
-        : m_promise(p)
-    { }
-
-    void operator()(const QPromiseError& error) const
+    template <typename E>
+    void operator()(E&& error) const
     {
-        auto p = m_promise;
-        if (p.isPending()) {
-            p.m_d->rejected = true;
-            p.m_d->resolved = true;
-            p.m_d->error = error;
-            p.dispatch();
+        Q_ASSERT(!m_promise.isNull());
+        if (m_promise->isPending()) {
+            m_promise->m_d->reject(std::forward<E>(error));
+            m_promise->m_d->dispatch();
         }
     }
 
 private:
-    QPromise<T> m_promise;
+    QSharedPointer<QPromise<T> > m_promise;
 };
 
-template <typename T>
-template <typename TFulfilled, typename TRejected>
-inline typename QtPromisePrivate::PromiseHandler<T, TFulfilled>::Promise
-QPromiseBase<T>::then(TFulfilled fulfilled, TRejected rejected)
-{
-    using namespace QtPromisePrivate;
-    using PromiseType = typename PromiseHandler<T, TFulfilled>::Promise;
-
-    PromiseType next([=](
-        const QPromiseResolve<typename PromiseType::Type>& resolve,
-        const QPromiseReject<typename PromiseType::Type>& reject) {
-        m_d->handlers << PromiseHandler<T, TFulfilled>::create(fulfilled, resolve, reject);
-        m_d->catchers << PromiseCatcher<T, TRejected>::create(rejected, resolve, reject);
-    });
-
-    if (m_d->resolved) {
-        dispatch();
-    }
-
-    return next;
-}
-
-template <typename T>
-template <typename TRejected>
-inline typename QtPromisePrivate::PromiseHandler<T, std::nullptr_t>::Promise
-QPromiseBase<T>::fail(TRejected rejected)
-{
-    return then(nullptr, rejected);
-}
-
-template <typename T>
-inline QPromise<T> QPromiseBase<T>::wait() const
-{
-    // @TODO wait timeout + global timeout
-    while (!m_d->resolved) {
-        QCoreApplication::processEvents(QEventLoop::AllEvents);
-        QCoreApplication::sendPostedEvents(nullptr, QEvent::DeferredDelete);
-    }
-
-    return *this;
-}
-
-template <typename T>
-template <typename E>
-inline QPromise<T> QPromiseBase<T>::reject(const E& error)
-{
-    return QPromise<T>([=](const QPromiseResolve<T>&) {
-        throw error;
-    });
-}
-
-template <typename T>
-inline QPromiseBase<T>::QPromiseBase()
-    : m_d(new QtPromisePrivate::PromiseData<T>())
-{
-}
-
 template <typename T>
 template <typename F, typename std::enable_if<QtPromisePrivate::ArgsOf<F>::count == 1, int>::type>
 inline QPromiseBase<T>::QPromiseBase(F resolver)
     : m_d(new QtPromisePrivate::PromiseData<T>())
 {
-    auto resolve = QPromiseResolve<T>(*this);
-    auto reject = QPromiseReject<T>(*this);
+    QPromiseResolve<T> resolve(*this);
+    QPromiseReject<T> reject(*this);
 
     try {
         resolver(resolve);
@@ -152,8 +98,8 @@ template <typename F, typename std::enable_if<QtPromisePrivate::ArgsOf<F>::count
 inline QPromiseBase<T>::QPromiseBase(F resolver)
     : m_d(new QtPromisePrivate::PromiseData<T>())
 {
-    auto resolve = QPromiseResolve<T>(*this);
-    auto reject = QPromiseReject<T>(*this);
+    QPromiseResolve<T> resolve(*this);
+    QPromiseReject<T> reject(*this);
 
     try {
         resolver(resolve, reject);
@@ -163,33 +109,59 @@ inline QPromiseBase<T>::QPromiseBase(F resolver)
 }
 
 template <typename T>
-inline void QPromiseBase<T>::dispatch()
+template <typename TFulfilled, typename TRejected>
+inline typename QtPromisePrivate::PromiseHandler<T, TFulfilled>::Promise
+QPromiseBase<T>::then(const TFulfilled& fulfilled, const TRejected& rejected) const
 {
     using namespace QtPromisePrivate;
+    using PromiseType = typename PromiseHandler<T, TFulfilled>::Promise;
 
-    Q_ASSERT(m_d->resolved);
+    PromiseType next([&](
+        const QPromiseResolve<typename PromiseType::Type>& resolve,
+        const QPromiseReject<typename PromiseType::Type>& reject) {
+        m_d->addHandler(PromiseHandler<T, TFulfilled>::create(fulfilled, resolve, reject));
+        m_d->addCatcher(PromiseCatcher<T, TRejected>::create(rejected, resolve, reject));
+    });
 
-    typename PromiseData<T>::HandlerList handlers;
-    typename PromiseData<T>::CatcherList catchers;
-
-    handlers.swap(m_d->handlers);
-    catchers.swap(m_d->catchers);
-
-    if (m_d->rejected) {
-        const QPromiseError error = m_d->error;
-        qtpromise_defer([=]() {
-            for (auto catcher: catchers) {
-                catcher(error);
-            }
-        });
-    } else {
-        notify(handlers);
+    if (!m_d->isPending()) {
+        m_d->dispatch();
     }
+
+    return next;
+}
+
+template <typename T>
+template <typename TRejected>
+inline typename QtPromisePrivate::PromiseHandler<T, std::nullptr_t>::Promise
+QPromiseBase<T>::fail(TRejected&& rejected) const
+{
+    return then(nullptr, std::forward<TRejected>(rejected));
+}
+
+template <typename T>
+inline QPromise<T> QPromiseBase<T>::wait() const
+{
+    // @TODO wait timeout + global timeout
+    while (m_d->isPending()) {
+        QCoreApplication::processEvents(QEventLoop::AllEvents);
+        QCoreApplication::sendPostedEvents(nullptr, QEvent::DeferredDelete);
+    }
+
+    return *this;
+}
+
+template <typename T>
+template <typename E>
+inline QPromise<T> QPromiseBase<T>::reject(E&& error)
+{
+    return QPromise<T>([&](const QPromiseResolve<T>&, const QPromiseReject<T>& reject) {
+        reject(std::forward<E>(error));
+    });
 }
 
 template <typename T>
 template <typename THandler>
-inline QPromise<T> QPromise<T>::finally(THandler handler)
+inline QPromise<T> QPromise<T>::finally(THandler handler) const
 {
     return this->then([=](const T& res) {
         return QPromise<void>::resolve().then(handler).then([=](){
@@ -220,7 +192,7 @@ inline QPromise<QVector<T> > QPromise<T>::all(const QVector<QPromise<T> >& promi
         QSharedPointer<QVector<T> > results(new QVector<T>(count));
 
         for (int i=0; i<count; ++i) {
-            QPromise<T>(promises[i]).then([=](const T& res) mutable {
+            promises[i].then([=](const T& res) mutable {
                 (*results)[i] = res;
                 if (--(*remaining) == 0) {
                     resolve(*results);
@@ -236,26 +208,15 @@ inline QPromise<QVector<T> > QPromise<T>::all(const QVector<QPromise<T> >& promi
 }
 
 template <typename T>
-inline QPromise<T> QPromise<T>::resolve(const T& value)
+inline QPromise<T> QPromise<T>::resolve(T&& value)
 {
-    return QPromise<T>([=](const QPromiseResolve<T>& resolve) {
-        resolve(value);
-    });
-}
-
-template <typename T>
-inline void QPromise<T>::notify(const typename QtPromisePrivate::PromiseData<T>::HandlerList& handlers) const
-{
-    const T value = this->m_d->value;
-    QtPromisePrivate::qtpromise_defer([handlers, value]() {
-        for (auto handler: handlers) {
-            handler(value);
-        }
+    return QPromise<T>([&](const QPromiseResolve<T>& resolve) {
+       resolve(std::forward<T>(value));
     });
 }
 
 template <typename THandler>
-inline QPromise<void> QPromise<void>::finally(THandler handler)
+inline QPromise<void> QPromise<void>::finally(THandler handler) const
 {
     return this->then([=]() {
         return QPromise<void>::resolve().then(handler).then([](){});
@@ -281,7 +242,7 @@ inline QPromise<void> QPromise<void>::all(const QVector<QPromise<void> >& promis
         QSharedPointer<int> remaining(new int(promises.size()));
 
         for (const auto& promise: promises) {
-            QPromise<void>(promise).then([=]() {
+            promise.then([=]() {
                 if (--(*remaining) == 0) {
                     resolve();
                 }
@@ -302,13 +263,4 @@ inline QPromise<void> QPromise<void>::resolve()
     });
 }
 
-inline void QPromise<void>::notify(const typename QtPromisePrivate::PromiseData<void>::HandlerList& handlers) const
-{
-    QtPromisePrivate::qtpromise_defer([handlers]() {
-        for (const auto& handler: handlers) {
-            handler();
-        }
-    });
-}
-
 } // namespace QtPromise
diff --git a/src/qtpromise/qpromise_p.h b/src/qtpromise/qpromise_p.h
index 28c71f4..8626854 100644
--- a/src/qtpromise/qpromise_p.h
+++ b/src/qtpromise/qpromise_p.h
@@ -7,6 +7,7 @@
 
 // Qt
 #include <QTimer>
+#include <QSharedPointer>
 #include <QSharedData>
 #include <QVector>
 
@@ -26,9 +27,9 @@ class QPromiseReject;
 namespace QtPromisePrivate {
 
 template <typename F>
-inline void qtpromise_defer(F f)
+inline void qtpromise_defer(F&& f)
 {
-    QTimer::singleShot(0, f);
+    QTimer::singleShot(0, std::forward<F>(f));
 }
 
 template <typename T>
@@ -46,11 +47,11 @@ template <typename T>
 struct PromiseFulfill
 {
     static void call(
-        const T& value,
+        T&& value,
         const QtPromise::QPromiseResolve<T>& resolve,
         const QtPromise::QPromiseReject<T>&)
     {
-        resolve(value);
+        resolve(std::move(value));
     }
 };
 
@@ -58,7 +59,7 @@ template <typename T>
 struct PromiseFulfill<QtPromise::QPromise<T> >
 {
     static void call(
-        QtPromise::QPromise<T> promise,
+        const QtPromise::QPromise<T>& promise,
         const QtPromise::QPromiseResolve<T>& resolve,
         const QtPromise::QPromiseReject<T>& reject)
     {
@@ -76,7 +77,10 @@ template <>
 struct PromiseFulfill<QtPromise::QPromise<void> >
 {
     template <typename TPromise, typename TResolve, typename TReject>
-    static void call(TPromise promise, TResolve resolve, TReject reject)
+    static void call(
+        const TPromise& promise,
+        const TResolve& resolve,
+        const TReject& reject)
     {
         promise.then(
             [=]() {
@@ -95,11 +99,10 @@ struct PromiseDispatch
     using ResType = Unqualified<TRes>;
 
     template <typename THandler, typename TResolve, typename TReject>
-    static void call(const T& value, THandler handler, TResolve resolve, TReject reject)
+    static void call(const T& value, THandler handler, const TResolve& resolve, const TReject& reject)
     {
         try {
-            const ResType res = handler(value);
-            PromiseFulfill<ResType>::call(res, resolve, reject);
+            PromiseFulfill<ResType>::call(handler(value), resolve, reject);
         } catch (...) {
             reject(std::current_exception());
         }
@@ -112,7 +115,7 @@ struct PromiseDispatch<T, void>
     using Promise = QtPromise::QPromise<void>;
 
     template <typename THandler, typename TResolve, typename TReject>
-    static void call(const T& value, THandler handler, TResolve resolve, TReject reject)
+    static void call(const T& value, THandler handler, const TResolve& resolve, const TReject& reject)
     {
         try {
             handler(value);
@@ -130,11 +133,10 @@ struct PromiseDispatch<void, TRes>
     using ResType = Unqualified<TRes>;
 
     template <typename THandler, typename TResolve, typename TReject>
-    static void call(THandler handler, TResolve resolve, TReject reject)
+    static void call(THandler handler, const TResolve& resolve, const TReject& reject)
     {
         try {
-            const ResType res = handler();
-            PromiseFulfill<ResType>::call(res, resolve, reject);
+            PromiseFulfill<ResType>::call(handler(), resolve, reject);
         } catch (...) {
             reject(std::current_exception());
         }
@@ -147,7 +149,7 @@ struct PromiseDispatch<void, void>
     using Promise = QtPromise::QPromise<void>;
 
     template <typename THandler, typename TResolve, typename TReject>
-    static void call(THandler handler, TResolve resolve, TReject reject)
+    static void call(THandler handler, const TResolve& resolve, const TReject& reject)
     {
         try {
             handler();
@@ -165,10 +167,13 @@ struct PromiseHandler
     using Promise = typename PromiseDispatch<T, ResType>::Promise;
 
     template <typename TResolve, typename TReject>
-    static std::function<void(T)> create(THandler handler, TResolve resolve, TReject reject)
+    static std::function<void(const T&)> create(
+        const THandler& handler,
+        const TResolve& resolve,
+        const TReject& reject)
     {
         return [=](const T& value) {
-            PromiseDispatch<T, ResType>::call(value, handler, resolve, reject);
+            PromiseDispatch<T, ResType>::call(value, std::move(handler), resolve, reject);
         };
     }
 };
@@ -180,7 +185,10 @@ struct PromiseHandler<T, THandler, void>
     using Promise = typename PromiseDispatch<T, ResType>::Promise;
 
     template <typename TResolve, typename TReject>
-    static std::function<void(T)> create(THandler handler, TResolve resolve, TReject reject)
+    static std::function<void(const T&)> create(
+        const THandler& handler,
+        const TResolve& resolve,
+        const TReject& reject)
     {
         return [=](const T&) {
             PromiseDispatch<void, ResType>::call(handler, resolve, reject);
@@ -195,7 +203,10 @@ struct PromiseHandler<void, THandler, void>
     using Promise = typename PromiseDispatch<void, ResType>::Promise;
 
     template <typename TResolve, typename TReject>
-    static std::function<void()> create(THandler handler, TResolve resolve, TReject reject)
+    static std::function<void()> create(
+        const THandler& handler,
+        const TResolve& resolve,
+        const TReject& reject)
     {
         return [=]() {
             PromiseDispatch<void, ResType>::call(handler, resolve, reject);
@@ -209,12 +220,15 @@ struct PromiseHandler<T, std::nullptr_t, void>
     using Promise = QtPromise::QPromise<T>;
 
     template <typename TResolve, typename TReject>
-    static std::function<void(T)> create(std::nullptr_t, TResolve resolve, TReject reject)
+    static std::function<void(const T&)> create(
+        std::nullptr_t,
+        const TResolve& resolve,
+        const TReject& reject)
     {
         return [=](const T& value) {
             // 2.2.7.3. If onFulfilled is not a function and promise1 is fulfilled,
             // promise2 must be fulfilled with the same value as promise1.
-            PromiseFulfill<T>::call(value, resolve, reject);
+            PromiseFulfill<T>::call(std::move(T(value)), resolve, reject);
         };
     }
 };
@@ -225,7 +239,10 @@ struct PromiseHandler<void, std::nullptr_t, void>
     using Promise = QtPromise::QPromise<void>;
 
     template <typename TResolve, typename TReject>
-    static std::function<void()> create(std::nullptr_t, TResolve resolve, TReject)
+    static std::function<void()> create(
+        std::nullptr_t,
+        const TResolve& resolve,
+        const TReject&)
     {
         return [=]() {
             // 2.2.7.3. If onFulfilled is not a function and promise1 is fulfilled,
@@ -238,11 +255,13 @@ struct PromiseHandler<void, std::nullptr_t, void>
 template <typename T, typename THandler, typename TArg = typename ArgsOf<THandler>::first>
 struct PromiseCatcher
 {
-    using Functor = std::function<void(QtPromise::QPromiseError)>;
     using ResType = typename std::result_of<THandler(TArg)>::type;
 
     template <typename TResolve, typename TReject>
-    static Functor create(THandler handler, TResolve resolve, TReject reject)
+    static std::function<void(const QtPromise::QPromiseError&)> create(
+        const THandler& handler,
+        const TResolve& resolve,
+        const TReject& reject)
     {
         return [=](const QtPromise::QPromiseError& error) {
             try {
@@ -259,11 +278,13 @@ struct PromiseCatcher
 template <typename T, typename THandler>
 struct PromiseCatcher<T, THandler, void>
 {
-    using Functor = std::function<void(QtPromise::QPromiseError)>;
     using ResType = typename std::result_of<THandler()>::type;
 
     template <typename TResolve, typename TReject>
-    static Functor create(THandler handler, TResolve resolve, TReject reject)
+    static std::function<void(const QtPromise::QPromiseError&)> create(
+        const THandler& handler,
+        const TResolve& resolve,
+        const TReject& reject)
     {
         return [=](const QtPromise::QPromiseError& error) {
             try {
@@ -278,10 +299,11 @@ struct PromiseCatcher<T, THandler, void>
 template <typename T>
 struct PromiseCatcher<T, std::nullptr_t, void>
 {
-    using Functor = std::function<void(QtPromise::QPromiseError)>;
-
     template <typename TResolve, typename TReject>
-    static Functor create(std::nullptr_t, TResolve, TReject reject)
+    static std::function<void(const QtPromise::QPromiseError&)> create(
+        std::nullptr_t,
+        const TResolve&,
+        const TReject& reject)
     {
         return [=](const QtPromise::QPromiseError& error) {
             // 2.2.7.4. If onRejected is not a function and promise1 is rejected,
@@ -291,33 +313,142 @@ struct PromiseCatcher<T, std::nullptr_t, void>
     }
 };
 
-template <class T>
-struct PromiseDataBase: public QSharedData
-{
-    using ErrorType = QtPromise::QPromiseError;
-    using CatcherList = QVector<std::function<void(ErrorType)> >;
+template <typename T> class PromiseData;
 
-    bool resolved;
-    bool rejected;
-    ErrorType error;
-    CatcherList catchers;
+template <typename T>
+class PromiseDataBase: public QSharedData
+{
+public:
+    using Error = QtPromise::QPromiseError;
+    using Catcher = std::function<void(const Error&)>;
+
+    virtual ~PromiseDataBase() {}
+
+    bool isFulfilled() const { return m_settled && m_error.isNull(); }
+    bool isRejected() const { return m_settled && !m_error.isNull(); }
+    bool isPending() const { return !m_settled; }
+
+    void addCatcher(Catcher catcher)
+    {
+        m_catchers.append(std::move(catcher));
+    }
+
+    void reject(Error error)
+    {
+        Q_ASSERT(m_error.isNull());
+        m_error.reset(new Error(std::move(error)));
+        setSettled();
+    }
+
+    void dispatch()
+    {
+        Q_ASSERT(!isPending());
+
+        if (isFulfilled()) {
+            notify();
+            return;
+        }
+
+        Q_ASSERT(isRejected());
+        QSharedPointer<Error> error = m_error;
+        QVector<Catcher> catchers(std::move(m_catchers));
+        for (const auto& catcher: catchers) {
+            qtpromise_defer([=]() {
+                catcher(*error);
+            });
+        }
+    }
+
+protected:
+    void setSettled()
+    {
+        Q_ASSERT(!m_settled);
+        m_settled = true;
+    }
+
+    virtual void notify() = 0;
+
+private:
+    bool m_settled = false;
+    QVector<Catcher> m_catchers;
+    QSharedPointer<Error> m_error;
 };
 
 template <typename T>
-struct PromiseData: PromiseDataBase<T>
+class PromiseData: public PromiseDataBase<T>
 {
-    using HandlerList = QVector<std::function<void(T)> >;
+public:
+    using Handler = std::function<void(const T&)>;
 
-    HandlerList handlers;
-    T value;
+    void addHandler(Handler handler)
+    {
+        m_handlers.append(std::move(handler));
+    }
+
+    const T& value() const
+    {
+        Q_ASSERT(!m_value.isNull());
+        return *m_value;
+    }
+
+    void resolve(T&& value)
+    {
+        Q_ASSERT(m_value.isNull());
+        m_value.reset(new T(std::move(value)));
+        this->setSettled();
+    }
+
+    void resolve(const T& value)
+    {
+        Q_ASSERT(m_value.isNull());
+        m_value.reset(new T(value));
+        this->setSettled();
+    }
+
+    void notify() Q_DECL_OVERRIDE
+    {
+        Q_ASSERT(this->isFulfilled());
+        QSharedPointer<T> value(m_value);
+        QVector<Handler> handlers(std::move(m_handlers));
+        for (const auto& handler: handlers) {
+            qtpromise_defer([=]() {
+                handler(*value);
+            });
+        }
+    }
+
+private:
+    QVector<Handler> m_handlers;
+    QSharedPointer<T> m_value;
 };
 
 template <>
-struct PromiseData<void>: PromiseDataBase<void>
+class PromiseData<void>: public PromiseDataBase<void>
 {
-    using HandlerList = QVector<std::function<void()> >;
+public:
+    using Handler = std::function<void()>;
 
-    HandlerList handlers;
+    void addHandler(Handler handler)
+    {
+        m_handlers.append(std::move(handler));
+    }
+
+    void resolve() { setSettled(); }
+
+protected:
+    void notify() Q_DECL_OVERRIDE
+    {
+        Q_ASSERT(isFulfilled());
+        QVector<Handler> handlers(std::move(m_handlers));
+        for (const auto& handler: handlers) {
+            qtpromise_defer([=]() {
+                handler();
+            });
+        }
+    }
+
+private:
+    QVector<Handler> m_handlers;
 };
 
 } // namespace QtPromise
diff --git a/src/qtpromise/qpromiseerror.h b/src/qtpromise/qpromiseerror.h
index e21a148..2c96fdb 100644
--- a/src/qtpromise/qpromiseerror.h
+++ b/src/qtpromise/qpromiseerror.h
@@ -9,32 +9,48 @@ namespace QtPromise {
 class QPromiseError
 {
 public:
-    QPromiseError()
-        : exception(nullptr)
-    { }
-
     template <typename T>
     QPromiseError(const T& value)
-        : exception(nullptr)
     {
         try {
             throw value;
         } catch(...) {
-            exception = std::current_exception();
+            m_exception = std::current_exception();
         }
     }
 
     QPromiseError(const std::exception_ptr& exception)
-        : exception(exception)
+        : m_exception(exception)
     { }
 
+    QPromiseError(const QPromiseError& error)
+        : m_exception(error.m_exception)
+    { }
+
+    QPromiseError(QPromiseError&& other)
+        : m_exception(nullptr)
+    {
+        swap(other);
+    }
+
+    QPromiseError& operator =(QPromiseError other)
+    {
+        swap(other);
+        return *this;
+    }
+
+    void swap(QPromiseError& other)
+    {
+        qSwap(m_exception, other.m_exception);
+    }
+
     void rethrow() const
     {
-        std::rethrow_exception(exception);
+        std::rethrow_exception(m_exception);
     }
 
 private:
-    std::exception_ptr exception;
+    std::exception_ptr m_exception;
 };
 
 } // namespace QtPromise
diff --git a/src/qtpromise/qpromisefuture.h b/src/qtpromise/qpromisefuture.h
index ff97599..d08f06c 100644
--- a/src/qtpromise/qpromisefuture.h
+++ b/src/qtpromise/qpromisefuture.h
@@ -48,8 +48,7 @@ struct PromiseFulfill<QFuture<T> >
                     watcher->waitForFinished();
                     reject(QtPromise::QPromiseCanceledException());
                 } else {
-                    T res = watcher->result();
-                    PromiseFulfill<T>::call(res, resolve, reject);
+                    PromiseFulfill<T>::call(watcher->result(), resolve, reject);
                 }
             } catch(...) {
                 reject(std::current_exception());
diff --git a/src/qtpromise/qpromisehelpers.h b/src/qtpromise/qpromisehelpers.h
index f901733..972729e 100644
--- a/src/qtpromise/qpromisehelpers.h
+++ b/src/qtpromise/qpromisehelpers.h
@@ -7,14 +7,14 @@
 namespace QtPromise {
 
 template <typename T>
-typename QtPromisePrivate::PromiseDeduce<T>::Type qPromise(const T& value)
+typename QtPromisePrivate::PromiseDeduce<T>::Type qPromise(T&& value)
 {
     using namespace QtPromisePrivate;
     using Promise = typename PromiseDeduce<T>::Type;
-    return Promise([=](
+    return Promise([&](
         const QPromiseResolve<typename Promise::Type>& resolve,
         const QPromiseReject<typename Promise::Type>& reject) {
-        PromiseFulfill<T>::call(value, resolve, reject);
+        PromiseFulfill<T>::call(std::forward<T>(value), resolve, reject);
     });
 }
 
diff --git a/tests/auto/auto.pro b/tests/auto/auto.pro
index 3b0b440..da47758 100644
--- a/tests/auto/auto.pro
+++ b/tests/auto/auto.pro
@@ -1,5 +1,6 @@
 TEMPLATE = subdirs
 SUBDIRS += \
+    benchmark \
     future \
     helpers \
     qpromise \
diff --git a/tests/auto/benchmark/benchmark.pro b/tests/auto/benchmark/benchmark.pro
new file mode 100644
index 0000000..cc5bd31
--- /dev/null
+++ b/tests/auto/benchmark/benchmark.pro
@@ -0,0 +1,5 @@
+QT += concurrent
+TARGET = tst_benchmark
+SOURCES += $$PWD/tst_benchmark.cpp
+
+include(../tests.pri)
diff --git a/tests/auto/benchmark/tst_benchmark.cpp b/tests/auto/benchmark/tst_benchmark.cpp
new file mode 100644
index 0000000..7d2587b
--- /dev/null
+++ b/tests/auto/benchmark/tst_benchmark.cpp
@@ -0,0 +1,223 @@
+// QtPromise
+#include <QtPromise>
+
+// Qt
+#include <QtConcurrent>
+#include <QtTest>
+
+using namespace QtPromise;
+
+class tst_benchmark: public QObject
+{
+    Q_OBJECT
+
+    void valueResolve();
+    void valueReject();
+    void valueThen();
+    void errorReject();
+private Q_SLOTS:
+    void errorThen();
+
+}; // class tst_benchmark
+
+QTEST_MAIN(tst_benchmark)
+#include "tst_benchmark.moc"
+
+struct Logs {
+    int ctor = 0;
+    int copy = 0;
+    int move = 0;
+    int refs = 0;
+
+    void reset() {
+        ctor = 0;
+        copy = 0;
+        move = 0;
+        refs = 0;
+    }
+};
+
+struct Logger
+{
+    Logger() { logs().ctor++; logs().refs++; }
+    Logger(const Logger&) { logs().copy++; logs().refs++; }
+    Logger(Logger&&) { logs().move++; logs().refs++; }
+    ~Logger() { logs().refs--; }
+
+    Logger& operator=(const Logger&) { logs().copy++; return *this; }
+    Logger& operator=(Logger&&) { logs().move++; return *this; }
+
+public: // STATICS
+    static Logs& logs() { static Logs logs; return logs; }
+};
+
+struct Data: public Logger
+{
+    Data(int v): Logger(), m_value(v) {}
+    int value() const { return m_value; }
+
+private:
+    int m_value;
+};
+
+void tst_benchmark::valueResolve()
+{
+    {   // should move the value when resolved by rvalue
+        Data::logs().reset();
+        QPromise<Data>([&](const QPromiseResolve<Data>& resolve) {
+            resolve(Data(42));
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 1);
+        QCOMPARE(Data::logs().copy, 0);
+        QCOMPARE(Data::logs().move, 1);     // move value to the promise data
+        QCOMPARE(Data::logs().refs, 0);
+    }
+    {   // should create one copy of the value when resolved by lvalue
+        Data::logs().reset();
+        QPromise<Data>([&](const QPromiseResolve<Data>& resolve) {
+            Data value(42);
+            resolve(value);
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 1);
+        QCOMPARE(Data::logs().copy, 1);     // copy value to the promise data
+        QCOMPARE(Data::logs().move, 0);
+        QCOMPARE(Data::logs().refs, 0);
+    }
+}
+
+void tst_benchmark::valueReject()
+{
+    {   // should not create any data if rejected
+        Data::logs().reset();
+        QPromise<Data>([&](const QPromiseResolve<Data>&, const QPromiseReject<Data>& reject) {
+            reject(QString("foo"));
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 0);
+        QCOMPARE(Data::logs().copy, 0);
+        QCOMPARE(Data::logs().move, 0);
+        QCOMPARE(Data::logs().refs, 0);
+    }
+}
+
+void tst_benchmark::valueThen()
+{
+    {   // should not copy value on continutation if fulfilled
+        int value = -1;
+        Data::logs().reset();
+        QPromise<Data>::resolve(Data(42)).then([&](const Data& res) {
+            value = res.value();
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 1);
+        QCOMPARE(Data::logs().copy, 0);
+        QCOMPARE(Data::logs().move, 1);     // move value to the promise data
+        QCOMPARE(Data::logs().refs, 0);
+        QCOMPARE(value, 42);
+    }
+    {   // should not create value on continutation if rejected
+        int value = -1;
+        QString error;
+        Data::logs().reset();
+        QPromise<Data>::reject(QString("foo")).then([&](const Data& res) {
+            value = res.value();
+        }, [&](const QString& err) {
+            error = err;
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 0);
+        QCOMPARE(Data::logs().copy, 0);
+        QCOMPARE(Data::logs().move, 0);     // move value to the promise data
+        QCOMPARE(Data::logs().refs, 0);
+        QCOMPARE(error, QString("foo"));
+        QCOMPARE(value, -1);
+    }
+    {   // should move the returned value when fulfilled
+        int value = -1;
+        Data::logs().reset();
+        QPromise<int>::resolve(42).then([&](int res) {
+            return Data(res+2);
+        }).then([&](const Data& res) {
+            value = res.value();
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 1);
+        QCOMPARE(Data::logs().copy, 0);
+        QCOMPARE(Data::logs().move, 1);     // move values to the next promise data
+        QCOMPARE(Data::logs().refs, 0);
+        QCOMPARE(value, 44);
+    }
+    {   // should not create any data if handler throws
+        Data::logs().reset();
+        QPromise<int>::resolve(42).then([&](int res) {
+            throw QString("foo");
+            return Data(res+2);
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 0);
+        QCOMPARE(Data::logs().copy, 0);
+        QCOMPARE(Data::logs().move, 0);
+        QCOMPARE(Data::logs().refs, 0);
+    }
+}
+
+void tst_benchmark::errorReject()
+{
+    {   // should create one copy of the error when rejected by rvalue
+        Data::logs().reset();
+        QPromise<int>([&](const QPromiseResolve<int>&, const QPromiseReject<int>& reject) {
+            reject(Data(42));
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 1);
+        QCOMPARE(Data::logs().copy, 1);     // copy value in std::exception_ptr
+        QCOMPARE(Data::logs().move, 0);
+        QCOMPARE(Data::logs().refs, 0);
+    }
+    {   // should create one copy of the error when rejected by lvalue (no extra copy)
+        Data::logs().reset();
+        QPromise<int>([&](const QPromiseResolve<int>&, const QPromiseReject<int>& reject) {
+            Data error(42);
+            reject(error);
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 1);
+        QCOMPARE(Data::logs().copy, 1);    // copy value to the promise data
+        QCOMPARE(Data::logs().move, 0);
+        QCOMPARE(Data::logs().refs, 0);
+    }
+}
+
+void tst_benchmark::errorThen()
+{
+    {   // should not copy error on continutation if rejected
+        int value = -1;
+        Data::logs().reset();
+        QPromise<void>::reject(Data(42)).fail([&](const Data& res) {
+            value = res.value();
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 1);
+        QCOMPARE(Data::logs().copy, 1);     // (initial) copy value in std::exception_ptr
+        QCOMPARE(Data::logs().move, 0);
+        QCOMPARE(Data::logs().refs, 0);
+        QCOMPARE(value, 42);
+    }
+    {   // should not copy error on continutation if rethrown
+        int value = -1;
+        Data::logs().reset();
+        QPromise<void>::reject(Data(42)).fail([](const Data&) {
+            throw;
+        }).fail([&](const Data& res) {
+            value = res.value();
+        }).wait();
+
+        QCOMPARE(Data::logs().ctor, 1);
+        QCOMPARE(Data::logs().copy, 1);     // (initial) copy value in std::exception_ptr
+        QCOMPARE(Data::logs().move, 0);
+        QCOMPARE(Data::logs().refs, 0);
+        QCOMPARE(value, 42);
+    }
+}