Avoid value copy when fulfilled from promise

src/qtpromise/qpromise.h

@@ -49,6 +49,7 @@ public: // STATIC
     inline static QPromise<T> reject(E&& error);
 
 protected:
+    friend struct QtPromisePrivate::PromiseFulfill<QPromise<T> >;
     friend class QPromiseResolve<T>;
     friend class QPromiseReject<T>;
 

src/qtpromise/qpromise.inl

@@ -12,28 +12,18 @@ public:
         : m_promise(new QPromise<T>(std::move(p)))
     { }
 
-    void operator()(const T& value) const
+    template <typename V>
+    void operator()(V&& value) const
     {
-        resolve(value);
+        Q_ASSERT(!m_promise.isNull());
+        if (m_promise->isPending()) {
+            m_promise->m_d->resolve(std::forward<V>(value));
+            m_promise->m_d->dispatch();
         }
-
-    void operator()(T&& value) const
-    {
-        resolve(std::move(value));
     }
 
 private:
     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 <>

src/qtpromise/qpromise_p.h

@@ -79,14 +79,18 @@ struct PromiseFulfill<QtPromise::QPromise<T> >
         const QtPromise::QPromiseResolve<T>& resolve,
         const QtPromise::QPromiseReject<T>& reject)
     {
-        promise.then(
-            [=](const T& value) {
-                resolve(value);
-            },
-            [=]() { // catch all
-                reject(std::current_exception());
+        if (promise.isFulfilled()) {
+            resolve(promise.m_d->value());
+        } else if (promise.isRejected()) {
+            reject(promise.m_d->error());
+        } else {
+            promise.then([=]() {
+                resolve(promise.m_d->value());
+            }, [=]() { // catch all
+                reject(promise.m_d->error());
             });
         }
+    }
 };
 
 template <>
@@ -98,14 +102,18 @@ struct PromiseFulfill<QtPromise::QPromise<void> >
         const TResolve& resolve,
         const TReject& reject)
     {
-        promise.then(
-            [=]() {
+        if (promise.isFulfilled()) {
             resolve();
-            },
-            [=]() { // catch all
-                reject(std::current_exception());
+        } else if (promise.isRejected()) {
+            reject(promise.m_d->error());
+        } else {
+            promise.then([=]() {
+                resolve();
+            }, [=]() { // catch all
+                reject(promise.m_d->error());
             });
         }
+    }
 };
 
 template <typename T, typename TRes>
@@ -377,6 +385,20 @@ public:
         setSettled();
     }
 
+    void reject(const QSharedPointer<Error>& error)
+    {
+        Q_ASSERT(isPending());
+        Q_ASSERT(m_error.isNull());
+        m_error = error;
+        this->setSettled();
+    }
+
+    const QSharedPointer<Error>& error() const
+    {
+        Q_ASSERT(isRejected());
+        return m_error;
+    }
+
     void dispatch()
     {
         if (isPending()) {
@@ -438,6 +460,7 @@ class PromiseData : public PromiseDataBase<T, void(const T&)>
 public:
     void resolve(T&& value)
     {
+        Q_ASSERT(this->isPending());
         Q_ASSERT(m_value.isNull());
         m_value.reset(new T(std::move(value)));
         this->setSettled();
@@ -445,11 +468,26 @@ public:
 
     void resolve(const T& value)
     {
+        Q_ASSERT(this->isPending());
         Q_ASSERT(m_value.isNull());
         m_value.reset(new T(value));
         this->setSettled();
     }
 
+    void resolve(const QSharedPointer<T>& value)
+    {
+        Q_ASSERT(this->isPending());
+        Q_ASSERT(m_value.isNull());
+        m_value = value;
+        this->setSettled();
+    }
+
+    const QSharedPointer<T>& value() const
+    {
+        Q_ASSERT(this->isFulfilled());
+        return m_value;
+    }
+
     void notify(const QVector<Handler>& handlers) Q_DECL_OVERRIDE
     {
         QSharedPointer<T> value(m_value);
@@ -473,7 +511,10 @@ class PromiseData<void> : public PromiseDataBase<void, void()>
     using Handler = typename PromiseDataBase<void, void()>::Handler;
 
 public:
-    void resolve() { setSettled(); }
+    void resolve()
+    {
+        setSettled();
+    }
 
 protected:
     void notify(const QVector<Handler>& handlers) Q_DECL_OVERRIDE

tests/auto/benchmark/tst_benchmark.cpp

@@ -14,6 +14,7 @@ private Q_SLOTS:
     void valueResolve();
     void valueReject();
     void valueThen();
+    void valueDelayed();
     void errorReject();
     void errorThen();
 
@@ -162,6 +163,36 @@ void tst_benchmark::valueThen()
     }
 }
 
+void tst_benchmark::valueDelayed()
+{
+    {   // should not copy the value on continutation if fulfilled
+        int value = -1;
+        Data::logs().reset();
+        QPromise<int>::resolve(42).then([&](int res) {
+            return QPromise<Data>::resolve(Data(res + 1));
+        }).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 input promise data
+        QCOMPARE(Data::logs().refs, 0);
+        QCOMPARE(value, 43);
+    }
+    {   // should not create value on continutation if rejected
+        Data::logs().reset();
+        QPromise<int>::resolve(42).then([&]() {
+            return QPromise<Data>::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::errorReject()
 {
     {   // should create one copy of the error when rejected by rvalue