Create any callable using QRunnable::create

The overhead of making new custom classes appears to be less than
constructing a generic std::function.

[ChangeLog][QtCore][QRunnable] QRunnable::create can now take
non-copyable functions as argument.

Task-number: QTBUG-112302
Change-Id: Ied870f13ca6c7eaa14ed6eff9c4e676c7b73881c
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

3 files changed, 72 insertions, 20 deletions

diff --git a/src/corelib/compat/removed_api.cpp b/src/corelib/compat/removed_api.cpp
index fdbe5de41c..e1d9c9eccf 100644
--- a/src/corelib/compat/removed_api.cpp
+++ b/src/corelib/compat/removed_api.cpp
@@ -509,6 +509,13 @@ QByteArray QMessageAuthenticationCode::hash(const QByteArray &msg, const QByteAr
                 qToByteArrayViewIgnoringNull(key), method);
 }
 
+#include "qrunnable.h"
+
+QRunnable *QRunnable::create(std::function<void()> functionToRun)
+{
+    return QRunnable::create<std::function<void()>>(std::move(functionToRun));
+}
+
 #include "qstring.h"
 
 qsizetype QString::toUcs4_helper(const ushort *uc, qsizetype length, uint *out)
diff --git a/src/corelib/thread/qrunnable.cpp b/src/corelib/thread/qrunnable.cpp
index 5bb354cbb1..718802010a 100644
--- a/src/corelib/thread/qrunnable.cpp
+++ b/src/corelib/thread/qrunnable.cpp
@@ -77,31 +77,17 @@ QRunnable::~QRunnable()
     \sa autoDelete(), QThreadPool
 */
 
-class FunctionRunnable : public QRunnable
-{
-    std::function<void()> m_functionToRun;
-public:
-    FunctionRunnable(std::function<void()> functionToRun) : m_functionToRun(std::move(functionToRun))
-    {
-    }
-    void run() override
-    {
-        m_functionToRun();
-    }
-};
-
 /*!
+    \fn template<typename Callable> QRunnable *QRunnable::create(Callable &&callableToRun);
     \since 5.15
 
-    Creates a QRunnable that calls \a functionToRun in run().
+    Creates a QRunnable that calls \a callableToRun in run().
 
     Auto-deletion is enabled by default.
 
+    \note In Qt versions prior to 6.6, this method took copyable functions only.
+
     \sa run(), autoDelete()
 */
-QRunnable *QRunnable::create(std::function<void()> functionToRun)
-{
-    return new FunctionRunnable(std::move(functionToRun));
-}
 
 QT_END_NAMESPACE
diff --git a/src/corelib/thread/qrunnable.h b/src/corelib/thread/qrunnable.h
index c77f8eea0a..131efdedb7 100644
--- a/src/corelib/thread/qrunnable.h
+++ b/src/corelib/thread/qrunnable.h
@@ -1,4 +1,4 @@
-// Copyright (C) 2016 The Qt Company Ltd.
+// Copyright (C) 2023 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
 #ifndef QRUNNABLE_H
@@ -6,6 +6,7 @@
 
 #include <QtCore/qglobal.h>
 #include <functional>
+#include <type_traits>
 
 QT_BEGIN_NAMESPACE
 
@@ -19,12 +20,70 @@ public:
 
     constexpr QRunnable() noexcept = default;
     virtual ~QRunnable();
+#if QT_CORE_REMOVED_SINCE(6, 6)
     static QRunnable *create(std::function<void()> functionToRun);
-
+#endif
+    template <typename Callable>
+    static QRunnable *create(Callable &&functionToRun);
     bool autoDelete() const { return m_autoDelete; }
     void setAutoDelete(bool autoDelete) { m_autoDelete = autoDelete; }
+
+protected:
+    // Type erasure, to only instantiate a non-virtual class per Callable:
+    class QGenericRunnableHelperBase
+    {
+        using OpFn = void(*)(const QGenericRunnableHelperBase *);
+        OpFn runFn;
+        OpFn destroyFn;
+    protected:
+        constexpr explicit QGenericRunnableHelperBase(OpFn fn, OpFn del) noexcept : runFn(fn), destroyFn(del) {}
+        ~QGenericRunnableHelperBase() = default;
+    public:
+        void run() { runFn(this); }
+        void destroy() { destroyFn(this); }
+    };
+
+    template <typename Callable>
+    class QGenericRunnableHelper : public QGenericRunnableHelperBase
+    {
+        Callable m_functionToRun;
+    public:
+        template <typename UniCallable>
+        QGenericRunnableHelper(UniCallable &&functionToRun) noexcept :
+              QGenericRunnableHelperBase(
+                      [](const QGenericRunnableHelperBase *that) { static_cast<const QGenericRunnableHelper*>(that)->m_functionToRun(); },
+                      [](const QGenericRunnableHelperBase *that) { delete static_cast<const QGenericRunnableHelper*>(that); }),
+              m_functionToRun(std::forward<UniCallable>(functionToRun))
+        {
+        }
+    };
+};
+
+class QGenericRunnable : public QRunnable
+{
+    QGenericRunnableHelperBase *runHelper;
+public:
+    QGenericRunnable(QGenericRunnableHelperBase *runner) noexcept: runHelper(runner)
+    {
+    }
+    ~QGenericRunnable() override
+    {
+        runHelper->destroy();
+    }
+    void run() override
+    {
+        runHelper->run();
+    }
 };
 
+template <typename Callable>
+QRunnable *QRunnable::create(Callable &&functionToRun)
+{
+    return new QGenericRunnable(
+            new QGenericRunnableHelper<std::decay_t<Callable>>(
+                    std::forward<Callable>(functionToRun)));
+}
+
 QT_END_NAMESPACE
 
 #endif