Add helper template for metacall event creation

Setting up the args and types arrays is cumbersome and error prone, and
we do it at least twice in qtbase. Provide a central implementation as a
variadic template function, and make it exception-safe with a unique_ptr
(the destructor of QMetaCallEvent will destroy the cloned arguments).

Change-Id: I5ff400467928446264eaedddb394691e9e23d22e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

3 files changed, 32 insertions, 24 deletions

diff --git a/src/corelib/kernel/qcoreapplication.cpp b/src/corelib/kernel/qcoreapplication.cpp
index 7c238af700..064a323507 100644
--- a/src/corelib/kernel/qcoreapplication.cpp
+++ b/src/corelib/kernel/qcoreapplication.cpp
@@ -2850,6 +2850,13 @@ void QCoreApplication::requestPermission(const QPermission &requestedPermission,
 
     Q_ASSERT(slotObj);
 
+    // Used as the signalID in the metacall event and only used to
+    // verify that we are not processing an unrelated event, not to
+    // emit the right signal. So using a value that can never clash
+    // with any signal index. Clang doesn't like this to be a static
+    // member of the PermissionReceiver.
+    static constexpr ushort PermissionReceivedID = 0xffff;
+
     // If we have a context object, then we dispatch the permission response
     // asynchronously through a received object that lives in the same thread
     // as the context object. Otherwise we call the functor synchronously when
@@ -2860,11 +2867,12 @@ void QCoreApplication::requestPermission(const QPermission &requestedPermission,
         PermissionReceiver(QtPrivate::QSlotObjectBase *slotObject, const QObject *context)
             : slotObject(slotObject), context(context)
         {}
+
     protected:
         bool event(QEvent *event) override {
             if (event->type() == QEvent::MetaCall) {
                 auto metaCallEvent = static_cast<QMetaCallEvent *>(event);
-                if (metaCallEvent->id() == ushort(-1)) {
+                if (metaCallEvent->id() == PermissionReceivedID) {
                     Q_ASSERT(slotObject);
                     // only execute if context object is still alive
                     if (context)
@@ -2898,18 +2906,8 @@ void QCoreApplication::requestPermission(const QPermission &requestedPermission,
             permission.m_status = status;
 
             if (receiver) {
-                const int nargs = 2;
-                auto metaCallEvent = new QMetaCallEvent(slotObj, qApp, ushort(-1), nargs);
-                Q_CHECK_PTR(metaCallEvent);
-                void **args = metaCallEvent->args();
-                QMetaType *types = metaCallEvent->types();
-                const auto voidType = QMetaType::fromType<void>();
-                const auto permissionType = QMetaType::fromType<QPermission>();
-                types[0] = voidType;
-                types[1] = permissionType;
-                args[0] = nullptr;
-                args[1] = permissionType.create(&permission);
-                Q_CHECK_PTR(args[1]);
+                auto metaCallEvent = QMetaCallEvent::create(slotObj, qApp,
+                                                            PermissionReceivedID, permission);
                 qApp->postEvent(receiver, metaCallEvent);
             } else {
                 void *argv[] = { nullptr, &permission };
diff --git a/src/corelib/kernel/qobject_p.h b/src/corelib/kernel/qobject_p.h
index f9bb93f960..d56e9aa60c 100644
--- a/src/corelib/kernel/qobject_p.h
+++ b/src/corelib/kernel/qobject_p.h
@@ -402,6 +402,26 @@ public:
 
     ~QMetaCallEvent() override;
 
+    template<typename ...Args>
+    static QMetaCallEvent *create(QtPrivate::QSlotObjectBase *slotObj, const QObject *sender,
+                                  int signal_index, Args ...argv)
+    {
+        auto metaCallEvent = std::make_unique<QMetaCallEvent>(slotObj, sender,
+                                                              signal_index, int(1 + sizeof...(Args)));
+
+        void **args = metaCallEvent->args();
+        QMetaType *types = metaCallEvent->types();
+        const std::array<void *, sizeof...(Args) + 1> argp{ nullptr, std::addressof(argv)... };
+        const std::array metaTypes{ QMetaType::fromType<void>(), QMetaType::fromType<Args>()... };
+        for (size_t i = 0; i < sizeof...(Args) + 1; ++i) {
+            types[i] = metaTypes[i];
+            args[i] = types[i].create(argp[i]);
+            Q_CHECK_PTR(!i || args[i]);
+        }
+
+        return metaCallEvent.release();
+    }
+
     inline int id() const { return d.method_offset_ + d.method_relative_; }
     inline const void * const* args() const { return d.args_; }
     inline void ** args() { return d.args_; }
diff --git a/src/network/kernel/qhostinfo.cpp b/src/network/kernel/qhostinfo.cpp
index 2283766b97..1631e97652 100644
--- a/src/network/kernel/qhostinfo.cpp
+++ b/src/network/kernel/qhostinfo.cpp
@@ -107,18 +107,8 @@ void QHostInfoResult::postResultsReady(const QHostInfo &info)
     auto result = new QHostInfoResult(this);
     Q_CHECK_PTR(result);
 
-    const int nargs = 2;
-    auto metaCallEvent = new QMetaCallEvent(slotObj, nullptr, signal_index, nargs);
+    auto metaCallEvent = QMetaCallEvent::create(slotObj, nullptr, signal_index, info);
     Q_CHECK_PTR(metaCallEvent);
-    void **args = metaCallEvent->args();
-    QMetaType *types = metaCallEvent->types();
-    auto voidType = QMetaType::fromType<void>();
-    auto hostInfoType = QMetaType::fromType<QHostInfo>();
-    types[0] = voidType;
-    types[1] = hostInfoType;
-    args[0] = nullptr;
-    args[1] = hostInfoType.create(&info);
-    Q_CHECK_PTR(args[1]);
     qApp->postEvent(result, metaCallEvent);
 }