Don't copy the functor object for each signal emission

The behavior was different in the variadic template code and in the C++98
code. The code without variadic template support was not copying the functor
object (e.g. a lambda) before calling it.
However, in the variadic template section, QtPrivate::FunctorCall::call
took the functor by value instead of by reference resulting in a copy.

QtPrivate::FunctorCall::call is a helper function for
QtPrivate::FunctionPointer::call which is only needed for variadic template
expension.

[ChangeLog][QtCore][QObject] If the compiler supports variadic templates,
no longer copy functor connected to a signal each time the signal is
emitted. Restoring the C++98 behavior.

Task-number: QTBUG-52542
Change-Id: I3ca20ef6910893b8a288e70af7de4c7b69502173
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 20 insertions, 1 deletions

diff --git a/src/corelib/kernel/qobjectdefs_impl.h b/src/corelib/kernel/qobjectdefs_impl.h
index d5574a4dd1..922b6bbb42 100644
--- a/src/corelib/kernel/qobjectdefs_impl.h
+++ b/src/corelib/kernel/qobjectdefs_impl.h
@@ -491,7 +491,7 @@ namespace QtPrivate {
     template <typename, typename, typename, typename> struct FunctorCall;
     template <int... II, typename... SignalArgs, typename R, typename Function>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, Function> {
-        static void call(Function f, void **arg) {
+        static void call(Function &f, void **arg) {
             f((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]);
         }
     };
diff --git a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
index f3334b1222..5b89ef3792 100644
--- a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
+++ b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
@@ -147,6 +147,7 @@ private slots:
     void qmlConnect();
     void exceptions();
     void noDeclarativeParentChangedOnDestruction();
+    void mutableFunctor();
 };
 
 struct QObjectCreatedOnShutdown
@@ -6420,6 +6421,24 @@ void tst_QObject::noDeclarativeParentChangedOnDestruction()
 #endif
 }
 
+struct MutableFunctor {
+    int count;
+    MutableFunctor() : count(0) {}
+    int operator()() { return ++count; }
+};
+
+void tst_QObject::mutableFunctor()
+{
+    ReturnValue o;
+    MutableFunctor functor;
+    QCOMPARE(functor.count, 0);
+    connect(&o, &ReturnValue::returnInt, functor);
+    QCOMPARE(emit o.returnInt(0), 1);
+    QCOMPARE(emit o.returnInt(0), 2); // each emit should increase the internal count
+
+    QCOMPARE(functor.count, 0); // but the original object should have been copied at connect time
+}
+
 // Test for QtPrivate::HasQ_OBJECT_Macro
 Q_STATIC_ASSERT(QtPrivate::HasQ_OBJECT_Macro<tst_QObject>::Value);
 Q_STATIC_ASSERT(!QtPrivate::HasQ_OBJECT_Macro<SiblingDeleter>::Value);