[libcamera-devel] [PATCH 2/2] libcamera: object: Support reference arguments in invokeMethod()

[Laurent Pinchart]

Invoking a method that takes a reference argument with
Object::invokeMethod() results in a compilation error:

../test/object-invoke.cpp:131:11: error: no matching member function for call to 'invokeMethod'
                object_.invokeMethod(&InvokedObject::methodWithReference,
                ~~~~~~~~^~~~~~~~~~~~
../include/libcamera/object.h:33:7: note: candidate template ignored: deduced conflicting types for parameter 'Args' (<const int &> vs. <int>)
        void invokeMethod(void (T::*func)(Args...), ConnectionType type, Args... args)

This is due to the fact that implicit type conversions (from value to
reference in this case) takes place after template argument type
deduction, during overload resolution. A similar issue would occur if
T::func took a long argument and invokeMethod() was called with an in
argument.

Fix this by specifying to sets of argument types in the invokeMethod()
template, one for the arguments to the invoked method, and one for the
arguments to invokeMethod() itself. The compiler can then first perform
type deduction separately, and implicit conversion in a second step.

Reported-by: Paul Elder <paul.elder at ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 include/libcamera/object.h | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/libcamera/object.h b/include/libcamera/object.h
index 86e0f7265865..21b70460b516 100644
--- a/include/libcamera/object.h
+++ b/include/libcamera/object.h
@@ -29,13 +29,15 @@ public:
 
 	void postMessage(std::unique_ptr<Message> msg);
 
-	template<typename T, typename... Args, typename std::enable_if<std::is_base_of<Object, T>::value>::type * = nullptr>
-	void invokeMethod(void (T::*func)(Args...), ConnectionType type, Args... args)
+	template<typename T, typename... FuncArgs, typename... Args,
+		 typename std::enable_if<std::is_base_of<Object, T>::value>::type * = nullptr>
+	void invokeMethod(void (T::*func)(FuncArgs...), ConnectionType type,
+			  Args... args)
 	{
 		T *obj = static_cast<T *>(this);
 		BoundMethodBase *method =
-			new BoundMemberMethod<T, Args...>(obj, this, func, type);
-		void *pack = new typename BoundMemberMethod<T, Args...>::PackType{ args... };
+			new BoundMemberMethod<T, FuncArgs...>(obj, this, func, type);
+		void *pack = new typename BoundMemberMethod<T, FuncArgs...>::PackType{ args... };
 
 		method->activatePack(pack, true);
 	}
-- 
Regards,

Laurent Pinchart