libcamera: base: bound_method: Move return value

Instead of copying, just move the returned value when the call is made
through an argument pack. This enables, e.g. `Object::invokeMethod()`
to be usable with functions returning types, such as`std::unique_ptr`,
that have no copy ctor/assignment. Since there are no other users of
the argument pack object, this is safe to do. Reference return types
are not supported, so a simple `std::move()` is sufficient.

Bug: https://bugs.libcamera.org/show_bug.cgi?id=273#c1
Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/base/bound_method.h

@@ -38,11 +38,6 @@ public:
 	{
 	}
 
-	R returnValue()
-	{
-		return ret_;
-	}
-
 	std::tuple<typename std::remove_reference_t<Args>...> args_;
 	R ret_;
 };
@@ -56,10 +51,6 @@ public:
 	{
 	}
 
-	void returnValue()
-	{
-	}
-
 	std::tuple<typename std::remove_reference_t<Args>...> args_;
 };
 
@@ -140,8 +131,10 @@ public:
 			return func_(args...);
 
 		auto pack = std::make_shared<PackType>(args...);
-		bool sync = BoundMethodBase::activatePack(pack, deleteMethod);
+		[[maybe_unused]] bool sync = BoundMethodBase::activatePack(pack, deleteMethod);
-		return sync ? pack->returnValue() : R();
+
+		if constexpr (!std::is_void_v<R>)
+			return sync ? std::move(pack->ret_) : R();
 	}
 
 	R invoke(Args... args) override
@@ -175,8 +168,10 @@ public:
 		}
 
 		auto pack = std::make_shared<PackType>(args...);
-		bool sync = BoundMethodBase::activatePack(pack, deleteMethod);
+		[[maybe_unused]] bool sync = BoundMethodBase::activatePack(pack, deleteMethod);
-		return sync ? pack->returnValue() : R();
+
+		if constexpr (!std::is_void_v<R>)
+			return sync ? std::move(pack->ret_) : R();
 	}
 
 	R invoke(Args... args) override

test/object-invoke.cpp

@@ -58,6 +58,19 @@ public:
 		return 42;
 	}
 
+	struct MoveOnly {
+		MoveOnly() = default;
+		MoveOnly(const MoveOnly &) = delete;
+		MoveOnly &operator=(const MoveOnly &) = delete;
+		MoveOnly(MoveOnly &&) = default;
+		MoveOnly &operator=(MoveOnly &&) = default;
+	};
+
+	MoveOnly methodWithReturnMoveOnly()
+	{
+		return {};
+	}
+
 private:
 	Status status_;
 	int value_;
@@ -186,6 +199,10 @@ protected:
 			return TestFail;
 		}
 
+		/* Test invoking a method that returns type with no copy ctor/assignment. */
+		object_.invokeMethod(&InvokedObject::methodWithReturnMoveOnly,
+				     ConnectionTypeBlocking);
+
 		return TestPass;
 	}