[libcamera-devel] [PATCH] libcamera: object: Avoid argument copies in invokeMethod()

[Laurent Pinchart]

Template argument deduction results in the lvalue and lvalue reference
arguments to the invokeMethod() function causing deduction of the Args
template type to a non-reference type. This results in the argument
being passed by value and copied.

Fix this by using a cv-unqualified rvalue reference parameter type. The
type is then deduced to an lvalue reference when the argument is an
lvalue or lvalue reference, due to a combination of the special template
argument deduction rule for rvalue reference parameter types:

  If P is an rvalue reference to a cv-unqualified template parameter
  (so-called forwarding reference), and the corresponding function call
  argument is an lvalue, the type lvalue reference to A is used in place
  of A for deduction.

  (https://en.cppreference.com/w/cpp/language/template_argument_deduction)

and the reference collapsing rule
(https://en.cppreference.com/w/cpp/language/reference#Reference_collapsing).

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 include/libcamera/base/object.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/libcamera/base/object.h b/include/libcamera/base/object.h
index 5c385ab4b140..ae2a275caf68 100644
--- a/include/libcamera/base/object.h
+++ b/include/libcamera/base/object.h
@@ -34,7 +34,7 @@ public:
 	template<typename T, typename R, typename... FuncArgs, typename... Args,
 		 typename std::enable_if_t<std::is_base_of<Object, T>::value> * = nullptr>
 	R invokeMethod(R (T::*func)(FuncArgs...), ConnectionType type,
-		       Args... args)
+		       Args&&... args)
 	{
 		T *obj = static_cast<T *>(this);
 		auto *method = new BoundMethodMember<T, R, FuncArgs...>(obj, this, func, type);

base-commit: d9a2a1f703273a28b001dee40fddd378bba7a1b6
-- 
Regards,

Laurent Pinchart