[libcamera-devel] [PATCH] libcamera: controls: Disable ControlValue<T> construction from unsupported T

[Jacopo Mondi]

Hi Laurent,

On Sat, Oct 24, 2020 at 01:00:20AM +0300, Laurent Pinchart wrote:
> The ControlValue<T> constructor for non-array values is a template
> function that participates in overload resolution for all T types that
> are not Span or std::string. Other T types that are not supported then
> result in a compilation error.
>
> This causes issues when calling an overloaded function that can accept
> both a ControlValue and a Span with an std::array<U> parameter. The
> first overload will be resolved using implicit construction of a
> ControlValue from the std::array<U>, while the second overload will be
> resolved using implicit construction of a Span<U> from the
> std::array<U>. This results in a compilation error due to an ambiguous
> function call.
>
> The first overload is invalid, selecting it would result in a
> compilation error in the ControlValue constructor, as the
> ControlValue<T> constructor doesn't support std::array<U> for type T.
> The compiler can't know about that, as overload resolution happens
> earlier.

mind->bend();

:)

>
> To fix it, we can disable the ControlValue<T> constructor for
> unsupported types T, moving the type check from compilation of the
> function to overload resolution. The constructor will not participate in
> overload resolution, and the call won't be ambiguous. The end result is
> the same for unsupported types, compilation will fail.
>
> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

Thanks, this should solve the issue I had with ControlInfo()
constructor that took a Span<ControlValue>

Reviewed-by: Jacopo Mondi <jacopo at jmondi.org>

Thanks
  j

> ---
>  include/libcamera/controls.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/include/libcamera/controls.h b/include/libcamera/controls.h
> index 3d7f4b0dc1a7..3b7f3347761e 100644
> --- a/include/libcamera/controls.h
> +++ b/include/libcamera/controls.h
> @@ -97,6 +97,7 @@ public:
>
>  #ifndef __DOXYGEN__
>  	template<typename T, typename std::enable_if_t<!details::is_span<T>::value &&
> +						       details::control_type<T>::value &&
>  						       !std::is_same<std::string, std::remove_cv_t<T>>::value,
>  						       std::nullptr_t> = nullptr>
>  	ControlValue(const T &value)
> --
> Regards,
>
> Laurent Pinchart
>