libcamera: v4l2_device: Avoid copy of V4L2ControlInfo

The V4L2Device::listControls() method creates an instance of
V4L2ControlInfo and then inserts it in the device's list of controls.
The insertion uses std::map::emplace(), but passes the V4L2ControlInfo,
resulting in a copy being performed (using the copy constructor of
V4L2ControlInfo).

Optimise this by really constructing the V4L2ControlInfo in-place. The
use of std::piecewise_construct is required for gcc-5 that seems to have
trouble with std::map::emplace() on non-copyable values in this case.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Jacopo Mondi <jacopo@jmondi.org>
Tested-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

1 files changed, 5 insertions, 4 deletions

diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
index fd4b9c6d..3bd82ff2 100644
--- a/src/libcamera/v4l2_device.cpp
+++ b/src/libcamera/v4l2_device.cpp
@@ -352,8 +352,7 @@ void V4L2Device::listControls()
 		    ctrl.flags & V4L2_CTRL_FLAG_DISABLED)
 			continue;
 
-		V4L2ControlInfo info(ctrl);
-		switch (info.type()) {
+		switch (ctrl.type) {
 		case V4L2_CTRL_TYPE_INTEGER:
 		case V4L2_CTRL_TYPE_BOOLEAN:
 		case V4L2_CTRL_TYPE_MENU:
@@ -364,12 +363,14 @@ void V4L2Device::listControls()
 			break;
 		/* \todo Support compound controls. */
 		default:
-			LOG(V4L2, Debug) << "Control type '" << info.type()
+			LOG(V4L2, Debug) << "Control type '" << ctrl.type
 					 << "' not supported";
 			continue;
 		}
 
-		controls_.emplace(ctrl.id, info);
+		controls_.emplace(std::piecewise_construct,
+				  std::forward_as_tuple(ctrl.id),
+				  std::forward_as_tuple(ctrl));
 	}
 }