libcamera: controls: Expose string controls as std::string_view

When retrieving the value from a `ControlValue` usually one of two
things happen: a small, trivially copyable object is returned by
value; or a view into the internal buffer is provided. This is true
for everything except strings, which are returned in `std::string`,
incurring the overhead of string construction.

To guarantee no potentially "expensive" copies, use `std::string_view`
pointing to the internal buffer to return the value. This is similar
to how other array-like types are returned with a `Span<>`.

This is an API break, but its scope is limited to just `properties::Model`.

Bug: https://bugs.libcamera.org/show_bug.cgi?id=256
Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/py/libcamera/py_helpers.cpp

@@ -47,7 +47,7 @@ py::object controlValueToPy(const ControlValue &cv)
 	case ControlTypeFloat:
 		return valueOrTuple<float>(cv);
 	case ControlTypeString:
-		return py::cast(cv.get<std::string>());
+		return py::cast(cv.get<std::string_view>());
 	case ControlTypeSize: {
 		const Size *v = reinterpret_cast<const Size *>(cv.data().data());
 		return py::cast(v);
@@ -88,7 +88,7 @@ ControlValue pyToControlValue(const py::object &ob, ControlType type)
 	case ControlTypeFloat:
 		return controlValueMaybeArray<float>(ob);
 	case ControlTypeString:
-		return ControlValue(ob.cast<std::string>());
+		return ControlValue(ob.cast<std::string_view>());
 	case ControlTypeRectangle:
 		return controlValueMaybeArray<Rectangle>(ob);
 	case ControlTypeSize: