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/apps/cam/capture_script.cpp

@@ -502,7 +502,7 @@ ControlValue CaptureScript::parseScalarControl(const ControlId *id,
 		break;
 	}
 	case ControlTypeString: {
-		value.set<std::string>(repr);
+		value.set<std::string_view>(repr);
 		break;
 	}
 	default:

src/apps/cam/main.cpp

@@ -345,7 +345,7 @@ std::string CamApp::cameraName(const Camera *camera)
 		 */
 		const auto &model = props.get(properties::Model);
 		if (model)
-			name += "'" + *model + "' ";
+			name.append("'").append(*model).append("' ");
 	}
 
 	name += "(" + camera->id() + ")";

src/apps/common/dng_writer.cpp

@@ -564,7 +564,7 @@ int DNGWriter::write(const char *filename, const Camera *camera,
 
 	const auto &model = cameraProperties.get(properties::Model);
 	if (model) {
-		TIFFSetField(tif, TIFFTAG_MODEL, model->c_str());
+		TIFFSetField(tif, TIFFTAG_MODEL, std::string(*model).c_str());
 		/* \todo set TIFFTAG_UNIQUECAMERAMODEL. */
 	}
 

src/apps/qcam/cam_select_dialog.cpp

@@ -114,8 +114,8 @@ void CameraSelectorDialog::updateCameraInfo(QString cameraId)
 		cameraLocation_->setText("Unknown");
 	}
 
-	const auto &model = properties.get(libcamera::properties::Model)
+	const auto model = properties.get(libcamera::properties::Model)
 				    .value_or("Unknown");
 
-	cameraModel_->setText(QString::fromStdString(model));
+	cameraModel_->setText(QString::fromUtf8(model.data(), model.length()));
 }

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: