[PATCH v2 3/3] libcamera: controls: Expose string controls as `std::string_view`
Jacopo Mondi
jacopo.mondi at ideasonboard.com
Fri May 2 12:11:49 CEST 2025
Hi Barnabás
On Thu, May 01, 2025 at 11:58:18AM +0200, Barnabás Pőcze wrote:
> 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`.
I've been a bit in two minds about this, until I realized we already
require c++17 for applications.
My understanding is that using string_view is no different than
returning a Span<> when it comes to lifetime of the referenced data,
so if we're fine with Span<> we're fine with string_view.
And as pointed out by Barnabas, if a user wants a string, it can be
constructed easily with the same cost as we have today.
>
> Bug: https://bugs.libcamera.org/show_bug.cgi?id=256
> Signed-off-by: Barnabás Pőcze <barnabas.pocze at ideasonboard.com>
> ---
> include/libcamera/control_ids.h.in | 1 +
> include/libcamera/controls.h | 15 ++++++++-------
> src/apps/cam/capture_script.cpp | 2 +-
> src/apps/cam/main.cpp | 2 +-
> src/apps/common/dng_writer.cpp | 2 +-
> src/apps/qcam/cam_select_dialog.cpp | 6 +++---
> src/py/libcamera/py_helpers.cpp | 4 ++--
> test/controls/control_value.cpp | 4 ++--
> utils/codegen/controls.py | 2 +-
> 9 files changed, 20 insertions(+), 18 deletions(-)
>
> diff --git a/include/libcamera/control_ids.h.in b/include/libcamera/control_ids.h.in
> index 5d0594c68..41997f79f 100644
> --- a/include/libcamera/control_ids.h.in
> +++ b/include/libcamera/control_ids.h.in
> @@ -13,6 +13,7 @@
> #include <map>
> #include <stdint.h>
> #include <string>
> +#include <string_view>
>
> #include <libcamera/controls.h>
>
> diff --git a/include/libcamera/controls.h b/include/libcamera/controls.h
> index 2ae4ec3d4..5a707a387 100644
> --- a/include/libcamera/controls.h
> +++ b/include/libcamera/controls.h
> @@ -13,6 +13,7 @@
> #include <set>
> #include <stdint.h>
> #include <string>
> +#include <string_view>
> #include <unordered_map>
> #include <vector>
>
> @@ -96,7 +97,7 @@ struct control_type<float> {
> };
>
> template<>
> -struct control_type<std::string> {
> +struct control_type<std::string_view> {
> static constexpr ControlType value = ControlTypeString;
> static constexpr std::size_t size = 0;
> };
> @@ -138,7 +139,7 @@ public:
> #ifndef __DOXYGEN__
> template<typename T, 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::is_same<std::string_view, std::remove_cv_t<T>>::value,
> std::nullptr_t> = nullptr>
> ControlValue(const T &value)
> : type_(ControlTypeNone), numElements_(0)
> @@ -148,7 +149,7 @@ public:
> }
>
> template<typename T, std::enable_if_t<details::is_span<T>::value ||
> - std::is_same<std::string, std::remove_cv_t<T>>::value,
> + std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> std::nullptr_t> = nullptr>
> #else
> template<typename T>
> @@ -182,7 +183,7 @@ public:
>
> #ifndef __DOXYGEN__
> template<typename T, std::enable_if_t<!details::is_span<T>::value &&
> - !std::is_same<std::string, std::remove_cv_t<T>>::value,
> + !std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> std::nullptr_t> = nullptr>
> T get() const
> {
> @@ -193,7 +194,7 @@ public:
> }
>
> template<typename T, std::enable_if_t<details::is_span<T>::value ||
> - std::is_same<std::string, std::remove_cv_t<T>>::value,
> + std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> std::nullptr_t> = nullptr>
> #else
> template<typename T>
> @@ -210,7 +211,7 @@ public:
>
> #ifndef __DOXYGEN__
> template<typename T, std::enable_if_t<!details::is_span<T>::value &&
> - !std::is_same<std::string, std::remove_cv_t<T>>::value,
> + !std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> std::nullptr_t> = nullptr>
> void set(const T &value)
> {
> @@ -219,7 +220,7 @@ public:
> }
>
> template<typename T, std::enable_if_t<details::is_span<T>::value ||
> - std::is_same<std::string, std::remove_cv_t<T>>::value,
> + std::is_same<std::string_view, std::remove_cv_t<T>>::value,
> std::nullptr_t> = nullptr>
> #else
> template<typename T>
> diff --git a/src/apps/cam/capture_script.cpp b/src/apps/cam/capture_script.cpp
> index fdf82efc0..bc4c7c3a0 100644
> --- a/src/apps/cam/capture_script.cpp
> +++ b/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:
> diff --git a/src/apps/cam/main.cpp b/src/apps/cam/main.cpp
> index fa266eca6..157d238d7 100644
> --- a/src/apps/cam/main.cpp
> +++ b/src/apps/cam/main.cpp
> @@ -337,7 +337,7 @@ std::string CamApp::cameraName(const Camera *camera)
> */
> const auto &model = props.get(properties::Model);
Does this need to be a reference now ?
> if (model)
> - name = "'" + *model + "' ";
> + name.append("'").append(*model).append("' ");
> }
>
> name += "(" + camera->id() + ")";
> diff --git a/src/apps/common/dng_writer.cpp b/src/apps/common/dng_writer.cpp
> index ac4619511..8d57023e1 100644
> --- a/src/apps/common/dng_writer.cpp
> +++ b/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);
likewise
> if (model) {
> - TIFFSetField(tif, TIFFTAG_MODEL, model->c_str());
> + TIFFSetField(tif, TIFFTAG_MODEL, std::string(*model).c_str());
> /* \todo set TIFFTAG_UNIQUECAMERAMODEL. */
> }
>
> diff --git a/src/apps/qcam/cam_select_dialog.cpp b/src/apps/qcam/cam_select_dialog.cpp
> index 6b6d0713c..9f25ba091 100644
> --- a/src/apps/qcam/cam_select_dialog.cpp
> +++ b/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)
> - .value_or("Unknown");
> + const auto model = properties.get(libcamera::properties::Model)
> + .value_or("Unknown");
>
> - cameraModel_->setText(QString::fromStdString(model));
> + cameraModel_->setText(QString::fromUtf8(model.data(), model.length()));
> }
> diff --git a/src/py/libcamera/py_helpers.cpp b/src/py/libcamera/py_helpers.cpp
> index 1ad1d4c1a..8c55ef845 100644
> --- a/src/py/libcamera/py_helpers.cpp
> +++ b/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:
> diff --git a/test/controls/control_value.cpp b/test/controls/control_value.cpp
> index 5084fd0cf..032050a77 100644
> --- a/test/controls/control_value.cpp
> +++ b/test/controls/control_value.cpp
> @@ -318,7 +318,7 @@ protected:
> /*
> * String type.
> */
> - std::string string{ "libcamera" };
> + std::string_view string{ "libcamera" };
> value.set(string);
Here's one thing: we now require users to use string_view when setting
a control, and we anyway end up copying it's content into the
ControlValue.
Should we provide a set() overload for std::string ? Is it worth it ?
> if (value.isNone() || !value.isArray() ||
> value.type() != ControlTypeString ||
> @@ -327,7 +327,7 @@ protected:
> return TestFail;
> }
>
> - if (value.get<std::string>() != string) {
> + if (value.get<std::string_view>() != string) {
For get() instead I think it's fine always returning a view
> cerr << "Control value mismatch after setting to string" << endl;
> return TestFail;
> }
> diff --git a/utils/codegen/controls.py b/utils/codegen/controls.py
> index e51610481..9399727bd 100644
> --- a/utils/codegen/controls.py
> +++ b/utils/codegen/controls.py
> @@ -111,7 +111,7 @@ class Control(object):
> size = self.__data.get('size')
>
> if typ == 'string':
> - return 'std::string'
> + return 'std::string_view'
>
> if self.__size is None:
> return typ
> --
> 2.49.0
>
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