[PATCH v2 3/3] libcamera: controls: Expose string controls as `std::string_view`
Barnabás Pőcze
barnabas.pocze at ideasonboard.com
Fri May 2 12:22:13 CEST 2025
Hi
2025. 05. 02. 12:11 keltezéssel, Jacopo Mondi írta:
> 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 ?
I think that could be useful, although I would defer it because there is only
a single string-valued control (properties::Model). And if we want to support
multiple "input" types for a single control types, then I think there are
better ways but those need potentially more changes.
Regards,
Barnabás Pőcze
>
>> 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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