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

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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>
This commit is contained in:
Barnabás Pőcze
2025-04-16 13:57:06 +02:00
parent b320b20db7
commit f84522d7cd
9 changed files with 19 additions and 17 deletions
Download Patch File Download Diff File Expand all files Collapse all files

1
include/libcamera/control_ids.h.in
View File

@@ -13,6 +13,7 @@
#include <map>
#include <stdint.h>
#include <string>
#include <string_view>
#include <libcamera/controls.h>

15
include/libcamera/controls.h
View File

@@ -12,6 +12,7 @@
#include <optional>
#include <stdint.h>
#include <string>
#include <string_view>
#include <unordered_map>
#include <vector>
@@ -95,7 +96,7 @@ 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;
};
@@ -137,7 +138,7 @@ 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)
@@ -147,7 +148,7 @@ 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>
@@ -181,7 +182,7 @@ 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
{
@@ -192,7 +193,7 @@ 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>
@@ -209,7 +210,7 @@ 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)
{
@@ -218,7 +219,7 @@ 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>

2
src/apps/cam/capture_script.cpp
View File

@@ -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:

2
src/apps/cam/main.cpp
View File

@@ -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() + ")";

2
src/apps/common/dng_writer.cpp
View File

@@ -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. */
}

4
src/apps/qcam/cam_select_dialog.cpp
View File

@@ -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()));
}

4
src/py/libcamera/py_helpers.cpp
View File

@@ -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:

4
test/controls/control_value.cpp
View File

@@ -318,7 +318,7 @@ protected:
/*
* String type.
*/
std::string string{ "libcamera" };
std::string_view string{ "libcamera" };
value.set(string);
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) {
cerr << "Control value mismatch after setting to string" << endl;
return TestFail;
}

2
utils/codegen/controls.py
View File

@@ -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