[PATCH 1/3] libcamera: Implement YamlEmitter

[Laurent Pinchart]

On Fri, Nov 29, 2024 at 09:35:06AM +0100, Jacopo Mondi wrote:
> On Wed, Nov 27, 2024 at 08:04:06AM +0200, Laurent Pinchart wrote:
> > On Wed, Nov 06, 2024 at 06:58:51PM +0100, Jacopo Mondi wrote:
> > > Implement a helpers to allow outputting text in YAML format.
> >
> > s/helpers/helper/
> >
> > > The class of helpers allows to create list and dictionaries and emit
> >
> > s/list/lists/
> >
> > > scalar values starting from a YamlRoot object initialized with
> > > a file path.
> > >
> > > Signed-off-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>
> > > ---
> > >  include/libcamera/internal/meson.build    |   1 +
> > >  include/libcamera/internal/yaml_emitter.h | 164 ++++++
> > >  src/libcamera/meson.build                 |   1 +
> > >  src/libcamera/yaml_emitter.cpp            | 577 ++++++++++++++++++++++
> > >  4 files changed, 743 insertions(+)
> > >  create mode 100644 include/libcamera/internal/yaml_emitter.h
> > >  create mode 100644 src/libcamera/yaml_emitter.cpp
> > >
> > > diff --git a/include/libcamera/internal/meson.build b/include/libcamera/internal/meson.build
> > > index 1c5eef9cab80..7533b075fde2 100644
> > > --- a/include/libcamera/internal/meson.build
> > > +++ b/include/libcamera/internal/meson.build
> > > @@ -41,6 +41,7 @@ libcamera_internal_headers = files([
> > >      'v4l2_pixelformat.h',
> > >      'v4l2_subdevice.h',
> > >      'v4l2_videodevice.h',
> > > +    'yaml_emitter.h',
> > >      'yaml_parser.h',
> > >  ])
> > >
> > > diff --git a/include/libcamera/internal/yaml_emitter.h b/include/libcamera/internal/yaml_emitter.h
> > > new file mode 100644
> > > index 000000000000..78196a7f147f
> > > --- /dev/null
> > > +++ b/include/libcamera/internal/yaml_emitter.h
> > > @@ -0,0 +1,164 @@
> > > +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> > > +/*
> > > + * Copyright (C) 2024, Ideas On Board Oy
> > > + *
> > > + * libcamera YAML emitter helper
> > > + */
> > > +
> > > +#pragma once
> > > +
> > > +#include <memory>
> > > +#include <string>
> > > +#include <string_view>
> > > +
> > > +#include <libcamera/base/class.h>
> > > +#include <libcamera/base/file.h>
> > > +
> > > +#include <yaml.h>
> > > +
> > > +namespace libcamera {
> > > +
> > > +class YamlDict;
> > > +class YamlEvent;
> > > +class YamlList;
> > > +class YamlRoot;
> > > +class YamlScalar;
> > > +
> > > +class YamlEmitter final
> > > +{
> > > +public:
> > > +	~YamlEmitter();
> > > +
> > > +	static YamlRoot root(const std::string &path);
> > > +
> > > +private:
> > > +	friend class YamlOutput;
> > > +	friend class YamlRoot;
> > > +
> > > +	LIBCAMERA_DISABLE_COPY(YamlEmitter)
> > > +
> > > +	YamlEmitter(const std::string &path);
> > > +
> > > +	void logError();
> > > +	void init();
> > > +	int emit();
> > > +
> > > +	File file_;
> > > +	yaml_event_t event_;
> > > +	yaml_emitter_t emitter_;
> > > +};
> > > +
> > > +class YamlOutput
> > > +{
> > > +public:
> > > +	bool valid() const { return !!emitter_; }
> > > +
> > > +protected:
> > > +	YamlOutput() = default;
> > > +
> > > +	YamlOutput(YamlEmitter *emitter)
> > > +		: emitter_(emitter)
> > > +	{
> > > +	}
> > > +
> > > +	YamlOutput &operator=(YamlOutput &&other)
> > > +	{
> > > +		emitter_ = other.emitter_;
> > > +		other.emitter_ = nullptr;
> > > +
> > > +		return *this;
> > > +	}
> > > +
> > > +	int emitScalar(std::string_view scalar);
> > > +	int emitMappingStart();
> > > +	int emitMappingEnd();
> > > +	int emitSequenceStart();
> > > +	int emitSequenceEnd();
> > > +
> > > +	YamlScalar scalar();
> > > +	YamlDict dict();
> > > +	YamlList list();
> > > +
> > > +	YamlEmitter *emitter_ = nullptr;
> > > +	yaml_event_t event_;
> > > +
> > > +private:
> > > +	LIBCAMERA_DISABLE_COPY(YamlOutput)
> > > +};
> > > +
> > > +class YamlRoot : public YamlOutput
> > > +{
> > > +public:
> > > +	YamlRoot() = default;
> > > +	~YamlRoot();
> > > +
> > > +	YamlRoot &operator=(YamlRoot &&other) = default;
> > > +
> > > +	YamlList list();
> > > +	YamlDict dict();
> > > +
> > > +private:
> > > +	LIBCAMERA_DISABLE_COPY(YamlRoot);
> > > +
> > > +	friend class YamlEmitter;
> > > +
> > > +	YamlRoot(std::unique_ptr<YamlEmitter> emitter)
> > > +		: YamlOutput(emitter.get()), emitterRoot_(std::move(emitter))
> > > +	{
> > > +	}
> > > +
> > > +	std::unique_ptr<YamlEmitter> emitterRoot_;
> > > +};
> > > +
> > > +class YamlScalar : public YamlOutput
> > > +{
> > > +public:
> > > +	YamlScalar() = default;
> > > +	~YamlScalar() = default;
> > > +
> > > +	void operator=(std::string_view scalar);
> > > +
> > > +private:
> > > +	friend class YamlOutput;
> > > +
> > > +	YamlScalar(YamlEmitter *emitter);
> > > +};
> > > +
> > > +class YamlList : public YamlOutput
> > > +{
> > > +public:
> > > +	YamlList() = default;
> > > +	~YamlList();
> > > +
> > > +	YamlList &operator=(YamlList &&other) = default;
> > > +
> > > +	YamlList list();
> > > +	YamlDict dict();
> > > +	void scalar(std::string_view scalar);
> > > +
> > > +private:
> > > +	friend class YamlOutput;
> > > +
> > > +	YamlList(YamlEmitter *emitter);
> > > +};
> > > +
> > > +class YamlDict : public YamlOutput
> > > +{
> > > +public:
> > > +	YamlDict() = default;
> > > +	~YamlDict();
> > > +
> > > +	YamlDict &operator=(YamlDict &&other) = default;
> > > +
> > > +	YamlList list(std::string_view key);
> > > +	YamlDict dict(std::string_view key);
> > > +
> > > +	YamlScalar operator[](std::string_view key);
> > > +
> > > +private:
> > > +	friend class YamlOutput;
> > > +
> > > +	YamlDict(YamlEmitter *emitter);
> > > +};
> > > +
> > > +} /* namespace libcamera */
> > > diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
> > > index aa9ab0291854..5b8af4103085 100644
> > > --- a/src/libcamera/meson.build
> > > +++ b/src/libcamera/meson.build
> > > @@ -51,6 +51,7 @@ libcamera_internal_sources = files([
> > >      'v4l2_pixelformat.cpp',
> > >      'v4l2_subdevice.cpp',
> > >      'v4l2_videodevice.cpp',
> > > +    'yaml_emitter.cpp',
> > >      'yaml_parser.cpp',
> > >  ])
> > >
> > > diff --git a/src/libcamera/yaml_emitter.cpp b/src/libcamera/yaml_emitter.cpp
> > > new file mode 100644
> > > index 000000000000..fa3de91ce988
> > > --- /dev/null
> > > +++ b/src/libcamera/yaml_emitter.cpp
> > > @@ -0,0 +1,577 @@
> > > +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> > > +/*
> > > + * Copyright (C) 2024, Ideas On Board Oy
> > > + *
> > > + * libcamera YAML emitter helper
> > > + */
> > > +
> > > +#include "libcamera/internal/yaml_emitter.h"
> > > +
> > > +#include <libcamera/base/log.h>
> >
> > Missing
> >
> > #include <libcamera/base/span.h>
> >
> > You should also include errno.h.
> >
> > > +
> > > +/**
> > > + * \file yaml_emitter.h
> > > + * \brief A YAML emitter helper
> > > + *
> > > + * The YAML emitter helpers allow users to emit output in YAML format.
> >
> > I would outline here the key difference with a YAML writer.
> >
> >  * Unlike YAML writers that operate on a fully populated representation of the
> >  * data, the YAML emitter outputs YAML data on the fly. It is suitable for
> >  * outputting large amount of data with low overhead and no runtime heap
> >  * allocation.
> 
> I like this
> 
> > > + *
> > > + * To emit YAML users of the these helper classes create a root node with
> > > + *
> > > + * \code
> > > +	std::string filePath("...");
> > > +	auto root = YamlEmitter::root(filePath);
> > > +   \endcode
> > > + *
> > > + * and start populating it with dictionaries and lists with the YamlRoot::dict()
> > > + * and YamlRoot::list() functions.
> >
> > I would write "and start emitting dictionaries and lists", "populating"
> > sounds more like a YAML writer.
> >
> ditto
> 
> > > + *
> > > + * The classes part of this file implement RAII-style handling of YAML
> > > + * events. By creating a YamlList and YamlDict instance the associated YAML
> > > + * sequence start and mapping start events are emitted and once the instances
> > > + * gets destroyed the corresponding sequence end and mapping end events are
> > > + * emitted.
> > > + *
> > > + * From an initialized YamlRoot instance is possible to create YAML list and
> > > + * dictionaries.
> > > + *
> > > + * \code
> > > +	YamlDict dict = root.dict();
> > > +	YamlList list = root.list();
> > > +   \endcode
> > > + *
> > > + * YamlDict instances can be populated with scalars associated with a key
> > > + *
> > > + * \code
> > > +	dict["key"] = "value";
> > > +   \endcode
> > > + *
> > > + * and it is possible to create lists and dictionaries, associated with a key
> > > + *
> > > + * \code
> > > +	YamlDict subDict = dict.dict("newDict");
> > > +	YamlList subList = dict.list("newList");
> > > +   \endcode
> > > + *
> > > + * YamlList instances can be populated with scalar elements
> > > + *
> > > + * \code
> > > +	list.scalar("x");
> > > +	list.scalar("y");
> > > +   \endcode
> > > + *
> > > + * and with dictionaries and lists too
> > > + *
> > > + * \code
> > > +	YamlDict subDict = list.dict();
> > > +	YamlList subList = list.list();
> > > +   \endcode
> >
> > The biggest issue with the API is that it can be easily misused:
> >
> > 	YamlDict list1 = dict.list("foo");
> > 	YamlDict list2 = dict.list("bar");
> > 	list1.scalar("0");
> >
> > I'm still annoyed that I haven't found a neat way to prevent this. I
> > wonder if we could make it a bit safer by at least catching incorrect
> > usage at runtime. What I'm thinking is
> 
> Yes, unfortunately I don't think we can easily catch this at build
> time
> 
> > - When creating a child, recording the child pointer in the parent, and
> >   the parent pointer in the child
> > - If the parent already has a child when a new child is created, reset
> >   the parent pointer of the previous child. This makes the previous
> >   child invalid.
> > - Any invalidation of a child needs to be propagated to its children.
> > - Any operation on an invalid child would be caught and logged.
> > - When a child is destroyed, if its parent is not null, reset the child
> >   pointer in the parent to null.
> > - If a parent is destroyed while it has a child pointer, reset the
> >   child's parent pointer to null and log an error.
> >
> 
> That should be enough, I wonder if we can make access to an invalid
> childer a compiler error...

I'd love to, but haven't found a nice way to do so :-S

> > > + */
> > > +
> > > +namespace libcamera {
> > > +
> > > +LOG_DEFINE_CATEGORY(YamlEmitter)
> > > +
> > > +namespace {
> > > +
> > > +int yamlWrite(void *data, unsigned char *buffer, size_t size)
> > > +{
> > > +	File *file = static_cast<File *>(data);
> > > +
> > > +	Span<unsigned char> buf{ buffer, size };
> > > +	ssize_t ret = file->write(buf);
> > > +	if (ret < 0) {
> > > +		LOG(YamlEmitter, Error) << "Write error : " << strerror(ret);
> >
> > s/error :/error:/
> >
> > > +		return 0;
> > > +	}
> > > +
> > > +	return 1;
> > > +}
> > > +
> > > +} /* namespace */
> > > +
> > > +/**
> > > + * \class YamlEmitter
> > > + *
> > > + * YAML helper classes entry point. This class allows to create a YamlRoot
> > > + * instances, using the YamlEmitter::root() function, that users can populate
> >
> > s/instances/instance/
> >
> > > + * with lists, dictionaries and scalars.
> > > + */
> > > +
> > > +YamlEmitter::YamlEmitter(const std::string &path)
> > > +{
> > > +	std::string filePath(path);
> > > +	file_.setFileName(filePath);
> >
> > I don't think you need the local variable, you can write
> >
> > 	file_.setFileName(path);
> >
> > > +	file_.open(File::OpenModeFlag::WriteOnly);
> > > +}
> > > +
> > > +/**
> > > + * \brief Destroy the YamlEmitter
> > > + */
> >
> > I think you can drop this, the documentation doesn't bring much, and the
> > function isn't part of the API exposed to users anyway.
> 
> Isn't it a public function ??

It is public, but isn't called by users. The YamlEmitter instance is
created internally by YamlEmitter::root(), and stored in the YamlRoot
class. The user never interacts with the YamlEmitter instance directly.

> > > +YamlEmitter::~YamlEmitter()
> > > +{
> > > +	yaml_event_delete(&event_);
> > > +	yaml_emitter_delete(&emitter_);
> > > +}
> > > +
> > > +/**
> > > + * \brief Create an initialized instance of YamlRoot
> > > + * \param[in] path The YAML output file path
> > > + *
> > > + * Create an initialized instance of the YamlRoot class that users can start
> > > + * using and populating with scalers, lists and dictionaries.
> >
> > s/scalers/scalars/
> >
> > > + *
> > > + * \return An initialized YamlRoot instance
> > > + */
> > > +YamlRoot YamlEmitter::root(const std::string &path)
> >
> > I asked in the review of the preview version if we should pass a
> > std::ostream to make this more generic. I'm fine ignoring it for now, as
> > it's unclear if we would have use cases for that.
> 
> I might have missed this. I recall I had a string_view most probably
> here. Let's post-pone it for when we have such a use case.
> 
> > > +{
> > > +	std::unique_ptr<YamlEmitter> emitter{ new YamlEmitter(path) };
> > > +
> > > +	emitter->init();
> > > +
> > > +	return YamlRoot(std::move(emitter));
> > > +}
> > > +
> > > +void YamlEmitter::logError()
> > > +{
> > > +	switch (emitter_.error) {
> > > +	case YAML_MEMORY_ERROR:
> > > +		LOG(YamlEmitter, Error)
> > > +			<< "Memory error: Not enough memory for emitting";
> > > +		break;
> > > +
> > > +	case YAML_WRITER_ERROR:
> > > +		LOG(YamlEmitter, Error)
> > > +			<< "Writer error: " << emitter_.problem;
> > > +		break;
> > > +
> > > +	case YAML_EMITTER_ERROR:
> > > +		LOG(YamlEmitter, Error)
> > > +			<< "Emitter error: " << emitter_.problem;
> > > +		break;
> > > +
> > > +	default:
> > > +		LOG(YamlEmitter, Error) << "Internal problem";
> >
> > "Internal error" ?
> 
> ack
> 
> > > +		break;
> > > +	}
> > > +}
> > > +
> > > +void YamlEmitter::init()
> > > +{
> > > +	yaml_emitter_initialize(&emitter_);
> > > +	yaml_emitter_set_output(&emitter_, yamlWrite, &file_);
> > > +
> > > +	yaml_stream_start_event_initialize(&event_, YAML_UTF8_ENCODING);
> > > +	emit();
> > > +
> > > +	yaml_document_start_event_initialize(&event_, NULL, NULL, NULL, 0);
> >
> > nullptr ? Same elsewhere.
> >
> > > +	emit();
> > > +}
> > > +
> > > +int YamlEmitter::emit()
> > > +{
> > > +	int ret = yaml_emitter_emit(&emitter_, &event_);
> > > +	if (!ret) {
> > > +		logError();
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +/**
> > > + * \class YamlOutput
> > > + *
> > > + * The YamlOutput base class. From this class are derived the YamlScalar,
> > > + * YamlList and YamlDict classes which are meant to be used by users of the
> > > + * YAML emitter helpers.
> > > + *
> > > + * The YamlOutput base class provides functions to create YAML lists and
> > > + * dictionaries and to populate them.
> > > + *
> > > + * Instances of this class cannot be instantiated directly by applications.
> >
> > s/Instances of this/This/
> > s/ by applications// as none of this is exposed to applications :-)
> 
> Yeah, I mean users of this class
> 
> > > + */
> > > +
> > > +/**
> > > + * \fn YamlOutput::valid()
> > > + * \brief Check if a YamlOutput instance has been correctly initialized
> > > + * \return True if the instance has been initialized, false otherwise
> > > + */
> >
> > We usually document constructors first.
> >
> > > +
> > > +/**
> > > + * \fn YamlOutput::YamlOutput(YamlEmitter *emitter)
> > > + * \brief Create a YamlOutput instance with an associated emitter
> > > + * \param[in] emitter The YAML emitter
> > > + */
> > > +
> > > +/**
> > > + * \fn YamlOutput &YamlOutput::operator=(YamlOutput &&other)
> > > + * \brief The move-assignment operator
> > > + * \param[in] other The instance to be moved
> > > + */
> > > +
> > > +/**
> > > + * \brief Emit \a scalar as a YAML scalar
> > > + * \param[in] scalar The element to emit
> > > + * \return 0 in case of success, a negative error value otherwise
> > > + */
> > > +int YamlOutput::emitScalar(std::string_view scalar)
> > > +{
> > > +	if (!valid())
> > > +		return -EINVAL;
> > > +
> > > +	const yaml_char_t *value = reinterpret_cast<const yaml_char_t *>
> > > +				   (scalar.data());
> > > +	yaml_scalar_event_initialize(&emitter_->event_, NULL, NULL, value,
> > > +				     scalar.length(), true, false,
> > > +				     YAML_PLAIN_SCALAR_STYLE);
> > > +	return emitter_->emit();
> > > +}
> > > +
> > > +/**
> > > + * \brief Emit the mapping start YAML event
> > > + * \return 0 in case of success, a negative error value otherwise
> > > + */
> > > +int YamlOutput::emitMappingStart()
> > > +{
> > > +	if (!valid())
> > > +		return -EINVAL;
> > > +
> > > +	yaml_mapping_start_event_initialize(&emitter_->event_, NULL, NULL,
> > > +					    true, YAML_BLOCK_MAPPING_STYLE);
> > > +	return emitter_->emit();
> > > +}
> > > +
> > > +/**
> > > + * \brief Emit the mapping end YAML event
> > > + * \return 0 in case of success, a negative error value otherwise
> > > + */
> > > +int YamlOutput::emitMappingEnd()
> > > +{
> > > +	if (!valid())
> > > +		return -EINVAL;
> > > +
> > > +	yaml_mapping_end_event_initialize(&emitter_->event_);
> > > +	return emitter_->emit();
> > > +}
> > > +
> > > +/**
> > > + * \brief Emit the sequence start YAML event
> > > + * \return 0 in case of success, a negative error value otherwise
> > > + */
> > > +int YamlOutput::emitSequenceStart()
> > > +{
> > > +	if (!valid())
> > > +		return -EINVAL;
> > > +
> > > +	yaml_sequence_start_event_initialize(&emitter_->event_, NULL, NULL,
> > > +					     true, YAML_BLOCK_SEQUENCE_STYLE);
> > > +	return emitter_->emit();
> > > +}
> > > +
> > > +/**
> > > + * \brief Emit the sequence end YAML event
> > > + * \return 0 in case of success, a negative error value otherwise
> > > + */
> > > +int YamlOutput::emitSequenceEnd()
> > > +{
> > > +	if (!valid())
> > > +		return -EINVAL;
> > > +
> > > +	yaml_sequence_end_event_initialize(&emitter_->event_);
> > > +	return emitter_->emit();
> > > +}
> > > +
> > > +/**
> > > + * \brief Create a scalar instance
> > > + * \return An instance of YamlScalar
> > > + */
> > > +YamlScalar YamlOutput::scalar()
> > > +{
> > > +	return YamlScalar(emitter_);
> > > +}
> > > +
> > > +/**
> > > + * \brief Create a dictionary instance
> > > + * \return An instance of YamlDict
> > > + */
> > > +YamlDict YamlOutput::dict()
> > > +{
> > > +	return YamlDict(emitter_);
> > > +}
> > > +
> > > +/**
> > > + * \brief Create a list instance
> > > + * \return An instance of YamlList
> > > + */
> > > +YamlList YamlOutput::list()
> > > +{
> > > +	return YamlList(emitter_);
> > > +}
> > > +
> > > +/**
> > > + * \var YamlOutput::emitter_
> > > + * \brief The emitter used by this YamlObject to output YAML events
> > > + */
> > > +
> > > +/**
> > > + * \var YamlOutput::event_
> > > + * \brief The YAML event used by this YamlObject
> > > + */
> > > +
> > > +/**
> > > + * \class YamlRoot
> > > + *
> > > + * The YAML root node. A valid YamlRoot instance can only be created using the
> > > + * YamlEmitter::root() function. The typical initialization pattern of users of
> > > + * this class is similar to the one in the following example:
> > > + *
> > > + * \code
> > > +	class YamlUser
> > > +	{
> > > +	public:
> > > +		YamlUser();
> > > +
> > > +	private:
> > > +		YamlRool root_;
> > > +	};
> > > +
> > > +	YamlUser::YamlUser()
> > > +	{
> > > +		root_ = YamlEmitter::root("/path/to/yaml/file.yml");
> > > +	}
> > > +   \endcode
> > > + *
> > > + * A YamlRoot element can be populated with list and dictionaries.
> > > + */
> > > +
> > > +/**
> > > + * \fn YamlRoot::YamlRoot()
> > > + * \brief Construct a YamlRoot instance without initializing it
> > > + *
> > > + * A YamlRoot instance can be created in non-initialized state typically to be
> > > + * stored as a class member by the users of this class. In order to start using
> > > + * and populating the YamlRoot instance a valid and initialized instance,
> > > + * created using the YamlEmitter::root() function, has to be move-assigned to
> > > + * the non-initialized instance.
> > > + *
> > > + * \code
> > > +	YamlRoot root;
> > > +
> > > +	root = YamlEmitter::root("/path/to/yaml/file.yml");
> > > +   \endcode
> > > + */
> > > +
> > > +/**
> > > + * \brief Delete a YamlRoot
> > > + */
> > > +YamlRoot::~YamlRoot()
> > > +{
> > > +	if (!valid())
> > > +		return;
> > > +
> > > +	yaml_document_end_event_initialize(&emitter_->event_, 0);
> > > +	emitterRoot_->emit();
> > > +
> > > +	yaml_stream_end_event_initialize(&emitter_->event_);
> > > +	emitterRoot_->emit();
> > > +}
> > > +
> > > +/**
> > > + * \fn YamlRoot &YamlRoot::operator=(YamlRoot &&other)
> > > + * \brief Move assignment operator
> > > + *
> > > + * Move-assign a YamlRoot instance. This function is typically used to assign an
> > > + * initialized instance returned by YamlEmitter::root() to a non-initialized
> > > + * one.
> > > + *
> > > + * \return A reference to this instance of YamlRoot
> > > + */
> > > +
> > > +/**
> > > + * \copydoc YamlOutput::dict()
> > > + */
> > > +YamlDict YamlRoot::dict()
> > > +{
> > > +	int ret = emitMappingStart();
> > > +	if (ret)
> > > +		return {};
> > > +
> > > +	return YamlOutput::dict();
> > > +}
> > > +
> > > +/**
> > > + * \copydoc YamlOutput::list()
> > > + */
> > > +YamlList YamlRoot::list()
> > > +{
> > > +	int ret = emitSequenceStart();
> > > +	if (ret)
> > > +		return {};
> > > +
> > > +	return YamlOutput::list();
> > > +}
> > > +
> > > +/**
> > > + * \class YamlScalar
> > > + *
> > > + * A YamlScalar can be assigned to an std::string_view to emit them as YAML
> > > + * elements.
> > > + */
> > > +
> > > +/**
> > > + * \brief Create a YamlScalar instance
> > > + */
> > > +YamlScalar::YamlScalar(YamlEmitter *emitter)
> > > +	: YamlOutput(emitter)
> > > +{
> > > +}
> > > +
> > > +/**
> > > + * \brief Emit \a scalar as a YAML scalar
> > > + * \param[in] scalar The element to emit in the YAML output
> > > + */
> > > +void YamlScalar::operator=(std::string_view scalar)
> > > +{
> > > +	emitScalar(scalar);
> > > +}
> > > +
> > > +/**
> > > + * \class YamlList
> > > + *
> > > + * A YamlList can be populated with scalars and allows to create nested lists
> > > + * and dictionaries.
> > > + */
> > > +
> > > +/**
> > > + * \brief Create a YamlList
> > > + */
> > > +YamlList::YamlList(YamlEmitter *emitter)
> > > +	: YamlOutput(emitter)
> > > +{
> > > +}
> > > +
> > > +/**
> > > + * \brief Destroy a YamlList instance
> > > + */
> > > +YamlList::~YamlList()
> > > +{
> > > +	emitSequenceEnd();
> > > +}
> > > +
> > > +/**
> > > + * \fn YamlList &YamlList::operator=(YamlList &&other)
> > > + * \brief Move-assignment operator
> > > + * \param[inout] other The instance to move
> > > + */
> > > +
> > > +/**
> > > + * \brief Append \a scalar to the list
> > > + * \param[in] scalar The element to append to the list
> > > + */
> > > +void YamlList::scalar(std::string_view scalar)
> > > +{
> > > +	emitScalar(scalar);
> > > +}
> > > +
> > > +/**
> > > + * \copydoc YamlOutput::list()
> > > + */
> > > +YamlList YamlList::list()
> > > +{
> > > +	int ret = emitSequenceStart();
> > > +	if (ret)
> > > +		return {};
> > > +
> > > +	return YamlOutput::list();
> > > +}
> > > +
> > > +/**
> > > + * \copydoc YamlOutput::dict()
> > > + */
> > > +YamlDict YamlList::dict()
> > > +{
> > > +	int ret = emitMappingStart();
> > > +	if (ret)
> > > +		return {};
> > > +
> > > +	return YamlOutput::dict();
> > > +}
> > > +
> > > +/**
> > > + * \class YamlDict
> > > + *
> > > + * A YamlDict can be populated with scalars using operator[] and allows to
> > > + * create other lists and dictionaries associated with a key.
> > > + */
> > > +
> > > +/**
> > > + * \fn YamlDict::YamlDict()
> > > + * \brief Create a non-initialized instance of a YamlDict
> > > + */
> > > +
> > > +YamlDict::YamlDict(YamlEmitter *emitter)
> > > +	: YamlOutput(emitter)
> > > +{
> > > +}
> > > +
> > > +/**
> > > + * \brief Destroy a YamlDict instance
> > > + */
> > > +YamlDict::~YamlDict()
> > > +{
> > > +	emitMappingEnd();
> > > +}
> > > +
> > > +/**
> > > + * \fn YamlDict &YamlDict::operator=(YamlDict &&other)
> > > + * \brief Move-assignment operator
> > > + * \param[inout] other The instance to move
> >
> > I think we usually use param[in] for move constructors and assignment
> > operators. Granted, it's a bit borderline.
> 
> ack
> 
> > > + */
> > > +
> > > +/**
> > > + * \copydoc YamlOutput::list()
> >
> > That won't document the key parameter.
> >
> > > + */
> > > +YamlList YamlDict::list(std::string_view key)
> > > +{
> > > +	int ret = emitScalar(key);
> > > +	if (ret)
> > > +		return {};
> > > +
> > > +	ret = emitSequenceStart();
> > > +	if (ret)
> > > +		return {};
> > > +
> > > +	return YamlOutput::list();
> > > +}
> > > +
> > > +/**
> > > + * \copydoc YamlOutput::dict()
> >
> > Same here.
> 
> I'll manually copy the documentation
> 
> > > + */
> > > +YamlDict YamlDict::dict(std::string_view key)
> > > +{
> > > +	int ret = emitScalar(key);
> > > +	if (ret)
> > > +		return {};
> > > +
> > > +	ret = emitMappingStart();
> > > +	if (ret)
> > > +		return {};
> > > +
> > > +	return YamlOutput::dict();
> > > +}
> > > +
> > > +/**
> > > + * \brief Create a scalar associated with \a key in the dictionary
> > > + * \param[in] key The key associated with the newly created scalar
> > > + * \return A YamlScalar that application can use to output text
> > > + */
> > > +YamlScalar YamlDict::operator[](std::string_view key)
> >
> > I'm still not entirely thrilled by this, for a few reasons. None are
> > showstoppers, you can decide what to do.
> >
> > First there's an asymmetry in the API. To emit scalars on lists you have
> > a scalar() function, while here you use the [] operator. Similarly, in
> > the YamlDict class, you have explicitly named functions to emit a list
> > or dictionary, but not for scalars. Now asymmetries are not always bad,
> > sometimes specific APIs are best for specific jobs.
> 
> To be hones I find the usage of operator[] to emit a scalar
> (associated with a key) quite neat as it directly connect to the
> 'dictionary' API (or a map, as this is C++ and not Python).
> 
> You're right, it's asymmetric, but in lists you can just emit a
> scalar, in dict you always associate it (as well as anything else)
> with a key. Operator[] makes sure you pass a key in :)
> 
> >
> > Another API design point that bothers me possibly a bit more is the
> > difference in behaviour between YamlDict::operator[]() and
> > std::map::operator[](). Using the [] operators, users could expect that
> >
> > 	dict["foo"] = "bar";
> > 	dict["foo"] = "baz";
> >
> > will result in the second statement overwriting the first one. The
> > YamlDict::operator[]() API is a bit misleading. As it's an internal
> > class it's less of an issue than it would be for the libcamera public
> > API, but still.
> 
> As we don't retain data but emit them as soon as we can, yes, this
> won't work as std::map.
> 
> >
> > Finally, returning a YamlScalar object opens the door for misuse. The
> > following code will compile, but is invalid:
> >
> > 	YamlScalar scalar = dict["foo"];
> > 	dict["bar"] = "0";
> > 	scalar = "1";
> >
> > Using an explicit function would solve all this:
> >
> > 	dict.scalar("foo", "0");
> >
> 
> I can indeed use a 'scalar' function if it's safer

I think it would be :-)

> > The YamlScalar class could then be moved from the .h file to the .cpp
> > file, and possibly even dropped completely.
> 
> yes, users now do not need to manage scalars directly now
> 
> > > +{
> > > +	int ret = emitScalar(key);
> > > +	if (ret)
> > > +		return {};
> > > +
> > > +	return YamlOutput::scalar();
> > > +}
> > > +
> > > +} /* namespace libcamera */

-- 
Regards,

Laurent Pinchart