38dd90307a
When inserting an element with emplace(), the element is constructed
in-place with the parameters to the emplace() method being forwarded to
the constructor of the element. For std::map containers, the element is
an std::pair<const Key, T>. The constructors of std::pair<T1, T2> fall
into three categories:
(1) Default, copy and move constructors (and related versions)
(2) Constructors that take lvalue or rvalue references to T1 and T2
(3) A forwarding constructor that forwards parameters to the
constructors of T1 and T2
The first category isn't useful in most cases for std::map::emplace(),
as the caller usually doesn't have an existing std::pair<const Key, T>
for the element to be inserted.
The constructor from the third category is useful to avoid constructing
intermediate Key or T instances when the caller doesn't have them
available. This constructor takes two std::tuple arguments that contain
the arguments for the Key and T constructors, respectively. Due to
template deduction rules, usage of such a constructor couldn't be
deduced by the compiler automatically in all cases, so the constructor
takes a first argument of type std::piecewise_construct_t that lets the
caller force the usage ot the forwarding constructor (also known for
this reason as the piecewise constructor). The caller uses a construct
such as
map.emplace(std::piecewise_construct,
std::forward_as_tuple(args_for_Key, ...),
std::forward_as_tuple(args_for_T, ...));
This syntax is a bit heavy, but is required to construct Key and T
in-place from arguments to their non-default constructor (it is also the
only std::pair non-default constructor that can be used for non-copyable
non-movable types).
When the caller of std::map::emplace() already has references to a Key
and a T, they can be passed to the std::pair piecewise constructor, and
this will create std::tuple instance to wrap the Key and T references
arguments to ultimately pass them to the Key and T copy constructors.
map.emplace(std::piecewise_construct,
std::forward_as_tuple(Key_value),
std::forward_as_tuple(T_value));
While this mechanism works, it's unnecessary complex. A constructor of
std::pair that takes references to Key and T can be used without any
performance penalty, as it will also call the copy constructor of Key
and T. In this case we can use a simpler constructor of std::pair, and
thus a simpler call of std::map::emplace.
map.emplace(Key_value, T_value);
We have a couple occurrences of this above misuse of piecewise
construction. Simplify them, which simplifies the code and reduces the
generated code size.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
.. section-begin-libcamera
===========
libcamera
===========
**A complex camera support library for Linux, Android, and ChromeOS**
Cameras are complex devices that need heavy hardware image processing
operations. Control of the processing is based on advanced algorithms that must
run on a programmable processor. This has traditionally been implemented in a
dedicated MCU in the camera, but in embedded devices algorithms have been moved
to the main CPU to save cost. Blurring the boundary between camera devices and
Linux often left the user with no other option than a vendor-specific
closed-source solution.
To address this problem the Linux media community has very recently started
collaboration with the industry to develop a camera stack that will be
open-source-friendly while still protecting vendor core IP. libcamera was born
out of that collaboration and will offer modern camera support to Linux-based
systems, including traditional Linux distributions, ChromeOS and Android.
.. section-end-libcamera
.. section-begin-getting-started
Getting Started
---------------
To fetch the sources, build and install:
::
git clone git://linuxtv.org/libcamera.git
cd libcamera
meson build
ninja -C build install
Dependencies
~~~~~~~~~~~~
The following Debian/Ubuntu packages are required for building libcamera.
Other distributions may have differing package names:
A C++ toolchain: [required]
Either {g++, clang}
for libcamera: [required]
meson ninja-build python3-yaml
for device hotplug enumeration: [optional]
pkg-config libudev-dev
for qcam: [optional]
qtbase5-dev libqt5core5a libqt5gui5 libqt5widgets5
for documentation: [optional]
python3-sphinx doxygen
.. section-end-getting-started