pw_status/guide.rst

.. _module-pw_status-guide:

====================
Get started & guides
====================
.. pigweed-module-subpage::
   :name: pw_status
   :tagline: pw_status: Exception-free error propagation for embedded

.. _module-pw_status-get-started:

-----------
Get started
-----------
To deploy ``pw_status``, depend on the library:

.. tab-set::

   .. tab-item:: Bazel

      Add ``@pigweed//pw_status`` to the ``deps`` list in your Bazel target:

      .. code-block::

         cc_library("...") {
           # ...
           deps = [
             # ...
             "@pigweed//pw_status",
             # ...
           ]
         }

      This assumes that your Bazel ``WORKSPACE`` has a `repository
      <https://bazel.build/concepts/build-ref#repositories>`_ named ``@pigweed``
      that points to the upstream Pigweed repository.

   .. tab-item:: GN

      Add ``$dir_pw_status`` to the ``deps`` list in your ``pw_executable()``
      build target:

      .. code-block::

         pw_executable("...") {
           # ...
           deps = [
             # ...
             "$dir_pw_status",
             # ...
           ]
         }

   .. tab-item:: CMake

      Add ``pw_status`` to your ``pw_add_library`` or similar CMake target:

      .. code-block::

         pw_add_library(my_library STATIC
           HEADERS
             ...
           PRIVATE_DEPS
             # ...
             pw_status
             # ...
         )

   .. tab-item:: Zephyr

      There are two ways to use ``pw_status`` from a Zephyr project:

      * Depend on ``pw_status`` in your CMake target (see CMake tab). This is
        the Pigweed team's suggested approach since it enables precise CMake
        dependency analysis.

      * Add ``CONFIG_PIGWEED_STATUS=y`` to the Zephyr project's configuration,
        which causes ``pw_status`` to become a global dependency and have the
        includes exposed to all targets. The Pigweed team does not recommend
        this approach, though it is the typical Zephyr solution.

Then use the status object or try macros:

.. code-block:: cpp

   #include "pw_status/try.h"
   #include "pw_status/status.h"

   pw::Status MyOperation() {
     PW_TRY(SubOp1());
     PW_TRY(SubOp2());
     // ...
     return pw::OkStatus();
   }

------
Guides
------

Tracking the first error encountered
------------------------------------
In some contexts it is useful to track the first error encountered while
allowing execution to continue. Manually writing out ``if`` statements to check
and then assign quickly becomes verbose, and doesn't explicitly highlight the
intended behavior of "latching" to the first error.

.. admonition:: **No**: Track status manually across calls
   :class: error

   .. code-block:: cpp

     Status overall_status;
     for (Sector& sector : sectors) {
       Status erase_status = sector.Erase();
       if (!overall_status.ok()) {
         overall_status = erase_status;
       }

       if (erase_status.ok()) {
         Status header_write_status = sector.WriteHeader();
         if (!overall_status.ok()) {
           overall_status = header_write_status;
         }
       }
     }
     return overall_status;

:cpp:class:`pw::Status` has a :cpp:func:`pw::Status::Update()` helper function
that does exactly this to reduce visual clutter and succinctly highlight the
intended behavior.

.. admonition:: **Yes**: Track status with :cpp:func:`pw::Status::Update()`
   :class: checkmark

   .. code-block:: cpp

     Status overall_status;
     for (Sector& sector : sectors) {
       Status erase_status = sector.Erase();
       overall_status.Update(erase_status);

       if (erase_status.ok()) {
         overall_status.Update(sector.WriteHeader());
       }
     }
     return overall_status;

----------------------------------
Jointly reporting status with size
----------------------------------
``pw::StatusWithSize`` (``pw_status/status_with_size.h``) is a convenient,
efficient class for reporting a status along with an unsigned integer value.
It is similar to the ``pw::Result<T>`` class, but it stores both a size and a
status, regardless of the status value, and only supports a limited range (27
bits).

``pw::StatusWithSize`` values may be created with functions similar to
``pw::Status``. For example,

.. code-block:: cpp

   // An OK StatusWithSize with a size of 123.
   StatusWithSize(123)

   // A NOT_FOUND StatusWithSize with a size of 0.
   StatusWithSize::NotFound()

   // A RESOURCE_EXHAUSTED StatusWithSize with a size of 10.
   StatusWithSize::ResourceExhausted(10)

-----------------------------------
Reducing error handling boilerplate
-----------------------------------
Manual error handling through return codes is easy to understand and
straightforward to write, but leads to verbose code. To reduce boilerplate,
Pigweed has the ``PW_TRY`` (``pw_status/try.h``) macro, easing development of
functions checking or returning ``pw::Status`` and ``pw::StatusWithSize``
objects. The ``PW_TRY`` and ``PW_TRY_WITH_SIZE`` macros call a function and do
an early return if the function's return status is not :c:enumerator:`OK`.

Example:

.. code-block:: cpp

   Status PwTryExample() {
     PW_TRY(FunctionThatReturnsStatus());
     PW_TRY(FunctionThatReturnsStatusWithSize());

     // Do something, only executed if both functions above return OK.
   }

   StatusWithSize PwTryWithSizeExample() {
     PW_TRY_WITH_SIZE(FunctionThatReturnsStatus());
     PW_TRY_WITH_SIZE(FunctionThatReturnsStatusWithSize());

     // Do something, only executed if both functions above return OK.
   }

``PW_TRY_ASSIGN`` is for working with ``pw::StatusWithSize`` objects in in
functions that return Status. It is similar to ``PW_TRY`` with the addition of
assigning the size from the ``pw::StatusWithSize`` on ok.

.. code-block:: cpp

   Status PwTryAssignExample() {
     size_t size_value
     PW_TRY_ASSIGN(size_value, FunctionThatReturnsStatusWithSize());

     // Do something that uses size_value. size_value is only assigned and this
     // following code executed if the PW_TRY_ASSIGN function above returns OK.
   }