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= Project Connected Home over IP Software

:plusplus: ++

== Doxygen Best Practices, Conventions, and Style

=== Comments

Due to Project CHIP’s infrastructure nature, it will be consumed by
other teams, both inside and outside Project CHIP. Therefore it is
critical that how it works, how it behaves, and how it is
interfaced with are **clearly** documented.

In support of this effort Project CHIP uses
http://www.doxygen.org/[Doxygen] to
markup (or markdown) all C, C{plusplus}, Objective C, Objective C{plusplus}, Perl,
Python, and Java code to:

* Detail what the various source and header files are and how they fit
into the broader context.
* Detail what the various C{plusplus} / Objective C{plusplus} namespaces are.
* Detail what the constants, C preprocessor definitions, and
enumerations are.
* Detail what the globals are and how they are to be used.
* Detail what the free function and object / class methods are and how
they are to be used, what their parameters are, and what their return
values are.
* Detail any other important technical information or theory of
operation unique and relevant to the stack that is not otherwise
captured in architecture, design, or protocol documentation.

==== File

Every C, C{plusplus}, Objective C, Objective C{plusplus}, Perl, Python, Shell, and Java
source file should, at minimum, have a standard, boilerplate Project
CHIP file header that also describes what the file is and how, if
applicable, it fits into the broader implementation.

Canonical examples for C, C{plusplus}, Objective C, and Objective C{plusplus} and
Python, Perl, and shell are shown in Listing 1 and Listing 2 below.

[source,C]
----
/*
 *    Copyright (c) <Create year>[-<Last modified year>] Project CHIP Authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *        http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License. 
 */

/**
 *    @file
 *        <Brief description>
 *
 *      [<Detailed description>]
 */
----
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*Listing 1.* Standard, boilerplate Project CHIP file header for C, C{plusplus},
Objective C, and Objective C{plusplus}..

[source,perl]
----
#
#    Copyright (c) <Create year>[-<Last modified year>] Project CHIP Authors.
#
#    Licensed under the Apache License, Version 2.0 (the "License");
#    you may not use this file except in compliance with the License.
#    You may obtain a copy of the License at
#
#        http://www.apache.org/licenses/LICENSE-2.0
#
#    Unless required by applicable law or agreed to in writing, software
#    distributed under the License is distributed on an "AS IS" BASIS,
#    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
#    See the License for the specific language governing permissions and
#    limitations under the License.
#

##
#    @file
#        <Brief description>
#
#      [<Detailed description>]
#
----
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*Listing 2.* Standard, boilerplate Project CHIP file header for Perl,
Python, shell, and make.

where:

* _<Create year>_ is the year the file was created.
* _<Last modified year>_ is, optionally, the year the file was last
modified if it is different from _<Create year>_.
* _<Brief description>_ is a succinct description of what the file is.
* _<Detailed description>_ is, optionally, a more in-depth description of
what the file is and how it fits into the broader context.

For header files, a good prologue for _<Brief description>_ is "This file
defines...", describing what is being defined or declared. Likewise, for
source files, a good prologue for _<Brief description>_ is "This file
implements...", describing what is being implemented. Usually,
copy-and-pasting the brief description from the header to the source and
changing the prologue from "defines" to "implements" is sufficient.

The _<Detailed description>_, if present, could be a link to one or more
of the architecture, design, or protocol specifications or some more in
depth but still succinct information about where the file and what it
defines or implements fit into the broader design or implementation.

===== Motivation and Rationale

The motivation and rationale for this is not from a legal perspective
and as a consequence is not in opposition to guidance from legal.
However, when Project CHIP provides a substantial amount of code as
reference code and as an SDK to third-parties and other work group member
companies, this makes it very clear—and consistently so—what code belongs
to and is authored by Project CHIP and what is not.

==== Functions and Methods

Every non-trival or non-default public, and ideally private, free function
and class method should likewise have a prologue comment that:

* Briefly describes what it is and what it does.
* Describes in detail, optionally, what it is and what it does.
* Describes the purpose, function, and influence of each parameter as
well as whether it is an input, an output, or both.
* Describes the return value or values, if present, and the expected range or
constraints of it.

The goal of function and method comments are to not simply echo back to the
reader what is in the API signature but to provide value-added insight. Also,
remember that what may be intuitive and self-evident to you as a domain expert
today may not be to your future self or others who are uninitiated in the domain.
The project's success will be measured by the ease, speed, and breadth of its adoption.
Your comments, alongside examples, will contribute toward this goal. When in doubt,
err on the side of being generous with your comments rather than parsimonious.

An example is shown in Listing 3 below for C, C{plusplus}, Objective C, and
Objective C{plusplus}. Adapt as appropriate for Perl, Python and Shell.

[source,C]
----
/**
 * Parse and attempt to convert a string to a 64-bit unsigned integer,
 * applying the appropriate interpretation based on the base parameter.
 *
 * @param[in]  str     A pointer to a NULL-terminated C string representing
 *                     the integer to parse.
 * @param[out] output  A reference to storage for a 64-bit unsigned integer
 *                     to which the parsed value will be stored on success.
 * @param[in]  base    The base according to which the string should be
 *                     interpreted and parsed. If 0 or 16, the string may
 *                     be hexadecimal and prefixed with "0x". Otherwise, a 0
 *                     is implied as 10 unless a leading 0 is encountered in
 *                     which case 8 is implied.
 *
 * @retval  0 on success.
 * @retval  #EINVAL if the given base contains an unsupported value or if no
 *          conversion was performed.
 * @retval  #ERANGE if the resulting value was out of range.
 */
----
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*Listing 3.* Standard Doxygen-compatible free function or method comment
for C, C{plusplus}, Objective C, and Objective C{plusplus}.

In addition, developers should well document the bodies of their
functions and methods, describing the overall flow, design intent, error
handling nuances, historical bugs encountered and resolved, and so
forth. While these types of comments do not typically become part of the
external documentation, they are invaluable to future maintainers of the
code.

==== Other

===== Dos

* *Do* use the '@' Doxygen markup style rather than the '\' markup style.
* *Do* also consider consulting tips on
http://centerforplainlanguage.org/5-steps-to-plain-language/[Plain
Language] for additional style and tone input.
* *Do* use consistent terminology and lingo.
* *Do* properly paragraph justify and wrap your documentation.

** See your editor's documentation on how to do this (for example, M-q in Emacs).

===== Don'ts

* *Do not* forget to document your files, enumerations, constants,
classes, objects, namespaces, functions, and methods.
* *Do not* include the file name in any Doxygen file comments or
directives.

** Your editor knows the [.underline]#file name#, source code control knows the file
name, and you know the file name.
** When it changes on the file system, having to change it in the file
comments is simply an added burden.

* *Do not* include [.underline]#your name# in any Doxygen comments or directives.

** Source code control knows who you are and what file revisions you own.
** We do not want Project CHIP consumers knowing who you are and calling
or e-mailing you directly for support.

* *Do not* include the [.underline]#modification date# the file was last changed in
Doxygen comments or directives, [.underline]#except for the copyright header#.

** Source code control knows when the file was last changed and the date
other revisions were made.

* *Do not* include subjective or opinionated commentary or expose
proprietary and confidential information not relevant to the code or
APIs.

** This content *will be* published to and for consumption by members, the
CHIP community, and the general public.

== Revision History

[cols="^1,^1,<2,<3",options="header"]
|===
|Revision |Date |Modified By |Description
|1 |2020-06-11 |Grant Erickson |Initial revision.
|===

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_Project Connect Home over IP Public Information_