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<h1>WPILib C++ Style Guide (Based on the <a href=http://google-styleguide.googlecode.com/svn/trunk/cppguide.html>Google C++ Style Guide</a>)</h1>
<div class="horizontal_toc" id="tocDiv"></div>
<div class="main_body">
<h2 class="ignoreLink" id="Background">Background</h2>
<p><strong>This guide is a work in progress.</strong>
We are currently working on getting this guide updated to
a point where it is useful for WPILib developers to use.</p>
<p>C++ is one of the two main languages (Java being the other)
used in WPILib; in order to maintain consistency and keep the
maintenance of the code manageable, we use this style guide.</p>
<p>There are two main overarching purposes to this guide. The first
is to act as a normal C++ style guide (both in terms fo formatting
and programming practices) for C++ developers of WPILib.
The other purpose is to help Java programmers who may
know a moderate amount of C++ but may not be fully
up to date with things like C++11 and so may not even
realize that certain C++ features exist.</p>
<p>This style guide is a heavily modified version of the
<a href=http://google-styleguide.googlecode.com/svn/trunk/cppguide.html>
Google C++ Style Guide</a>. The Google Style Guide has
a lot of good points and is a good read, but in order
to cut the style guide down to a more readable size and to
focus mroe on WPILib-specific information, we have
altetered the original style guide in several ways.</p>
<p>One way in which we <em>haven't</em> done much to
alter the original style guide is to keep the vast
majority of the formatting/naming/etc. related
information intact. This is both so that we
do not have to write up our own standards and so
that existing tools such as clang-format and
the Google eclipse format configuration files
can work out of the box. All of these things
should be relatively non-controversial and do not
require much discussion.</p>
<p>Where we deviate more from the original guide is
in the style of the code itself. At the moment (ie,
when we first created this modified version), we
deleted all of the sections of the original guide
which mandate particular programming practices
such as forbidding exceptions, multiple inheritance,
etc. However, as time goes on, we gradually add in more
information along this lines, either by copying
directly from Google's Style Guide or by writing
our own decisions and best practices, some of which
may be very WPILib-specific.</p>
<p>As the original guide makes very clear, consistency
is extremely important to keeping the code base
manageable, and so we encourage that, wherever
reasonable, that you keep everything consistent
with whatever the standard style is.</p>
<p>Along with just C++ style, it is also important
to keep in mind that WPILib consists of both a C++
and Java half. In order to keep things consistent
and easier for users, we ask that, in general,
Java and C++ be kept as consistent with one another
as reasonable. This includes everything from using
two spaces for indentation in both language to
keeping the inheritance structure essentially the
same in both. Although the two do not have to be
precisely the same, it does mean that if there is
something that you are doing which will be imposssible
to reproduce in some way in Java, then you may
want to reconsider.</p>
<p>One final thing to remember is that High School
students with relatively little experience programming
are the main user for this code, and throwing the full
brunt of C++ at a student just learning how to program
is likely not the best of ideas. As such, any
user-facing APIs should minimize the use of any
more complicated C++ features. As always,
use your judgement and ask others in cases where
there is something which may violate anything
in this guide.</p>
<h2 id="Programming_Guidelines">Programming Guidelines</h2>
<p>C++ is a large, complicated language, and in order
to ensure that we stay consistent and maintain certain
best practices, we have certain rules. For the most part
these are common sense rules and in some cases exist
solely to point out features of C++ that someone more
familiar with Java may not realize even exist.</p>
<h3 id="Pointers">Pointers</h3>
<p>In general, we strongly discourage the use of
raw pointers in C++ code; instead, references or
STL pointers should be used where appropriate.
There are two exceptions to this rule:</p>
<ul>
<li>When interfacing with lower-level C code or
with any libraries which force you to use raw pointers.</li>
<li>In order to keep user interfaces consistent,
we may keep around deprecated functions which
take raw pointers. Any user-facing functions
which take raw pointers should be deprecated
using the
<a href=https://en.wikipedia.org/wiki/C%2B%2B14#The_attribute_.5B.5Bdeprecated.5D.5D><code>[[deprecated]]</code></a>
attribute and replaced with either references
or STL pointers.</li>
</ul>
<p>As of C++11, the following are options in the
place of raw pointers:</p>
<ul>
<li><code>std::unique_ptr</code> Should be used
when you still need to use a pointer, but you
only need one entity to own the pointer. The
<code>std::unique_ptr</code> will automatically
be deleted when there are no more references to
it.</li>
<li><code>std::shared_ptr</code> Should be used
when you still need to use a pointer and you
need many references to the object. When
there are zero remaining references to the
object, it will be deleted. Use <code>std::weak_ptr</code>
where necessary to avoid circular dependencies
or other potential issues.</li>
<li>L-value references (the traditional sort
of reference that has been around since before C++11)
should be used when you want to pass around a
reference to an object and want to guarantee
that it won't be null. Use const references
if you want to avoid copying a large object
but don't want to modify it.</li>
<li>R-value references were introduced in C++11
and allow for the use of <code>std::move</code>.
R-value references should be used where it makes
sense that a parameter to a function is having
its ownership passed from one place to another.
In general, R-value references are not inherently
bad, but they do introduce additional complexity
that may confuse people who are not familiar
with them.</li>
</ul>
<h3 id="Deprecation">Deprecation</h3>
<p>When updating APIs, make liberal use of the
<code>[[deprecated]]</code> attribute (although if
it is reasonable to simply remove any old interfaces
then do so) to indicate that users should no longer
use the function. Currently, this will cause warnings
in user code and errors in the WPILib build.</p>
<pre>
[[deprecated("This is a deprecated function; this text will be displayed when"
" the compiler throws a warning.")]]
void foo() {}
class [[deprecated("This is a deprecated class.")]] Foo {};
int bar [[deprecated("This is a deprecated variable.")]];
</pre>
<p>See <a href=http://josephmansfield.uk/articles/marking-deprecated-c++14.html>
here</a> for more information on deprecation.</p>
<h2 id="Header_Files">Header Files</h2>
<p>In general, every <code>.cc</code> file should have an
associated <code>.h</code> file. There are some common
exceptions, such as unittests and
small <code>.cpp</code> files containing just a
<code>main()</code> function.</p>
<p>Correct use of header files can make a huge difference to
the readability, size and performance of your code.</p>
<p>The following rules will guide you through the various
pitfalls of using header files.</p>
<a id="The_-inl.h_Files"></a>
<h3 id="Self_contained_Headers">Self-contained Headers</h3>
<div class="summary">
<p>Header files should be self-contained and end in <code>.h</code>. Files that
are meant for textual inclusion, but are not headers, should end in
<code>.inc</code>. Separate <code>-inl.h</code> headers are disallowed.</p>
</div>
<div class="stylebody">
<p>All header files should be self-contained. In other
words, users and refactoring tools should not have to adhere to special
conditions in order to include the header. Specifically, a
header should have <a href="#The__define_Guard">header guards</a>,
should include all other headers it needs, and should not require any
particular symbols to be defined.</p>
<p>There are rare cases where a file is not meant to be self-contained, but
instead is meant to be textually included at a specific point in the code.
Examples are files that need to be included multiple times or
platform-specific extensions that essentially are part of other headers. Such
files should use the file extension <code>.inc</code>.</p>
<p>If a template or inline function is declared in a <code>.h</code> file,
define it in that same file. The definitions of these constructs must
be included into every <code>.cc</code> file that uses them, or the
program may fail to link in some build configurations. Do not move these
definitions to separate <code>-inl.h</code> files.</p>
<p>As an exception, a function template that is explicitly
instantiated for all relevant sets of template arguments, or
that is a private member of a class, may
be defined in the only <code>.cc</code> file that
instantiates the template.</p>
</div>
<h3 id="The__define_Guard">The #define Guard</h3>
<div class="summary">
<p>All header files should have <code>#define</code> guards to
prevent multiple inclusion. The format of the symbol name
should be
<code><i>&lt;PROJECT&gt;</i>_<i>&lt;PATH&gt;</i>_<i>&lt;FILE&gt;</i>_H_</code>.</p>
</div>
<div class="stylebody">
<p>To guarantee uniqueness, they should
be based on the full path in a project's source tree. For
example, the file <code>foo/src/bar/baz.h</code> in
project <code>foo</code> should have the following
guard:</p>
<pre>#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_
...
#endif // FOO_BAR_BAZ_H_
</pre>
</div>
<h3 id="Forward_Declarations">Forward Declarations</h3>
<div class="summary">
<p>You may forward declare ordinary classes in order to avoid
unnecessary <code>#include</code>s.</p>
</div>
<div class="stylebody">
<div class="definition">
<p>A "forward declaration" is a declaration of a class,
function, or template without an associated definition.
<code>#include</code> lines can often be replaced with
forward declarations of whatever symbols are actually
used by the client code.</p>
</div>
<div class="pros">
<ul>
<li>Unnecessary <code>#include</code>s force the
compiler to open more files and process more
input.</li>
<li>They can also force your code to be recompiled more
often, due to changes in the header.</li>
</ul>
</div>
<div class="cons">
<ul>
<li>It can be difficult to determine the correct form
of a forward declaration in the presence of features
like templates, typedefs, default parameters, and using
declarations.</li>
<li>It can be difficult to determine whether a forward
declaration or a full <code>#include</code> is needed
for a given piece of code, particularly when implicit
conversion operations are involved. In extreme cases,
replacing an <code>#include</code> with a forward
declaration can silently change the meaning of
code.</li>
<li>Forward declaring multiple symbols from a header
can be more verbose than simply
<code>#include</code>ing the header.</li>
<li>Forward declarations of functions and templates can
prevent the header owners from making
otherwise-compatible changes to their APIs; for
example, widening a parameter type, or adding a
template parameter with a default value.</li>
<li>Forward declaring symbols from namespace
<code>std::</code> usually yields undefined
behavior.</li>
<li>Structuring code to enable forward declarations
(e.g. using pointer members instead of object members)
can make the code slower and more complex.</li>
<li>The practical efficiency benefits of forward
declarations are unproven.</li>
</ul>
</div>
<div class="decision">
<ul>
<li>When using a function declared in a header file,
always <code>#include</code> that header.</li>
<li>When using a class template, prefer to
<code>#include</code> its header file.</li>
<li>When using an ordinary class, relying on a forward
declaration is OK, but be wary of situations where a
forward declaration may be insufficient or incorrect;
when in doubt, just <code>#include</code> the
appropriate header.</li>
<li>Do not replace data members with pointers just to
avoid an <code>#include</code>.</li>
</ul>
<p>Please see <a href="#Names_and_Order_of_Includes">Names and Order
of Includes</a> for rules about when to #include a header.</p>
</div>
</div>
<h3 id="Inline_Functions">Inline Functions</h3>
<div class="summary">
<p>Define functions inline only when they are small, say, 10
lines or less.</p>
</div>
<div class="stylebody">
<div class="definition">
<p>You can declare functions in a way that allows the compiler to expand
them inline rather than calling them through the usual
function call mechanism.</p>
</div>
<div class="pros">
<p>Inlining a function can generate more efficient object
code, as long as the inlined function is small. Feel free
to inline accessors and mutators, and other short,
performance-critical functions.</p>
</div>
<div class="cons">
<p>Overuse of inlining can actually make programs slower.
Depending on a function's size, inlining it can cause the
code size to increase or decrease. Inlining a very small
accessor function will usually decrease code size while
inlining a very large function can dramatically increase
code size. On modern processors smaller code usually runs
faster due to better use of the instruction cache.</p>
</div>
<div class="decision">
<p>A decent rule of thumb is to not inline a function if
it is more than 10 lines long. Beware of destructors,
which are often longer than they appear because of
implicit member- and base-destructor calls!</p>
<p>Another useful rule of thumb: it's typically not cost
effective to inline functions with loops or switch
statements (unless, in the common case, the loop or
switch statement is never executed).</p>
<p>It is important to know that functions are not always
inlined even if they are declared as such; for example,
virtual and recursive functions are not normally inlined.
Usually recursive functions should not be inline. The
main reason for making a virtual function inline is to
place its definition in the class, either for convenience
or to document its behavior, e.g., for accessors and
mutators.</p>
</div>
</div>
<h3 id="Function_Parameter_Ordering">Function Parameter Ordering</h3>
<div class="summary">
<p>When defining a function, parameter order is: inputs, then
outputs.</p>
</div>
<div class="stylebody">
<p>Parameters to C/C++ functions are either input to the
function, output from the function, or both. Input
parameters are usually values or <code>const</code>
references, while output and input/output parameters will
be non-<code>const</code> pointers. When ordering
function parameters, put all input-only parameters before
any output parameters. In particular, do not add new
parameters to the end of the function just because they
are new; place new input-only parameters before the
output parameters.</p>
<p>This is not a hard-and-fast rule. Parameters that are
both input and output (often classes/structs) muddy the
waters, and, as always, consistency with related
functions may require you to bend the rule.</p>
</div>
<h3 id="Names_and_Order_of_Includes">Names and Order of Includes</h3>
<div class="summary">
<p>Use standard order for readability and to avoid hidden
dependencies: Related header, C library, C++ library, other libraries'
<code>.h</code>, your project's <code>.h</code>.</p>
</div>
<div class="stylebody">
<p>
All of a project's header files should be
listed as descendants of the project's source
directory without use of UNIX directory shortcuts
<code>.</code> (the current directory) or <code>..</code>
(the parent directory). For example,
<code>google-awesome-project/src/base/logging.h</code>
should be included as:</p>
<pre>#include "base/logging.h"
</pre>
<p>In <code><var>dir/foo</var>.cc</code> or
<code><var>dir/foo_test</var>.cc</code>, whose main
purpose is to implement or test the stuff in
<code><var>dir2/foo2</var>.h</code>, order your includes
as follows:</p>
<ol>
<li><code><var>dir2/foo2</var>.h</code>.</li>
<li>C system files.</li>
<li>C++ system files.</li>
<li>Other libraries' <code>.h</code>
files.</li>
<li>
Your project's <code>.h</code>
files.</li>
</ol>
<p>With the preferred ordering, if
<code><var>dir2/foo2</var>.h</code> omits any necessary
includes, the build of <code><var>dir/foo</var>.cc</code>
or <code><var>dir/foo</var>_test.cc</code> will break.
Thus, this rule ensures that build breaks show up first
for the people working on these files, not for innocent
people in other packages.</p>
<p><code><var>dir/foo</var>.cc</code> and
<code><var>dir2/foo2</var>.h</code> are usually in the same
directory (e.g. <code>base/basictypes_test.cc</code> and
<code>base/basictypes.h</code>), but may sometimes be in different
directories too.</p>
<p>Within each section the includes should be ordered
alphabetically. Note that older code might not conform to
this rule and should be fixed when convenient.</p>
<p>You should include all the headers that define the symbols you rely
upon (except in cases of <a href="#Forward_Declarations">forward
declaration</a>). If you rely on symbols from <code>bar.h</code>,
don't count on the fact that you included <code>foo.h</code> which
(currently) includes <code>bar.h</code>: include <code>bar.h</code>
yourself, unless <code>foo.h</code> explicitly demonstrates its intent
to provide you the symbols of <code>bar.h</code>. However, any
includes present in the related header do not need to be included
again in the related <code>cc</code> (i.e., <code>foo.cc</code> can
rely on <code>foo.h</code>'s includes).</p>
<p>For example, the includes in
<code>google-awesome-project/src/foo/internal/fooserver.cc</code>
might look like this:</p>
<pre>#include "foo/server/fooserver.h"
#include &lt;sys/types.h&gt;
#include &lt;unistd.h&gt;
#include &lt;hash_map&gt;
#include &lt;vector&gt;
#include "base/basictypes.h"
#include "base/commandlineflags.h"
#include "foo/server/bar.h"
</pre>
<p class="exception">Sometimes, system-specific code needs
conditional includes. Such code can put conditional
includes after other includes. Of course, keep your
system-specific code small and localized. Example:</p>
<pre>#include "foo/public/fooserver.h"
#include "base/port.h" // For LANG_CXX11.
#ifdef LANG_CXX11
#include &lt;initializer_list&gt;
#endif // LANG_CXX11
</pre>
</div>
<h2 id="Naming">Naming</h2>
<p>The most important consistency rules are those that govern
naming. The style of a name immediately informs us what sort of
thing the named entity is: a type, a variable, a function, a
constant, a macro, etc., without requiring us to search for the
declaration of that entity. The pattern-matching engine in our
brains relies a great deal on these naming rules.
</p>
<p>Naming rules are pretty arbitrary, but
we feel that
consistency is more important than individual preferences in this
area, so regardless of whether you find them sensible or not,
the rules are the rules.</p>
<h3 id="General_Naming_Rules">General Naming Rules</h3>
<div class="summary">
<p>Function names, variable names, and filenames should be
descriptive; eschew abbreviation.</p>
</div>
<div class="stylebody">
<p>Give as descriptive a name as possible, within reason.
Do not worry about saving horizontal space as it is far
more important to make your code immediately
understandable by a new reader. Do not use abbreviations
that are ambiguous or unfamiliar to readers outside your
project, and do not abbreviate by deleting letters within
a word.</p>
<pre>int price_count_reader; // No abbreviation.
int num_errors; // "num" is a widespread convention.
int num_dns_connections; // Most people know what "DNS" stands for.
</pre>
<pre class="badcode">int n; // Meaningless.
int nerr; // Ambiguous abbreviation.
int n_comp_conns; // Ambiguous abbreviation.
int wgc_connections; // Only your group knows what this stands for.
int pc_reader; // Lots of things can be abbreviated "pc".
int cstmr_id; // Deletes internal letters.
</pre>
</div>
<h3 id="File_Names">File Names</h3>
<div class="summary">
<p>Filenames should be all lowercase and can include
underscores (<code>_</code>) or dashes (<code>-</code>).
Follow the convention that your
project uses. If there is no consistent
local pattern to follow, prefer "_".</p>
</div>
<div class="stylebody">
<p>Examples of acceptable file names:</p>
<ul>
<li><code>my_useful_class.cc</code></li>
<li><code>my-useful-class.cc</code></li>
<li><code>myusefulclass.cc</code></li>
<li><code>myusefulclass_test.cc // _unittest and _regtest are deprecated.</code></li>
</ul>
<p>C++ files should end in <code>.cc</code> and header files should end in
<code>.h</code>. Files that rely on being textually included at specific points
should end in <code>.inc</code> (see also the section on
<a href="#Self_contained_Headers">self-contained headers</a>).</p>
<p>Do not use filenames that already exist in
<code>/usr/include</code>, such as <code>db.h</code>.</p>
<p>In general, make your filenames very specific. For
example, use <code>http_server_logs.h</code> rather than
<code>logs.h</code>. A very common case is to have a pair
of files called, e.g., <code>foo_bar.h</code> and
<code>foo_bar.cc</code>, defining a class called
<code>FooBar</code>.</p>
<p>Inline functions must be in a <code>.h</code> file. If
your inline functions are very short, they should go
directly into your <code>.h</code> file. </p>
</div>
<h3 id="Type_Names">Type Names</h3>
<div class="summary">
<p>Type names start with a capital letter and have a capital
letter for each new word, with no underscores:
<code>MyExcitingClass</code>, <code>MyExcitingEnum</code>.</p>
</div>
<div class="stylebody">
<p>The names of all types &#8212; classes, structs, typedefs,
and enums &#8212; have the same naming convention. Type names
should start with a capital letter and have a capital letter
for each new word. No underscores. For example:</p>
<pre>// classes and structs
class UrlTable { ...
class UrlTableTester { ...
struct UrlTableProperties { ...
// typedefs
typedef hash_map&lt;UrlTableProperties *, string&gt; PropertiesMap;
// enums
enum UrlTableErrors { ...
</pre>
</div>
<h3 id="Variable_Names">Variable Names</h3>
<div class="summary">
<p>The names of variables and data members are all lowercase, with
underscores between words. Data members of classes (but not structs)
additionally are prefixed with "m_". For instance:
<code>a_local_variable</code>, <code>a_struct_data_member</code>,
<code>m_a_class_data_member</code>.</p>
</div>
<div class="stylebody">
<h4 class="stylepoint_subsection">Common Variable names</h4>
<p>For example:</p>
<pre>string table_name; // OK - uses underscore.
string tablename; // OK - all lowercase.
</pre>
<pre class="badcode">string tableName; // Bad - mixed case.
</pre>
<h4 class="stylepoint_subsection">Class Data Members</h4>
<p>Data members of classes, both static and non-static, are
named like ordinary nonmember variables, but prefixed with a
"m_".</p>
<pre>class TableInfo {
...
private:
string m_table_name; // OK - m_ at beginning.
string m_tablename; // OK.
static Pool&lt;TableInfo&gt;* m_pool; // OK.
};
</pre>
<h4 class="stylepoint_subsection">Struct Data Members</h4>
<p>Data members of structs, both static and non-static,
are named like ordinary nonmember variables. They do not have
the preceding "m_" that data members in classes have.</p>
<pre>struct UrlTableProperties {
string name;
int num_entries;
static Pool&lt;UrlTableProperties&gt;* pool;
};
</pre>
<p>See <a href="#Structs_vs._Classes">Structs vs.
Classes</a> for a discussion of when to use a struct
versus a class.</p>
<h4 class="stylepoint_subsection">Global Variables</h4>
<p>There are no special requirements for global
variables, which should be rare in any case, but if you
use one, consider prefixing it with <code>g_</code> or
some other marker to easily distinguish it from local
variables.</p>
</div>
<h3 id="Constant_Names">Constant Names</h3>
<div class="summary">
<p>Use a <code>k</code> followed by mixed case, e.g.,
<code>kDaysInAWeek</code>, for constants defined globally or within a class.</p>
</div>
<div class="stylebody">
<p>As a convenience to the reader, compile-time constants of global or class scope
follow a different naming convention from other variables.
Use a <code>k</code> followed by words with uppercase first letters:</p>
<pre>const int kDaysInAWeek = 7;
</pre>
<p>This convention may optionally be used for compile-time constants of local scope;
otherwise the usual variable naming rules apply.
</p></div>
<h3 id="Function_Names">Function Names</h3>
<div class="summary">
<p>Regular functions have mixed case; accessors and mutators
match the name of the variable:
<code>MyExcitingFunction()</code>,
<code>MyExcitingMethod()</code>,
<code>my_exciting_member_variable()</code>,
<code>set_my_exciting_member_variable()</code>.</p>
</div>
<div class="stylebody">
<h4 class="stylepoint_subsection">Regular Functions</h4>
<p>Functions should start with a capital letter and have
a capital letter for each new word. No underscores.</p>
<p>If your function crashes upon an error, you should
append OrDie to the function name. This only applies to
functions which could be used by production code and to
errors that are reasonably likely to occur during normal
operation.</p>
<pre>AddTableEntry()
DeleteUrl()
OpenFileOrDie()
</pre>
<h4 class="stylepoint_subsection">Accessors and Mutators</h4>
<p>Accessors and mutators (get and set functions) should
match the name of the variable they are getting and
setting. This shows an excerpt of a class whose instance
variable is <code>num_entries_</code>.</p>
<pre>class MyClass {
public:
...
int num_entries() const { return num_entries_; }
void set_num_entries(int num_entries) { num_entries_ = num_entries; }
private:
int num_entries_;
};
</pre>
<p>You may also use lowercase letters for other very
short inlined functions. For example if a function were
so cheap you would not cache the value if you were
calling it in a loop, then lowercase naming would be
acceptable.</p>
</div>
<h3 id="Namespace_Names">Namespace Names</h3>
<div class="summary">
<p>Namespace names are all lower-case,
and based on project names and possibly their directory
structure: <code>google_awesome_project</code>.</p>
</div>
<div class="stylebody">
<p>See <a href="#Namespaces">Namespaces</a> for a
discussion of namespaces and how to name them.</p>
</div>
<h3 id="Enumerator_Names">Enumerator Names</h3>
<div class="summary">
<p>Enumerators should be named <i>either</i> like
<a href="#Constant_Names">constants</a> or like
<a href="#Macro_Names">macros</a>: either <code>kEnumName</code> or
<code>ENUM_NAME</code>.</p>
</div>
<div class="stylebody">
<p>Preferably, the individual enumerators should be named
like <a href="#Constant_Names">constants</a>. However, it
is also acceptable to name them like
<a href="Macro_Names">macros</a>. The enumeration name,
<code>UrlTableErrors</code> (and
<code>AlternateUrlTableErrors</code>), is a type, and
therefore mixed case.</p>
<pre>enum UrlTableErrors {
kOK = 0,
kErrorOutOfMemory,
kErrorMalformedInput,
};
enum AlternateUrlTableErrors {
OK = 0,
OUT_OF_MEMORY = 1,
MALFORMED_INPUT = 2,
};
</pre>
<p>Until January 2009, the style was to name enum values
like <a href="#Macro_Names">macros</a>. This caused
problems with name collisions between enum values and
macros. Hence, the change to prefer constant-style naming
was put in place. New code should prefer constant-style
naming if possible. However, there is no reason to change
old code to use constant-style names, unless the old
names are actually causing a compile-time problem.</p>
</div>
<h3 id="Macro_Names">Macro Names</h3>
<div class="summary">
<p>You're not really going to <a href="#Preprocessor_Macros">
define a macro</a>, are you? If you do, they're like this:
<code>MY_MACRO_THAT_SCARES_SMALL_CHILDREN</code>.</p>
</div>
<div class="stylebody">
<p>Please see the <a href="#Preprocessor_Macros">description
of macros</a>; in general macros should <em>not</em> be used.
However, if they are absolutely needed, then they should be
named with all capitals and underscores.</p>
<pre>#define ROUND(x) ...
#define PI_ROUNDED 3.0
</pre>
</div>
<h3 id="Exceptions_to_Naming_Rules">Exceptions to Naming Rules</h3>
<div class="summary">
<p>If you are naming something that is analogous to an
existing C or C++ entity then you can follow the existing
naming convention scheme.</p>
</div>
<div class="stylebody">
<dl>
<dt><code>bigopen()</code></dt>
<dd>function name, follows form of <code>open()</code></dd>
<dt><code>uint</code></dt>
<dd><code>typedef</code></dd>
<dt><code>bigpos</code></dt>
<dd><code>struct</code> or <code>class</code>, follows
form of <code>pos</code></dd>
<dt><code>sparse_hash_map</code></dt>
<dd>STL-like entity; follows STL naming conventions</dd>
<dt><code>LONGLONG_MAX</code></dt>
<dd>a constant, as in <code>INT_MAX</code></dd>
</dl>
</div>
<h2 id="Comments">Comments</h2>
<p>Though a pain to write, comments are absolutely vital to
keeping our code readable. The following rules describe what
you should comment and where. But remember: while comments are
very important, the best code is self-documenting. Giving
sensible names to types and variables is much better than using
obscure names that you must then explain through comments.</p>
<p>When writing your comments, write for your audience: the
next
contributor who will need to
understand your code. Be generous &#8212; the next
one may be you!</p>
<h3 id="Comment_Style">Comment Style</h3>
<div class="summary">
<p>Use either the <code>//</code> or <code>/* */</code>
syntax, as long as you are consistent.</p>
</div>
<div class="stylebody">
<p>You can use either the <code>//</code> or the <code>/*
*/</code> syntax; however, <code>//</code> is
<em>much</em> more common. Be consistent with how you
comment and what style you use where.</p>
</div>
<h3 id="File_Comments">File Comments</h3>
<div class="summary">
<p> Start each file with license
boilerplate, followed by a description of its
contents.</p>
</div>
<div class="stylebody">
<h4 class="stylepoint_subsection">Legal Notice and Author
Line</h4>
<p>Every file should contain license
boilerplate. Choose the appropriate boilerplate for the
license used by the project (for example, Apache 2.0,
BSD, LGPL, GPL).</p>
<p>If you make significant changes to a file with an
author line, consider deleting the author line.</p>
<h4 class="stylepoint_subsection">File Contents</h4>
<p>Every file should have a comment at the top describing
its contents.</p>
<p>Generally a <code>.h</code> file will describe the
classes that are declared in the file with an overview of
what they are for and how they are used. A
<code>.cc</code> file should contain more information
about implementation details or discussions of tricky
algorithms. If you feel the implementation details or a
discussion of the algorithms would be useful for someone
reading the <code>.h</code>, feel free to put it there
instead, but mention in the <code>.cc</code> that the
documentation is in the <code>.h</code> file.</p>
<p>Do not duplicate comments in both the <code>.h</code>
and the <code>.cc</code>. Duplicated comments
diverge.</p>
</div>
<h3 id="Class_Comments">Class Comments</h3>
<div class="summary">
<p>Every class definition should have an accompanying comment
that describes what it is for and how it should be used.</p>
</div>
<div class="stylebody">
<pre>// Iterates over the contents of a GargantuanTable. Sample usage:
// GargantuanTableIterator* iter = table-&gt;NewIterator();
// for (iter-&gt;Seek("foo"); !iter-&gt;done(); iter-&gt;Next()) {
// process(iter-&gt;key(), iter-&gt;value());
// }
// delete iter;
class GargantuanTableIterator {
...
};
</pre>
<p>If you have already described a class in detail in the
comments at the top of your file feel free to simply
state "See comment at top of file for a complete
description", but be sure to have some sort of
comment.</p>
<p>Document the synchronization assumptions the class
makes, if any. If an instance of the class can be
accessed by multiple threads, take extra care to document
the rules and invariants surrounding multithreaded
use.</p>
</div>
<h3 id="Function_Comments">Function Comments</h3>
<div class="summary">
<p>Declaration comments describe use of the function; comments
at the definition of a function describe operation.</p>
</div>
<div class="stylebody">
<h4 class="stylepoint_subsection">Function Declarations</h4>
<p>Every function declaration should have comments
immediately preceding it that describe what the function
does and how to use it. These comments should be
descriptive ("Opens the file") rather than imperative
("Open the file"); the comment describes the function, it
does not tell the function what to do. In general, these
comments do not describe how the function performs its
task. Instead, that should be left to comments in the
function definition.</p>
<p>Types of things to mention in comments at the function
declaration:</p>
<ul>
<li>What the inputs and outputs are.</li>
<li>For class member functions: whether the object
remembers reference arguments beyond the duration of
the method call, and whether it will free them or
not.</li>
<li>If the function allocates memory that the caller
must free.</li>
<li>Whether any of the arguments can be a null
pointer.</li>
<li>If there are any performance implications of how a
function is used.</li>
<li>If the function is re-entrant. What are its
synchronization assumptions?</li>
</ul>
<p>Here is an example:</p>
<pre>// Returns an iterator for this table. It is the client's
// responsibility to delete the iterator when it is done with it,
// and it must not use the iterator once the GargantuanTable object
// on which the iterator was created has been deleted.
//
// The iterator is initially positioned at the beginning of the table.
//
// This method is equivalent to:
// Iterator* iter = table-&gt;NewIterator();
// iter-&gt;Seek("");
// return iter;
// If you are going to immediately seek to another place in the
// returned iterator, it will be faster to use NewIterator()
// and avoid the extra seek.
Iterator* GetIterator() const;
</pre>
<p>However, do not be unnecessarily verbose or state the
completely obvious. Notice below that it is not necessary
to say "returns false otherwise" because this is
implied.</p>
<pre>// Returns true if the table cannot hold any more entries.
bool IsTableFull();
</pre>
<p>When commenting constructors and destructors, remember
that the person reading your code knows what constructors
and destructors are for, so comments that just say
something like "destroys this object" are not useful.
Document what constructors do with their arguments (for
example, if they take ownership of pointers), and what
cleanup the destructor does. If this is trivial, just
skip the comment. It is quite common for destructors not
to have a header comment.</p>
<h4 class="stylepoint_subsection">Function Definitions</h4>
<p>If there is anything tricky about how a function does
its job, the function definition should have an
explanatory comment. For example, in the definition
comment you might describe any coding tricks you use,
give an overview of the steps you go through, or explain
why you chose to implement the function in the way you
did rather than using a viable alternative. For instance,
you might mention why it must acquire a lock for the
first half of the function but why it is not needed for
the second half.</p>
<p>Note you should <em>not</em> just repeat the comments
given with the function declaration, in the
<code>.h</code> file or wherever. It's okay to
recapitulate briefly what the function does, but the
focus of the comments should be on how it does it.</p>
</div>
<h3 id="Variable_Comments">Variable Comments</h3>
<div class="summary">
<p>In general the actual name of the variable should be
descriptive enough to give a good idea of what the variable
is used for. In certain cases, more comments are required.</p>
</div>
<div class="stylebody">
<h4 class="stylepoint_subsection">Class Data Members</h4>
<p>Each class data member (also called an instance
variable or member variable) should have a comment
describing what it is used for. If the variable can take
sentinel values with special meanings, such as a null
pointer or -1, document this. For example:</p>
<pre>private:
// Keeps track of the total number of entries in the table.
// Used to ensure we do not go over the limit. -1 means
// that we don't yet know how many entries the table has.
int num_total_entries_;
</pre>
<h4 class="stylepoint_subsection">Global Variables</h4>
<p>As with data members, all global variables should have
a comment describing what they are and what they are used
for. For example:</p>
<pre>// The total number of tests cases that we run through in this regression test.
const int kNumTestCases = 6;
</pre>
</div>
<h3 id="Implementation_Comments">Implementation Comments</h3>
<div class="summary">
<p>In your implementation you should have comments in tricky,
non-obvious, interesting, or important parts of your code.</p>
</div>
<div class="stylebody">
<h4 class="stylepoint_subsection">Explanatory Comments</h4>
<p>Tricky or complicated code blocks should have comments
before them. Example:</p>
<pre>// Divide result by two, taking into account that x
// contains the carry from the add.
for (int i = 0; i &lt; result-&gt;size(); i++) {
x = (x &lt;&lt; 8) + (*result)[i];
(*result)[i] = x &gt;&gt; 1;
x &amp;= 1;
}
</pre>
<h4 class="stylepoint_subsection">Line Comments</h4>
<p>Also, lines that are non-obvious should get a comment
at the end of the line. These end-of-line comments should
be separated from the code by 2 spaces. Example:</p>
<pre>// If we have enough memory, mmap the data portion too.
mmap_budget = max&lt;int64&gt;(0, mmap_budget - index_-&gt;length());
if (mmap_budget &gt;= data_size_ &amp;&amp; !MmapData(mmap_chunk_bytes, mlock))
return; // Error already logged.
</pre>
<p>Note that there are both comments that describe what
the code is doing, and comments that mention that an
error has already been logged when the function
returns.</p>
<p>If you have several comments on subsequent lines, it
can often be more readable to line them up:</p>
<pre>DoSomething(); // Comment here so the comments line up.
DoSomethingElseThatIsLonger(); // Two spaces between the code and the comment.
{ // One space before comment when opening a new scope is allowed,
// thus the comment lines up with the following comments and code.
DoSomethingElse(); // Two spaces before line comments normally.
}
vector&lt;string&gt; list{// Comments in braced lists describe the next element ..
"First item",
// .. and should be aligned appropriately.
"Second item"};
DoSomething(); /* For trailing block comments, one space is fine. */
</pre>
<h4 class="stylepoint_subsection">nullptr/NULL, true/false, 1, 2, 3...</h4>
<p>When you pass in a null pointer, boolean, or literal
integer values to functions, you should consider adding a
comment about what they are, or make your code
self-documenting by using constants. For example,
compare:</p>
<pre class="badcode">bool success = CalculateSomething(interesting_value,
10,
false,
NULL); // What are these arguments??
</pre>
<p>versus:</p>
<pre>bool success = CalculateSomething(interesting_value,
10, // Default base value.
false, // Not the first time we're calling this.
NULL); // No callback.
</pre>
<p>Or alternatively, constants or self-describing variables:</p>
<pre>const int kDefaultBaseValue = 10;
const bool kFirstTimeCalling = false;
Callback *null_callback = NULL;
bool success = CalculateSomething(interesting_value,
kDefaultBaseValue,
kFirstTimeCalling,
null_callback);
</pre>
<h4 class="stylepoint_subsection">Don'ts</h4>
<p>Note that you should <em>never</em> describe the code
itself. Assume that the person reading the code knows C++
better than you do, even though he or she does not know
what you are trying to do:</p>
<pre class="badcode">// Now go through the b array and make sure that if i occurs,
// the next element is i+1.
... // Geez. What a useless comment.
</pre>
</div>
<h3 id="Punctuation,_Spelling_and_Grammar">Punctuation, Spelling and Grammar</h3>
<div class="summary">
<p>Pay attention to punctuation, spelling, and grammar; it is
easier to read well-written comments than badly written
ones.</p>
</div>
<div class="stylebody">
<p>Comments should be as readable as narrative text, with
proper capitalization and punctuation. In many cases,
complete sentences are more readable than sentence
fragments. Shorter comments, such as comments at the end
of a line of code, can sometimes be less formal, but you
should be consistent with your style.</p>
<p>Although it can be frustrating to have a code reviewer
point out that you are using a comma when you should be
using a semicolon, it is very important that source code
maintain a high level of clarity and readability. Proper
punctuation, spelling, and grammar help with that
goal.</p>
</div>
<h3 id="TODO_Comments">TODO Comments</h3>
<div class="summary">
<p>Use <code>TODO</code> comments for code that is temporary,
a short-term solution, or good-enough but not perfect.</p>
</div>
<div class="stylebody">
<p><code>TODO</code>s should include the string
<code>TODO</code> in all caps, followed by the
name, e-mail address, or other
identifier of the person
with the best context
about the problem referenced by the <code>TODO</code>. The
main purpose is to have a consistent <code>TODO</code> that
can be searched to find out how to get more details upon
request. A <code>TODO</code> is not a commitment that the
person referenced will fix the problem. Thus when you create
a <code>TODO</code>, it is almost always your
name
that is given.</p>
<div>
<pre>// TODO(kl@gmail.com): Use a "*" here for concatenation operator.
// TODO(Zeke) change this to use relations.
</pre>
</div>
<p>If your <code>TODO</code> is of the form "At a future
date do something" make sure that you either include a
very specific date ("Fix by November 2005") or a very
specific event ("Remove this code when all clients can
handle XML responses.").</p>
</div>
<h3 id="Deprecation_Comments">Deprecation Comments</h3>
<div class="summary">
<p>Mark deprecated interface points with <code>DEPRECATED</code>
comments.</p>
</div>
<div class="stylebody">
<p>You can mark an interface as deprecated by writing a
comment containing the word <code>DEPRECATED</code> in
all caps. The comment goes either before the declaration
of the interface or on the same line as the
declaration.</p>
<p>After the word
<code>DEPRECATED</code>, write your name, e-mail address,
or other identifier in parentheses.</p>
<p>A deprecation comment must include simple, clear
directions for people to fix their callsites. In C++, you
can implement a deprecated function as an inline function
that calls the new interface point.</p>
<p>Marking an interface point <code>DEPRECATED</code>
will not magically cause any callsites to change. If you
want people to actually stop using the deprecated
facility, you will have to fix the callsites yourself or
recruit a crew to help you.</p>
<p>New code should not contain calls to deprecated
interface points. Use the new interface point instead. If
you cannot understand the directions, find the person who
created the deprecation and ask them for help using the
new interface point.</p>
</div>
<h2 id="Formatting">Formatting</h2>
<p>Coding style and formatting are pretty arbitrary, but a
project is much easier to follow
if everyone uses the same style. Individuals may not agree with every
aspect of the formatting rules, and some of the rules may take
some getting used to, but it is important that all
project contributors follow the
style rules so that
they can all read and understand
everyone's code easily.</p>
<p>To help you format code correctly, we've
created a
<a href="http://google-styleguide.googlecode.com/svn/trunk/google-c-style.el">
settings file for emacs</a>.</p>
<h3 id="Line_Length">Line Length</h3>
<div class="summary">
<p>Each line of text in your code should be at most 80
characters long.</p>
</div>
<div class="stylebody">
<p>We recognize that this rule is
controversial, but so much existing code already adheres
to it, and we feel that consistency is important.</p>
<div class="pros">
<p>Those who favor this rule
argue that it is rude to force them to resize
their windows and there is no need for anything longer.
Some folks are used to having several code windows
side-by-side, and thus don't have room to widen their
windows in any case. People set up their work environment
assuming a particular maximum window width, and 80
columns has been the traditional standard. Why change
it?</p>
</div>
<div class="cons">
<p>Proponents of change argue that a wider line can make
code more readable. The 80-column limit is an hidebound
throwback to 1960s mainframes; modern equipment has wide screens that
can easily show longer lines.</p>
</div>
<div class="decision">
<p> 80 characters is the maximum.</p>
<p class="exception">If a comment line contains an example
command or a literal URL longer than 80 characters, that
line may be longer than 80 characters for ease of cut and
paste.</p>
<p class="exception">A raw-string literal may have content
that exceeds 80 characters. Except for test code, such literals
should appear near top of a file.</p>
<p class="exception">An <code>#include</code> statement with a
long path may exceed 80 columns.</p>
<p class="exception">You needn't be concerned about
<a href="#The__define_Guard">header guards</a> that exceed
the maximum length. </p>
</div>
</div>
<h3 id="Non-ASCII_Characters">Non-ASCII Characters</h3>
<div class="summary">
<p>Non-ASCII characters should be rare, and must use UTF-8
formatting.</p>
</div>
<div class="stylebody">
<p>You shouldn't hard-code user-facing text in source,
even English, so use of non-ASCII characters should be
rare. However, in certain cases it is appropriate to
include such words in your code. For example, if your
code parses data files from foreign sources, it may be
appropriate to hard-code the non-ASCII string(s) used in
those data files as delimiters. More commonly, unittest
code (which does not need to be localized) might
contain non-ASCII strings. In such cases, you should use
UTF-8, since that is an encoding
understood by most tools able to handle more than just
ASCII.</p>
<p>Hex encoding is also OK, and encouraged where it
enhances readability &#8212; for example,
<code>"\xEF\xBB\xBF"</code>, or, even more simply,
<code>u8"\uFEFF"</code>, is the Unicode zero-width
no-break space character, which would be invisible if
included in the source as straight UTF-8.</p>
<p>Use the <code>u8</code> prefix
to guarantee that a string literal containing
<code>\uXXXX</code> escape sequences is encoded as UTF-8.
Do not use it for strings containing non-ASCII characters
encoded as UTF-8, because that will produce incorrect
output if the compiler does not interpret the source file
as UTF-8. </p>
<p>You shouldn't use the C++11 <code>char16_t</code> and
<code>char32_t</code> character types, since they're for
non-UTF-8 text. For similar reasons you also shouldn't
use <code>wchar_t</code> (unless you're writing code that
interacts with the Windows API, which uses
<code>wchar_t</code> extensively).</p>
</div>
<h3 id="Spaces_vs._Tabs">Spaces vs. Tabs</h3>
<div class="summary">
<p>Use only spaces, and indent 2 spaces at a time.</p>
</div>
<div class="stylebody">
<p>We use spaces for indentation. Do not use tabs in your
code. You should set your editor to emit spaces when you
hit the tab key.</p>
</div>
<h3 id="Function_Declarations_and_Definitions">Function Declarations and Definitions</h3>
<div class="summary">
<p>Return type on the same line as function name, parameters
on the same line if they fit. Wrap parameter lists which do
not fit on a single line as you would wrap arguments in a
function call.</p>
</div>
<div class="stylebody">
<p>Functions look like this:</p>
<pre>ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) {
DoSomething();
...
}
</pre>
<p>If you have too much text to fit on one line:</p>
<pre>ReturnType ClassName::ReallyLongFunctionName(Type par_name1, Type par_name2,
Type par_name3) {
DoSomething();
...
}
</pre>
<p>or if you cannot fit even the first parameter:</p>
<pre>ReturnType LongClassName::ReallyReallyReallyLongFunctionName(
Type par_name1, // 4 space indent
Type par_name2,
Type par_name3) {
DoSomething(); // 2 space indent
...
}
</pre>
<p>Some points to note:</p>
<ul>
<li>If you cannot fit the return type and the function
name on a single line, break between them.</li>
<li>If you break after the return type of a function
declaration or definition, do not indent.</li>
<li>The open parenthesis is always on the same line as
the function name.</li>
<li>There is never a space between the function name
and the open parenthesis.</li>
<li>There is never a space between the parentheses and
the parameters.</li>
<li>The open curly brace is always at the end of the
same line as the last parameter.</li>
<li>The close curly brace is either on the last line by
itself or (if other style rules permit) on the same
line as the open curly brace.</li>
<li>There should be a space between the close
parenthesis and the open curly brace.</li>
<li>All parameters should be named, with identical
names in the declaration and implementation.</li>
<li>All parameters should be aligned if possible.</li>
<li>Default indentation is 2 spaces.</li>
<li>Wrapped parameters have a 4 space indent.</li>
</ul>
<p>If some parameters are unused, comment out the
variable name in the function definition:</p>
<pre>// Always have named parameters in interfaces.
class Shape {
public:
virtual void Rotate(double radians) = 0;
};
// Always have named parameters in the declaration.
class Circle : public Shape {
public:
virtual void Rotate(double radians);
};
// Comment out unused named parameters in definitions.
void Circle::Rotate(double /*radians*/) {}
</pre>
<pre class="badcode">// Bad - if someone wants to implement later, it's not clear what the
// variable means.
void Circle::Rotate(double) {}
</pre>
</div>
<h3 id="Formatting_Lambda_Expressions">Lambda Expressions</h3>
<div class="summary">
<p>Format parameters and bodies as for any other function, and capture
lists like other comma-separated lists.</p>
</div>
<div class="stylebody">
<p>For by-reference captures, do not leave a space between the
ampersand (&amp;) and the variable name.</p>
<pre>int x = 0;
auto add_to_x = [&amp;x](int n) { x += n; };
</pre>
<p>Short lambdas may be written inline as function arguments.</p>
<pre>std::set&lt;int&gt; blacklist = {7, 8, 9};
std::vector&lt;int&gt; digits = {3, 9, 1, 8, 4, 7, 1};
digits.erase(std::remove_if(digits.begin(), digits.end(), [&amp;blacklist](int i) {
return blacklist.find(i) != blacklist.end();
}),
digits.end());
</pre>
</div>
<h3 id="Function_Calls">Function Calls</h3>
<div class="summary">
<p>Either write the call all on a single line, wrap the
arguments at the parenthesis, or start the arguments on a new
line indented by four spaces and continue at that 4 space
indent. In the absence of other considerations, use the
minimum number of lines, including placing multiple arguments
on each line where appropriate.</p>
</div>
<div class="stylebody">
<p>Function calls have the following format:</p>
<pre>bool retval = DoSomething(argument1, argument2, argument3);
</pre>
<p>If the arguments do not all fit on one line, they
should be broken up onto multiple lines, with each
subsequent line aligned with the first argument. Do not
add spaces after the open paren or before the close
paren:</p>
<pre>bool retval = DoSomething(averyveryveryverylongargument1,
argument2, argument3);
</pre>
<p>Arguments may optionally all be placed on subsequent
lines with a four space indent:</p>
<pre>if (...) {
...
...
if (...) {
DoSomething(
argument1, argument2, // 4 space indent
argument3, argument4);
}
</pre>
<p>Put multiple arguments on a single line to reduce the
number of lines necessary for calling a function unless
there is a specific readability problem. Some find that
formatting with strictly one argument on each line is
more readable and simplifies editing of the arguments.
However, we prioritize for the reader over the ease of
editing arguments, and most readability problems are
better addressed with the following techniques.</p>
<p>If having multiple arguments in a single line decreases
readability due to the complexity or confusing nature of the
expressions that make up some arguments, try creating
variables that capture those arguments in a descriptive name:</p>
<pre>int my_heuristic = scores[x] * y + bases[x];
bool retval = DoSomething(my_heuristic, x, y, z);
</pre>
<p>Or put the confusing argument on its own line with
an explanatory comment:</p>
<pre>bool retval = DoSomething(scores[x] * y + bases[x], // Score heuristic.
x, y, z);
</pre>
<p>If there is still a case where one argument is
significantly more readable on its own line, then put it on
its own line. The decision should be specific to the argument
which is made more readable rather than a general policy.</p>
<p>Sometimes arguments form a structure that is important
for readability. In those cases, feel free to format the
arguments according to that structure:</p>
<pre>// Transform the widget by a 3x3 matrix.
my_widget.Transform(x1, x2, x3,
y1, y2, y3,
z1, z2, z3);
</pre>
</div>
<h3 id="Braced_Initializer_List_Format">Braced Initializer List Format</h3>
<div class="summary">
<p>Format a <a href="#Braced_Initializer_List">braced initializer list</a>
exactly like you would format a function call in its place.</p>
</div>
<div class="stylebody">
<p>If the braced list follows a name (e.g. a type or
variable name), format as if the <code>{}</code> were the
parentheses of a function call with that name. If there
is no name, assume a zero-length name.</p>
<pre>// Examples of braced init list on a single line.
return {foo, bar};
functioncall({foo, bar});
pair&lt;int, int&gt; p{foo, bar};
// When you have to wrap.
SomeFunction(
{"assume a zero-length name before {"},
some_other_function_parameter);
SomeType variable{
some, other, values,
{"assume a zero-length name before {"},
SomeOtherType{
"Very long string requiring the surrounding breaks.",
some, other values},
SomeOtherType{"Slightly shorter string",
some, other, values}};
SomeType variable{
"This is too long to fit all in one line"};
MyType m = { // Here, you could also break before {.
superlongvariablename1,
superlongvariablename2,
{short, interior, list},
{interiorwrappinglist,
interiorwrappinglist2}};
</pre>
</div>
<h3 id="Conditionals">Conditionals</h3>
<div class="summary">
<p>Prefer no spaces inside parentheses. The <code>if</code>
and <code>else</code> keywords belong on separate lines.</p>
</div>
<div class="stylebody">
<p>There are two acceptable formats for a basic
conditional statement. One includes spaces between the
parentheses and the condition, and one does not.</p>
<p>The most common form is without spaces. Either is
fine, but <em>be consistent</em>. If you are modifying a
file, use the format that is already present. If you are
writing new code, use the format that the other files in
that directory or project use. If in doubt and you have
no personal preference, do not add the spaces.</p>
<pre>if (condition) { // no spaces inside parentheses
... // 2 space indent.
} else if (...) { // The else goes on the same line as the closing brace.
...
} else {
...
}
</pre>
<p>If you prefer you may add spaces inside the
parentheses:</p>
<pre>if ( condition ) { // spaces inside parentheses - rare
... // 2 space indent.
} else { // The else goes on the same line as the closing brace.
...
}
</pre>
<p>Note that in all cases you must have a space between
the <code>if</code> and the open parenthesis. You must
also have a space between the close parenthesis and the
curly brace, if you're using one.</p>
<pre class="badcode">if(condition) { // Bad - space missing after IF.
if (condition){ // Bad - space missing before {.
if(condition){ // Doubly bad.
</pre>
<pre>if (condition) { // Good - proper space after IF and before {.
</pre>
<p>Short conditional statements may be written on one
line if this enhances readability. You may use this only
when the line is brief and the statement does not use the
<code>else</code> clause.</p>
<pre>if (x == kFoo) return new Foo();
if (x == kBar) return new Bar();
</pre>
<p>This is not allowed when the if statement has an
<code>else</code>:</p>
<pre class="badcode">// Not allowed - IF statement on one line when there is an ELSE clause
if (x) DoThis();
else DoThat();
</pre>
<p>In general, curly braces are not required for
single-line statements, but they are allowed if you like
them; conditional or loop statements with complex
conditions or statements may be more readable with curly
braces. Some
projects require that an
<code>if</code> must always always have an accompanying
brace.</p>
<pre>if (condition)
DoSomething(); // 2 space indent.
if (condition) {
DoSomething(); // 2 space indent.
}
</pre>
<p>However, if one part of an
<code>if</code>-<code>else</code> statement uses curly
braces, the other part must too:</p>
<pre class="badcode">// Not allowed - curly on IF but not ELSE
if (condition) {
foo;
} else
bar;
// Not allowed - curly on ELSE but not IF
if (condition)
foo;
else {
bar;
}
</pre>
<pre>// Curly braces around both IF and ELSE required because
// one of the clauses used braces.
if (condition) {
foo;
} else {
bar;
}
</pre>
</div>
<h3 id="Loops_and_Switch_Statements">Loops and Switch Statements</h3>
<div class="summary">
<p>Switch statements may use braces for blocks. Annotate
non-trivial fall-through between cases.
Braces are optional for single-statement loops.
Empty loop bodies should use <code>{}</code> or <code>continue</code>.</p>
</div>
<div class="stylebody">
<p><code>case</code> blocks in <code>switch</code>
statements can have curly braces or not, depending on
your preference. If you do include curly braces they
should be placed as shown below.</p>
<p>If not conditional on an enumerated value, switch
statements should always have a <code>default</code> case
(in the case of an enumerated value, the compiler will
warn you if any values are not handled). If the default
case should never execute, simply
<code>assert</code>:</p>
<div>
<pre>switch (var) {
case 0: { // 2 space indent
... // 4 space indent
break;
}
case 1: {
...
break;
}
default: {
assert(false);
}
}
</pre>
</div>
<p> Braces are optional for single-statement loops.</p>
<pre>for (int i = 0; i &lt; kSomeNumber; ++i)
printf("I love you\n");
for (int i = 0; i &lt; kSomeNumber; ++i) {
printf("I take it back\n");
}
</pre>
<p>Empty loop bodies should use <code>{}</code> or
<code>continue</code>, but not a single semicolon.</p>
<pre>while (condition) {
// Repeat test until it returns false.
}
for (int i = 0; i &lt; kSomeNumber; ++i) {} // Good - empty body.
while (condition) continue; // Good - continue indicates no logic.
</pre>
<pre class="badcode">while (condition); // Bad - looks like part of do/while loop.
</pre>
</div>
<h3 id="Pointer_and_Reference_Expressions">Pointer and Reference Expressions</h3>
<div class="summary">
<p>No spaces around period or arrow. Pointer operators do not
have trailing spaces.</p>
</div>
<div class="stylebody">
<p>The following are examples of correctly-formatted
pointer and reference expressions:</p>
<pre>x = *p;
p = &amp;x;
x = r.y;
x = r-&gt;y;
</pre>
<p>Note that:</p>
<ul>
<li>There are no spaces around the period or arrow when
accessing a member.</li>
<li>Pointer operators have no space after the
<code>*</code> or <code>&amp;</code>.</li>
</ul>
<p>When declaring a pointer variable or argument, you may
place the asterisk adjacent to either the type or to the
variable name:</p>
<pre>// These are fine, space preceding.
char *c;
const string &amp;str;
// These are fine, space following.
char* c; // but remember to do "char* c, *d, *e, ...;"!
const string&amp; str;
</pre>
<pre class="badcode">char * c; // Bad - spaces on both sides of *
const string &amp; str; // Bad - spaces on both sides of &amp;
</pre>
<p>You should do this consistently within a single
file,
so, when modifying an existing file, use the style in
that file.</p>
</div>
<h3 id="Boolean_Expressions">Boolean Expressions</h3>
<div class="summary">
<p>When you have a boolean expression that is longer than the
<a href="#Line_Length">standard line length</a>, be
consistent in how you break up the lines.</p>
</div>
<div class="stylebody">
<p>In this example, the logical AND operator is always at
the end of the lines:</p>
<pre>if (this_one_thing &gt; this_other_thing &amp;&amp;
a_third_thing == a_fourth_thing &amp;&amp;
yet_another &amp;&amp; last_one) {
...
}
</pre>
<p>Note that when the code wraps in this example, both of
the <code>&amp;&amp;</code> logical AND operators are at
the end of the line. This is more common in Google code,
though wrapping all operators at the beginning of the
line is also allowed. Feel free to insert extra
parentheses judiciously because they can be very helpful
in increasing readability when used
appropriately. Also note that you should always use
the punctuation operators, such as
<code>&amp;&amp;</code> and <code>~</code>, rather than
the word operators, such as <code>and</code> and
<code>compl</code>.</p>
</div>
<h3 id="Return_Values">Return Values</h3>
<div class="summary">
<p>Do not needlessly surround the <code>return</code>
expression with parentheses.</p>
</div>
<div class="stylebody">
<p>Use parentheses in <code>return expr;</code> only
where you would use them in <code>x = expr;</code>.</p>
<pre>return result; // No parentheses in the simple case.
// Parentheses OK to make a complex expression more readable.
return (some_long_condition &amp;&amp;
another_condition);
</pre>
<pre class="badcode">return (value); // You wouldn't write var = (value);
return(result); // return is not a function!
</pre>
</div>
<h3 id="Variable_and_Array_Initialization">Variable and Array Initialization</h3>
<div class="summary">
<p>Your choice of <code>=</code>, <code>()</code>, or
<code>{}</code>.</p>
</div>
<div class="stylebody">
<p>You may choose between <code>=</code>,
<code>()</code>, and <code>{}</code>; the following are
all correct:</p>
<pre>int x = 3;
int x(3);
int x{3};
string name = "Some Name";
string name("Some Name");
string name{"Some Name"};
</pre>
<p>Be careful when using a braced initialization list <code>{...}</code>
on a type with an <code>std::initializer_list</code> constructor.
A nonempty <i>braced-init-list</i> prefers the
<code>std::initializer_list</code> constructor whenever
possible. Note that empty braces <code>{}</code> are special, and
will call a default constructor if available. To force the
non-<code>std::initializer_list</code> constructor, use parentheses
instead of braces.</p>
<pre>vector&lt;int&gt; v(100, 1); // A vector of 100 1s.
vector&lt;int&gt; v{100, 1}; // A vector of 100, 1.
</pre>
<p>Also, the brace form prevents narrowing of integral
types. This can prevent some types of programming
errors.</p>
<pre>int pi(3.14); // OK -- pi == 3.
int pi{3.14}; // Compile error: narrowing conversion.
</pre>
</div>
<h3 id="Preprocessor_Directives">Preprocessor Directives</h3>
<div class="summary">
<p>The hash mark that starts a preprocessor directive should
always be at the beginning of the line.</p>
</div>
<div class="stylebody">
<p>Even when preprocessor directives are within the body
of indented code, the directives should start at the
beginning of the line.</p>
<pre>// Good - directives at beginning of line
if (lopsided_score) {
#if DISASTER_PENDING // Correct -- Starts at beginning of line
DropEverything();
# if NOTIFY // OK but not required -- Spaces after #
NotifyClient();
# endif
#endif
BackToNormal();
}
</pre>
<pre class="badcode">// Bad - indented directives
if (lopsided_score) {
#if DISASTER_PENDING // Wrong! The "#if" should be at beginning of line
DropEverything();
#endif // Wrong! Do not indent "#endif"
BackToNormal();
}
</pre>
</div>
<h3 id="Class_Format">Class Format</h3>
<div class="summary">
<p>Sections in <code>public</code>, <code>protected</code> and
<code>private</code> order, each indented one space.</p>
</div>
<div class="stylebody">
<p>The basic format for a class declaration (lacking the
comments, see <a href="#Class_Comments">Class
Comments</a> for a discussion of what comments are
needed) is:</p>
<pre>class MyClass : public OtherClass {
public: // Note the 1 space indent!
MyClass(); // Regular 2 space indent.
explicit MyClass(int var);
~MyClass() {}
void SomeFunction();
void SomeFunctionThatDoesNothing() {
}
void set_some_var(int var) { some_var_ = var; }
int some_var() const { return some_var_; }
private:
bool SomeInternalFunction();
int some_var_;
int some_other_var_;
};
</pre>
<p>Things to note:</p>
<ul>
<li>Any base class name should be on the same line as
the subclass name, subject to the 80-column limit.</li>
<li>The <code>public:</code>, <code>protected:</code>,
and <code>private:</code> keywords should be indented
one space.</li>
<li>Except for the first instance, these keywords
should be preceded by a blank line. This rule is
optional in small classes.</li>
<li>Do not leave a blank line after these
keywords.</li>
<li>The <code>public</code> section should be first,
followed by the <code>protected</code> and finally the
<code>private</code> section.</li>
<li>See <a href="#Declaration_Order">Declaration
Order</a> for rules on ordering declarations within
each of these sections.</li>
</ul>
</div>
<h3 id="Constructor_Initializer_Lists">Constructor Initializer Lists</h3>
<div class="summary">
<p>Constructor initializer lists can be all on one line or
with subsequent lines indented four spaces.</p>
</div>
<div class="stylebody">
<p>There are two acceptable formats for initializer
lists:</p>
<pre>// When it all fits on one line:
MyClass::MyClass(int var) : some_var_(var), some_other_var_(var + 1) {}
</pre>
<p>or</p>
<pre>// When it requires multiple lines, indent 4 spaces, putting the colon on
// the first initializer line:
MyClass::MyClass(int var)
: some_var_(var), // 4 space indent
some_other_var_(var + 1) { // lined up
...
DoSomething();
...
}
</pre>
</div>
<h3 id="Namespace_Formatting">Namespace Formatting</h3>
<div class="summary">
<p>The contents of namespaces are not indented.</p>
</div>
<div class="stylebody">
<p><a href="#Namespaces">Namespaces</a> do not add an
extra level of indentation. For example, use:</p>
<pre>namespace {
void foo() { // Correct. No extra indentation within namespace.
...
}
} // namespace
</pre>
<p>Do not indent within a namespace:</p>
<pre class="badcode">namespace {
// Wrong. Indented when it should not be.
void foo() {
...
}
} // namespace
</pre>
<p>When declaring nested namespaces, put each namespace
on its own line.</p>
<pre>namespace foo {
namespace bar {
</pre>
</div>
<h3 id="Horizontal_Whitespace">Horizontal Whitespace</h3>
<div class="summary">
<p>Use of horizontal whitespace depends on location. Never put
trailing whitespace at the end of a line.</p>
</div>
<div class="stylebody">
<h4 class="stylepoint_subsection">General</h4>
<pre>void f(bool b) { // Open braces should always have a space before them.
...
int i = 0; // Semicolons usually have no space before them.
// Spaces inside braces for braced-init-list are optional. If you use them,
// put them on both sides!
int x[] = { 0 };
int x[] = {0};
// Spaces around the colon in inheritance and initializer lists.
class Foo : public Bar {
public:
// For inline function implementations, put spaces between the braces
// and the implementation itself.
Foo(int b) : Bar(), baz_(b) {} // No spaces inside empty braces.
void Reset() { baz_ = 0; } // Spaces separating braces from implementation.
...
</pre>
<p>Adding trailing whitespace can cause extra work for
others editing the same file, when they merge, as can
removing existing trailing whitespace. So: Don't
introduce trailing whitespace. Remove it if you're
already changing that line, or do it in a separate
clean-up
operation (preferably when no-one
else is working on the file).</p>
<h4 class="stylepoint_subsection">Loops and Conditionals</h4>
<pre>if (b) { // Space after the keyword in conditions and loops.
} else { // Spaces around else.
}
while (test) {} // There is usually no space inside parentheses.
switch (i) {
for (int i = 0; i &lt; 5; ++i) {
// Loops and conditions may have spaces inside parentheses, but this
// is rare. Be consistent.
switch ( i ) {
if ( test ) {
for ( int i = 0; i &lt; 5; ++i ) {
// For loops always have a space after the semicolon. They may have a space
// before the semicolon, but this is rare.
for ( ; i &lt; 5 ; ++i) {
...
// Range-based for loops always have a space before and after the colon.
for (auto x : counts) {
...
}
switch (i) {
case 1: // No space before colon in a switch case.
...
case 2: break; // Use a space after a colon if there's code after it.
</pre>
<h4 class="stylepoint_subsection">Operators</h4>
<pre>// Assignment operators always have spaces around them.
x = 0;
// Other binary operators usually have spaces around them, but it's
// OK to remove spaces around factors. Parentheses should have no
// internal padding.
v = w * x + y / z;
v = w*x + y/z;
v = w * (x + z);
// No spaces separating unary operators and their arguments.
x = -5;
++x;
if (x &amp;&amp; !y)
...
</pre>
<h4 class="stylepoint_subsection">Templates and Casts</h4>
<pre>// No spaces inside the angle brackets (&lt; and &gt;), before
// &lt;, or between &gt;( in a cast
vector&lt;string&gt; x;
y = static_cast&lt;char*&gt;(x);
// Spaces between type and pointer are OK, but be consistent.
vector&lt;char *&gt; x;
set&lt;list&lt;string&gt;&gt; x; // Permitted in C++11 code.
set&lt;list&lt;string&gt; &gt; x; // C++03 required a space in &gt; &gt;.
// You may optionally use symmetric spacing in &lt; &lt;.
set&lt; list&lt;string&gt; &gt; x;
</pre>
</div>
<h3 id="Vertical_Whitespace">Vertical Whitespace</h3>
<div class="summary">
<p>Minimize use of vertical whitespace.</p>
</div>
<div class="stylebody">
<p>This is more a principle than a rule: don't use blank
lines when you don't have to. In particular, don't put
more than one or two blank lines between functions,
resist starting functions with a blank line, don't end
functions with a blank line, and be discriminating with
your use of blank lines inside functions.</p>
<p>The basic principle is: The more code that fits on one
screen, the easier it is to follow and understand the
control flow of the program. Of course, readability can
suffer from code being too dense as well as too spread
out, so use your judgement. But in general, minimize use
of vertical whitespace.</p>
<p>Some rules of thumb to help when blank lines may be
useful:</p>
<ul>
<li>Blank lines at the beginning or end of a function
very rarely help readability.</li>
<li>Blank lines inside a chain of if-else blocks may
well help readability.</li>
</ul>
</div>
<h2 id="Exceptions_to_the_Rules">Exceptions to the Rules</h2>
<p>The coding conventions described above are mandatory.
However, like all good rules, these sometimes have exceptions,
which we discuss here.</p>
<div>
<h3 id="Existing_Non-conformant_Code">Existing Non-conformant Code</h3>
<div class="summary">
<p>You may diverge from the rules when dealing with code that
does not conform to this style guide.</p>
</div>
<div class="stylebody">
<p>If you find yourself modifying code that was written
to specifications other than those presented by this
guide, you may have to diverge from these rules in order
to stay consistent with the local conventions in that
code. If you are in doubt about how to do this, ask the
original author or the person currently responsible for
the code. Remember that <em>consistency</em> includes
local consistency, too.</p>
</div>
</div>
<h2 class="ignoreLink">Parting Words</h2>
<p>Use common sense and <em>BE CONSISTENT</em>.</p>
<p>If you are editing code, take a few minutes to look at the
code around you and determine its style. If they use spaces
around their <code>if</code> clauses, you should, too. If their
comments have little boxes of stars around them, make your
comments have little boxes of stars around them too.</p>
<p>The point of having style guidelines is to have a common
vocabulary of coding so people can concentrate on what you are
saying, rather than on how you are saying it. We present global
style rules here so people know the vocabulary. But local style
is also important. If code you add to a file looks drastically
different from the existing code around it, the discontinuity
throws readers out of their rhythm when they go to read it. Try
to avoid this.</p>
<p>OK, enough writing about writing code; the code itself is much
more interesting. Have fun!</p>
<hr>
<p style="text-align:right; font-style:italic;">Revision 4.45</p>
</div>
</body></html>