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C++ style guide: Difference between revisions

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C++ style guide (edit)

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== Formatting ==
== Formatting ==
=== Line Length ===
There is no fixed line length.  In general, strive for clarity and readability and use your own judgement.
Everyone has access to monitors with more than 80 columns, but even so, exceptionally long lines can be hard to read.  However, keeping code together on a line that is logically one unit does improve readability.


=== Indentation ===
=== Indentation ===


Use only spaces, and indent 2 spaces at a time.
* Use only spaces, with 2 spaces per indent.
* Tabs are prohibited.
 
==== Functions, class, struct, enum ====
 
The curly braces defining the beginning and end of the block should appear on their own line.
 
The braces should not be indented, i.e., they align at the same indentation level as the keyword such as {{codeline|class}}.
 
The body of the block is indented.
 
Note that class access specifiers {{codeline|public}}, {{codeline|protected}}, {{codeline|private}} are not indented.
 
Example:
 
<syntaxhighlight lang="cpp">
class MatrixType
{
public:
  enum matrix_type
  {
    Unknown = 0,
    Full,
    Rectangular
  };
 
}
</syntaxhighlight>
 
==== Control structures (if, while, ...) ====
 
When indenting, indent the statement after control structures (like {{codeline|if}}, {{codeline|while}}, etc.).


We use spaces for indentation. Absolutely do not use tabs in your code.
If there is a compound statement, indent ''both'' the curly braces and the body of the statement (so that the body gets indented by ''two'' indents).
You should probably set your editor to emit spaces when you hit the tab key.


When indenting, indent the statement after control
Example:
structures (like {{codeline|if}}, {{codeline|while}}, etc.).  If there
is a compound statement, indent ''both'' the curly braces and the
body of the statement (so that the body gets indented by ''two''
indents).  This format is known as "GNU style" and is an option for
some code formatting tools.  Example indenting:


<pre>
<syntaxhighlight lang="cpp">
if (have_args)
if (have_args)
   {
   {
Line 30: Line 61:
else
else
   idx.push_back (make_value_list (args, arg_nm, tmp));
   idx.push_back (make_value_list (args, arg_nm, tmp));
</pre>
</syntaxhighlight>
 
If you have nested {{codeline|if}} statements, use extra braces for extra clarification.
 
==== Switch statements ====
 
Indent ''both'' the curly braces and the body of the switch statement (so that the body gets indented by ''two'' indents).
 
However, the {{codeline|case}} statement is not doubly indented and instead aligns with the first brace.
 
<syntaxhighlight lang="cpp">
switch (info)
  {
  case -1:
    {
      cout << "function failed\n";
      return false;
    }
 
  case 0:
    return true;
  }
</syntaxhighlight>
 
==== #ifdef directives ====
 
Indent code that follows a conditional processor directive such as {{codeline|#ifdef}} or {{codeline|#else}}.
 
Example
 
<syntaxhighlight lang="cpp">
#if defined (HAVE_CONFIG_H)
#  include "config.h"
#endif
</syntaxhighlight>


If you have nested {{codeline|if}} statements, use extra braces for extra
The '#' character may also be placed with the directive rather than remaining in column 1 if this looks better.
clarification.
 
==== Split long expressions ====


Split long expressions in such a way that a continuation line starts
Split long expressions in such a way that a continuation line starts
Line 40: Line 106:
innermost braces enclosing the split.  Example:
innermost braces enclosing the split.  Example:


<pre>
<syntaxhighlight lang="cpp">
SVD::type type = ((nargout == 0 || nargout == 1)
SVD::type type = ((nargout == 0 || nargout == 1)
                   ? SVD::sigma_only
                   ? SVD::sigma_only
                   : (nargin == 2) ? SVD::economy : SVD::std);
                   : (nargin == 2) ? SVD::economy : SVD::std);
</pre>
</syntaxhighlight>


=== Non indenting whitespace ===
==== Optional braces ====


Consider putting extra braces around a multi-line expression to make it
Consider putting extra braces around a multi-line expression to make it
more readable, even if they are not necessary.  Also, do not hesitate to
more readable, even if they are not necessary.  Also, do not hesitate to
put extra braces anywhere if it improves clarity.
put extra braces anywhere if it improves clarity.
=== Pointer and Reference appearance ===
Declarations of pointers have the '*' character cuddled with the ''name'' of the variable.
<syntaxhighlight lang="cpp">
unsigned int *pointer_variable;
</syntaxhighlight>
However, references have the '&' character cuddled with the ''type'' of the variable.
<syntaxhighlight lang="cpp">
unsigned int& reference_variable;
</syntaxhighlight>
=== Miscellaneous ===


The negation operator is written with a space between the operator
The negation operator is written with a space between the operator
and its target, e.g., {{codeline|! A}}.
and its target, e.g., {{codeline|! A}}.
=== Line Length ===
Keep the length of source lines to 79 characters or less, for maximum
readability in the widest range of environments.  This is inherited from
the [https://www.gnu.org/prep/standards/standards.html#Formatting GNU Coding Standards].


=== Function headers ===
=== Function headers ===


Format function headers like this:
In general, in non-header files, format function headers like this:


<pre>
<syntaxhighlight lang="cpp">
static bool
static bool
matches_patterns (const string_vector& patterns, int pat_idx,
matches_patterns (const string_vector& patterns, int pat_idx,
                   int num_pat, const std::string& name)
                   int num_pat, const std::string& name)
</pre>
</syntaxhighlight>


The return type of the function and any modifiers are specified on the first
The return type of the function and any modifiers are specified on the first
line.  The function name on the second line should start in column 1, and
line.  The function name on the second line should start in column 1, and
multi-line argument lists should be aligned on the first char after the open
multi-line argument lists should be aligned on the first character after the open
parenthesis.  You should put a space before the left open parenthesis and after
parenthesis.  Put a space before the left open parenthesis and after
commas, for both function definitions and function calls.
commas, for both function definitions and function calls.
For header files, or in class definitions, it may look better not to split the return type from the rest of the function header.  Use your own judgement.
=== Class declarations ===
The access specifier ({{codeline|public}}, {{codeline|protected}}, {{codeline|private}}) should always be stated rather than relying on the C++ language defaults for a particular object (for example, "{{codeline|class}}" = "{{codeline|private}}").
Within a class, the different access blocks should appear in the order 1) {{codeline|public}}, 2) {{codeline|protected}}, 3) {{codeline|private}}.
Within an access block, member functions (methods) should be specified before member variables.  If there are both member functions and member variables use
    //--------
between the sections to visually separate the two categories.


=== Namespace ===
=== Namespace ===


All code should be in the octave namespace. This is an ongoing project.  We
All code should be in the {{codeline|octave}} namespace or in a namespace below it.
are still moving existing classes into namespaces but all new classes
 
should go somewhere into the "octave" namespace.  There is 1 extra level for namespaces
Namespaces should start and stop using the special macros {{codeline|OCTAVE_BEGIN_NAMESPACE(XXX)}} and {{codeline|OCTAVE_END_NAMESPACE(XXX)}}.  There is no indentation of code that is placed into namespaces using these macros.
inside octave to be used with care, we don't want too many namespaces.
 
Ask before creating a new namespace.
Example
 
{{Code|Use of namespace macros|<syntaxhighlight lang="cpp">
OCTAVE_BEGIN_NAMESPACE(octave)
 
OCTAVE_BEGIN_NAMESPACE(math)


* Indent namespaces as any other block.  Emacs and other editors can do this automatically.
template <typename T>
* Define namespace on the .cc files;
void
* Do not use "using X" directives;
umfpack_report_control (const double *Control);
* Do not declare anything on the std namespace;


{{Code|namespace style on a .h file|<pre>
OCTAVE_END_NAMESPACE(math)
// Note indentation
OCTAVE_END_NAMESPACE(octave)
namespace octave
</syntaxhighlight>}}
{
 
  namespace math
If bare namespace directives must be used, as occasionally is required in Qt code, then the code within the namespace should be indented.
  {
    class foo
    {
    public:
      foo (...);
    };
  }
}
</pre>}}


{{Code|namespace style on a .cc file|<pre>
{{Code|bare namespace usage|<syntaxhighlight lang="cpp">
// Note indentation and that functions are not defined
// Note indentation and that functions are not defined as "octave::math::foo:foo"
// as "octave::math::foo:foo"
namespace octave
namespace octave
{
{
Line 118: Line 202:
   }
   }
}
}
</pre>}}
</syntaxhighlight>}}
 
==== Other Guidelines ====
* Do not use {{codeline|using XXX;}} directives
* Do not declare anything on the {{codeline|std::}} namespace


== Naming ==
== Naming ==


Use lowercase names if possible. Uppercase is acceptable for variable
Use lowercase names if possible. Uppercase is acceptable for variable names consisting of 1-2 letters. Do not use mixed case (a.k.a. CamelCase) names.
names consisting of 1-2 letters. Do not use mixed case names.
 
=== Member Variables ===
 
Member variables should use the prefix "m_" whenever possible.
 
=== Class Variables ===
 
Class variables should use the prefix "s_" (for "static") whenever possible.
 
=== Filenames ===


As with m-files, the file name of a C++ source file containing a class should match the name of the class defined within the file.  For example, "password.h" defines the class "password" rather than "passwd.h" which is a common abbreviation for "password".


== Header Files ==
== Header Files ==
Line 162: Line 260:
== C++ features ==
== C++ features ==


=== C++11 features ===
=== references ===
 
Use references when passing variables that will be changed by a subroutine rather than the C-style method of passing pointers.
 
{| class="wikitable"
! style="color:green;" | good
! style="color:darkred;" | bad
|-
| <syntaxhighlight lang="c++">
void foo (int& a_ref)
{
  // foo changes content of `a_ref`
  a_ref = a_ref + 1;
}
 
void bar ()
{
  int a = 42;
  foo (a);
}
</syntaxhighlight>
| <syntaxhighlight lang="c++">
void foo (int *a_ptr)
{
  // foo changes content of `a_ptr`
  *a_ptr = *aptr + 1;
}
 
void bar ()
{
  int a = 42;
  foo (&a);
}
</syntaxhighlight>
|}
 
When passing variables that are large, but will not be changed in a subroutine (read-only), use {{codeline|const}} references.  This helps avoid overflowing the finite stack capacity of a program while still ensuring that read-only access is enforced.
 
{| class="wikitable"
! style="color:green;" | good
! style="color:darkred;" | bad
|-
| <syntaxhighlight lang="c++">
void foo (const std::string& str_ref)
{
  // foo does not change content of `str_ref`
}
 
void bar ()
{
  std::string str ("This is a large variable, however as a reference it will take up just 8 bytes on the stack when passed to the subroutine foo()");
  foo (str);
}
</syntaxhighlight>
| <syntaxhighlight lang="c++">
void foo (std::string str_copy)
{
  // foo does not change content of `str_copy`
}
 
void bar ()
{
  std::string str ("This is a large variable that will be copied on to the stack and passed as a temporary variable to the subroutine foo()");
  foo (str);
}
</syntaxhighlight>
|}
 
=== new/delete ===
 
Pointers that will be allocated memory with {{codeline|new}} should be initialized with the C++ literal {{codeline|nullptr}}, not the numerical value 0 or the macro {{codeline|NULL}}.
 
The {{codeline|delete}} keyword accepts {{codeline|nullptr}} and programmers should not put an {{codeline|if (ptr)}} guard around {{codeline|delete}}.
 
{| class="wikitable"
! style="color:green;" | good
! style="color:darkred;" | bad
|-
| <syntaxhighlight lang="c++">
delete ptr;
</syntaxhighlight>
| <syntaxhighlight lang="c++">
if (ptr)
  delete ptr;
</syntaxhighlight>
|}
 
=== lambda expressions ===


C++11 features are generally allowed. Check if the feature you want to
When capturing variables from the surrounding function, explicitly list the variables being captured rather than relying on a default capture by value (`[=]`) or by reference (`[&]`).  This more clearly captures the programmer's intent and makes the code more understandable.
use has been already used.  If not, ask on the mailing list.


=== std::string ===
=== std::string ===


When an empty string is required, use @qcode{""}, rather than creating an empty
When an empty string is required, use <code>""</code>, rather than creating an empty
string object with @code{std::string ()}.
string object with <code>std::string ()</code>.


=== auto ===
=== auto ===
Line 181: Line 365:
* Beware of copy when using {{codeline|auto}} in for loops. Pass by reference and use {{codeline|const}} unless you're dealing with simple types such as {{codeline|int}}. See [http://lists.gnu.org/archive/html/octave-maintainers/2016-06/msg00144.html 'auto' uses and for-range loops] on the maintainers mailing list for more details.
* Beware of copy when using {{codeline|auto}} in for loops. Pass by reference and use {{codeline|const}} unless you're dealing with simple types such as {{codeline|int}}. See [http://lists.gnu.org/archive/html/octave-maintainers/2016-06/msg00144.html 'auto' uses and for-range loops] on the maintainers mailing list for more details.


=== C++14 ===
=== C++ style casting ===
 
Always use one of the four C++ long style casting forms ({{codeline|static_cast}}, {{codeline|dynamic_cast}}, {{codeline|reinterpret_cast}}, {{codeline|const_cast}}) rather than C-style forms (type cast {{codeline|(new_type) variable}} or the function form {{codeline|new_type (variable)}}).
 
=== C++11 features ===
 
A C++11 compatible compiler is required for [[Building | building Octave]].  Please make use of all C++11 features.
 
=== C++14, C++17, C++20 features ===


Do not use C++14 features.  Octave is widely used in very old systems and we
Try to avoid C++14, C++17, or C++20 features.  Octave is widely used in very old systems and we want them to be able to use up to date versions of Octave.  Building a recent compiler in such systems is not a trivial task so the limitation must happen
want them to be able to use up to date versions of Octave.  Building a recent
compiler in such systems is not a trivial task so the limitation must happen
in Octave.
in Octave.


An exception: code that requires C++14 feature must also implement an
If the implementation using a C++14, C++17, or C++20 feature is very beneficial, make it optional via <code>configure</code> feature detection or also implement an alternative code in the absence of said feature.  In any case, please get in contact with the Octave maintainers on [https://octave.discourse.group/c/maintainers/7 Discourse].
alternative code in the absence of said feature.  In such case, use a
 
configure check.  This increases maintenance a lot, must be used sparsely,
<syntaxhighlight lang="cpp">
and requires approval from other maintainers.
#if defined (HAVE_THIS_C14_FEATURE)
  // code that really needs it
#else
  // alternative code in its absence
#endif
</syntaxhighlight>
 
== Doxygen ==
 
Doxygen documentation can be a great help when developing octave however the current state has a lot of room for improvement. For more information about Doxygen in Octave look at [[Doxygen]].
 
=== Doxygen Style Guide ===
 
Doxygen allows for a variety of commenting styles. In order to maintain uniformity across the entire project the following rules should be applied:
 
* For Doxygen comments use only {{codeline|//!}} and NOT {{codeline|/*! ... */}}, regardless of the size of the block comment
* Use {{codeline|@}} for any [https://www.stack.nl/~dimitri/doxygen/manual/commands.html Doxygen Special Commands]
* Use as little formatting as possible.  Restrict to [https://www.stack.nl/~dimitri/doxygen/manual/markdown.html Markdown] and avoid HTML-markup.
* Do NOT use the {{codeline|@brief}} command, the first sentence will automatically be used as the summary description.
* The first sentence should describe briefly what the function does and end with a period.
* Leave a blank line between the Doxygen comment and function definition.
 
An example of properly used Doxygen would look like:


#if defined (HAVE_THIS_C14_FEATURE)
<syntaxhighlight lang="cpp">
  // code that really needs it
//! Does something interesting with its arguments.
#else
//!
  // alternative code in its absence
//! Long comment with **bold** special commands.
#endif
//!
//! @param some_param Really should figure out what to do.
//! @param another_param Does something cool with @p some_param.
//!
//! And some example using syntax highlighting:
//!
//! @code{.cc}
//! double v = 1.0;
//! int n = 2;
//! some_function (v, n);
//! @endcode
 
void
some_function (double some_param, int another_param)
{
  // ...
}
</syntaxhighlight>


== Comments ==
== Comments ==
Line 205: Line 434:
The preferred comment mark for places that may need further attention is
The preferred comment mark for places that may need further attention is
with {{codeline|FIXME:}} comments.
with {{codeline|FIXME:}} comments.
[[Category:Development]]
Rik
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