mutter/doc/CODING_STYLE

Clutter Coding Style
-------------------------------------------------------------------------------

This document is intended to be a short description of the preferred
coding style to be used for the Clutter source code.

Coding style is a matter of consistency, readability and maintainance;
coding style is also completely arbitrary and a matter of taste. This
document will use examples at the very least to provide authoritative
and consistent answers to common questions regarding the coding style,
and will also try to identify the allowed exceptions.

The examples will show the preferred coding style; the negative examples
will be clearly identified. Please, don't submit code to Clutter that
looks like any of these.

Part of the rationales for these coding style rules are available either
in the kernel CodingStyle document or in Cairo's CODING_STYLE one.

When in doubt, check the surrounding code and try to imitate it.

Clutter provides an Uncrustify configuration file that tries to match
this document. Since automated tools are not a substitute for human eye,
they should not be entirely relied upon - but they can give an initial
layout for contributors.

+ Line width

The maximum line width for source files is 80 characters, whenever possible.
Longer lines are usually an indication that you either need a function
or a pre-processor macro.

+ Indentation

Each new level is indented 2 or more spaces than the previous level:

  if (condition)
    single_statement ();

This can only be achieved using space characters. It may not be achieved
using tab characters alone, or using a combination of spaces and tabs.

Do not change the editor's configuration to change the meaning of a
tab character (see below); code using tabs to indent will not be accepted
into Clutter.

Even if two spaces for each indentation level allows deeper nesting than
8 spaces, Clutter favours self-documenting function names that can take
quite some space. For this reason you should avoid deeply nested code.

+ Tab characters

The tab character must always be expanded to spaces. If a literal
tab must be used inside the source, the tab must always be interpreted
according to its traditional meaning:

	Advance to the next column which is a multiple of 8.
        [ these two lines should be aligned ]

+ Braces

Curly braces should not be used for single statement blocks:

  if (condition)
    single_statement ();
  else
    another_single_statement (arg1);

In case of multiple statements, curly braces should be put on another
indentation level:

  if (condition)
    {
      statement_1 ();
      statement_2 ();
      statement_3 ();
    }

The "no block for single statements" rule has only three exceptions:

  ①  if the single statement covers multiple lines, e.g. for functions with
     many arguments, and it is followed by else or else if:

  /* valid */
  if (condition)
    {
      a_single_statement_with_many_arguments (some_lengthy_argument,
                                              another_lengthy_argument,
                                              and_another_one,
                                              plus_one);
    }
  else
    another_single_statement (arg1, arg2);

  ②  if the condition is composed of many lines:

  /* valid */
  if (condition1 ||
      (condition2 && condition3) ||
      condition4 ||
      (condition5 && (condition6 || condition7)))
    {
      a_single_statement ();
    }

  ③  Nested if's, in which case the block should be placed on the
     outermost if:

  /* valid */
  if (condition)
    {
      if (another_condition)
        single_statement ();
      else
        another_single_statement ();
    }

  /* invalid */
  if (condition)
    if (another_condition)
      single_statement ();
    else if (yet_another_condition)
      another_single_statement ();

In general, new blocks should be placed on a new indentation level,
like:

  int retval = 0;

  statement_1 ();
  statement_2 ();

  {
    int var1 = 42;
    gboolean res = FALSE;

    res = statement_3 (var1);

    retval = res ? -1 : 1;
  }

While curly braces for function definitions should rest on a new line
they should not add an indentation level:

  /* valid */
  static void
  my_function (int argument)
  {
    do_my_things ();
  }

  /* invalid */
  static void
  my_function (int argument) {
    do_my_things ();
  }

  /* invalid */
  static void
  my_function (int argument)
    {
      do_my_things ();
    }

Curly braces must not be placed on the same line as a condition:

  /* invalid */
  if (condition) {
    statement_1 ();
    statement_2 ();
  }

+ Conditions

Do not check boolean values for equality:

  /* invalid */
  if (condition == TRUE)
    do_foo ();

  /* valid */
  if (another_condition)
    do_bar ();

Even if C handles NULL equality like a boolean, be explicit:

  /* valid */
  if (some_pointer == NULL)
    do_blah ();

  /* invalid */
  if (some_other_pointer)
    do_blurp ();

In case of conditions split over multiple lines, the logical operators should
always go at the end of the line:

  /* invalid */
  if (condition1
      || condition2
      || condition3)
    {
      do_foo ();
    }

  /* valid */
  if (condition1 &&
      condition2 &&
      (condition3 || (condition4 && condition5)))
    {
      do_blah ();
    }

+ Functions

Functions should be declared by placing the returned value on a separate
line from the function name:

  void
  my_function (void)
  {
  }

The arguments list must be broken into a new line for each argument,
with the argument names right aligned, taking into account pointers:

  void
  my_function (some_type_t     type,
               another_type_t *a_pointer,
               final_type_t    another_type)
  {
  }

The alignment also holds when invoking a function without breaking the
80 characters limit:

  align_function_arguments (first_argument,
                            second_argument,
                            third_argument);

To respect the 80 characters limit do not break the function name from
the arguments:

  /* invalid */
  a_very_long_function_name_with_long_parameters
    (argument_the_first, argument_the_second);

  /* valid */
  first_a = argument_the_first;
  second_a = argument_the_second;
  a_very_long_function_name_with_long_parameters (first_a, second_a);

+ Whitespace

Always put a space before a parenthesis but never after:

  /* valid */
  if (condition)
    do_my_things ();

  /* valid */
  switch (condition)
    {
    }

  /* invalid */
  if(condition)
    do_my_things();

  /* invalid */
  if ( condition )
    do_my_things ( );

A switch() should open a block on a new indentation level, and each case
should start on the same indentation level as the curly braces, with the
case block on a new indentation level:

  /* valid */
  switch (condition)
    {
    case FOO:
      do_foo ();
      break;

    case BAR:
      do_bar ();
      break;
    }

  /* invalid */
  switch (condition) {
    case FOO: do_foo (); break;
    case BAR: do_bar (); break;
  }

  /* invalid */
  switch (condition)
    {
    case FOO: do_foo ();
      break;
    case BAR: do_bar ();
      break;
    }

  /* invalid */
  switch (condition)
    {
      case FOO:
      do_foo ();
      break;
      case BAR:
      do_bar ();
      break;
    }

It is preferable, though not mandatory, to separate the various cases with
a newline:

  switch (condition)
    {
    case FOO:
      do_foo ();
      break;

    case BAR:
      do_bar ();
      break;

    default:
      do_default ();
    }

The 'break' statement for the default: case is not mandatory.

If a case block needs to declare new variables, the same rules as the
inner blocks (see above) apply; the break statement should be placed
outside of the inner block:

  switch (condition)
    {
    case FOO:
      {
        int foo;

        foo = do_foo ();
      }
      break;

    ...
    }

When declaring a structure type use newlines to separate logical sections
of the structure:

  struct _ClutterActorPrivate
  {
    /* fixed position */
    ClutterUnit fixed_x;
    ClutterUnit fixed_y;

    ClutterRequestMode request_mode;

    /* requisition sizes */
    ClutterUnit request_width_for_height;
    ClutterUnit request_min_width;
    ClutterUnit request_natural_width;
    ClutterUnit request_height_for_width;
    ClutterUnit request_min_height;
    ClutterUnit request_natural_height;

    ClutterActorBox allocation;

    ...
  };

Do not eliminate whitespace and newlines just because something would
fit on 80 characters:

  /* invalid */
  if (condition) foo (); else bar ();

Do eliminate trailing whitespace on any line, preferably as a separate
patch or commit. Never use empty lines at the beginning or at the end of
a file.

Do enable the default git pre-commit hook that detect trailing
whitespace for you and help you to avoid corrupting Clutter's tree with
it. Do that as follows:

  chmod a+x .git/hooks/pre-commit

You might also find the git-stripspace utility helpful which acts as a
filter to remove trailing whitespace as well as initial, final, and
duplicate blank lines.

+ Headers

Headers are special, for Clutter, in that they don't have to obey the
80 characters limit. The only major rule for headers is that the functions
definition should be vertically aligned in three columns:

  return value          function_name           (type   argument,
                                                 type   argument,
                                                 type   argument);

The maximum width of each column is given by the longest element in the
column:

  void         clutter_type_set_property (ClutterType  *type,
                                          const gchar  *value,
                                          GError      **error);
  const gchar *clutter_type_get_property (ClutterType  *type);

It is also possible to align the columns to the next tab:

  void          clutter_type_set_prop           (ClutterType *type,
                                                 gfloat       value);
  gfloat        clutter_type_get_prop           (ClutterType *type);
  gint          clutter_type_update_foobar      (ClutterType *type);

Public headers should never be included directly:

  #if !defined(__CLUTTER_H_INSIDE__) && !defined(CLUTTER_COMPILATION)
  #error "Only <clutter/clutter.h> can be included directly."
  #endif

Public headers should also have inclusion guards (for internal usage)
and C++ guards:

  #ifndef __CLUTTER_HEADER_H__
  #define __CLUTTER_HEADER_H__

  #include <clutter/clutter-actor.h>

  G_BEGIN_DECLS

  ...

  G_END_DECLS

  #endif /* __CLUTTER_HEADER_H__ */

+ Includes

Clutter source files should never include the global clutter.h header, but
instead include the individual headers that are needed. Every file must
include config.h first, then its own header, then other Clutter headers
that it needs, then system and third-party headers that it needs.

  /* valid */
  #include "config.h"

  #include "clutter-foo.h"

  #include "clutter-actor.h"
  #include "clutter-container.h"

  ...

  #include <string.h>

+ GObject

GObject classes definition and implementation require some additional
coding style notices.

Typedef declarations should be placed at the beginning of the file:

  typedef struct _ClutterActor          ClutterActor;
  typedef struct _ClutterActorPrivate   ClutterActorPrivate;
  typedef struct _ClutterActorClass     ClutterActorClass;

This includes enumeration types:

  typedef enum {
    CLUTTER_REQUEST_WIDTH_FOR_HEIGHT,
    CLUTTER_REQUEST_HEIGHT_FOR_WIDTH
  } ClutterRequestMode;

And callback types:

  typedef void (* ClutterCallback) (ClutterActor *actor,
                                    gpointer      user_data);

Instance structures should only contain the parent type and a pointer to a
private data structure, and they should be annotated as "private":

  struct _ClutterRectangle
  {
    /*< private >*/
    ClutterActor parent_instance;

    ClutterRectanglePrivate *priv;
  };

All the properties should be stored inside the private data structure, which
is defined inside the source file - or, if needed, inside a private header
file; the private header filename must end with "-private.h" and must not be
installed.

The private data structure should only be accessed internally using the
pointer inside the instance structure, and never using the
G_TYPE_INSTANCE_GET_PRIVATE() macro or the g_type_instance_get_private()
function.

Always use the G_DEFINE_TYPE(), G_DEFINE_TYPE_WITH_CODE() macros, or
their abstract variants G_DEFINE_ABSTRACT_TYPE() and
G_DEFINE_ABSTRACT_TYPE_WITH_CODE().

Avoid forward declaration for functions: use the G_DEFINE_* macros right
after the private types, variables and macros declarations.

Interface types should always have the dummy typedef for cast purposes:

        typedef struct _ClutterFoo              ClutterFoo;

The interface structure should have "Iface" postfixed to the dummy typedef:

        typedef struct _ClutterFooIface         ClutterFooIface;

Interfaces must have the following macros:

        - Macro:                                - Expands to:
        • CLUTTER_TYPE_<iface_name>             <iface_name>_get_type
        • CLUTTER_<iface_name>                  G_TYPE_CHECK_INSTANCE_CAST
        • CLUTTER_IS_<iface_name>               G_TYPE_CHECK_INSTANCE_TYPE
        • CLUTTER_<iface_name>_GET_IFACE        G_TYPE_INSTANCE_GET_INTERFACE

+ Memory allocation

When dynamically allocating data on the heap either use g_new() or,
if allocating multiple small data structures, g_slice_new().

Public structure types should always be returned after being zero-ed,
either explicitly for each member, or by using g_new0() or g_slice_new0().

+ Macros

Try to avoid private macros unless strictly necessary. Remember to #undef
them at the end of a block or a series of functions needing them.

Inline functions are usually preferable to private macros.

Public macros should not be used unless they evaluate to a constant.

+ Public API

Avoid exporting variables as public API, since this is cumbersome on some
platforms. It is always preferable to add getters and setters instead.

+ Private API

Non-exported functions that are needed in more than one source file
should be named "_clutter_...", and declared in a private header file.

Underscore-prefixed functions are never exported.

Non-exported functions that are only needed in one source file
should be declared static.

+ Documentation

All public APIs must have gtk-doc comments. For functions, these should
be placed in the source file, directly above the function.

  /* valid */
  /**
   * clutter_get_flow:
   * @actor: a #ClutterActor
   *
   * Gets the flow of an actor.
   *
   * Note that flows may be laminar or turbulent...
   *
   * Return value: (transfer none): the flow of @actor
   */
  ClutterFlow *
  clutter_get_flow (ClutterActor *actor)
  {
    ...
  }

Doc comments for macros, function types, class structs, etc should be
placed next to the definitions, typically in headers.

Section introductions should be placed in the source file they describe,
after the license header:

  /* valid */
  /**
   * SECTION:clutter-align-constraint
   * @Title: ClutterAlignConstraint
   * @Short_Description: A constraint aligning the position of an actor
   *
   * #ClutterAlignConstraint is a #ClutterConstraint that aligns the position
   * of the #ClutterActor to which it is applied to the size of another
   * #ClutterActor using an alignment factor
   *
   * [...]
   */

To properly document a new function, macro, function type or struct,
it needs to be listed in the clutter-sections.txt file.

To properly document a new class, it needs to be given its own section
in clutter-sections.txt, needs to be included in clutter-docs.xml, and the
get_type function needs to listed in clutter.types.

+ Old code

It is ok to update the style of a code block or function when you
are touching it anyway, but sweeping whitespace changes obscure the
git history and should be avoided.