     * Coding: (coding).           Coding Standards for GNUstep Libraries

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Coding Standards
****************

Introduction
============

This document explains the official coding standards which developers for GNUstep should follow. Note that these standards are in addition to GNU coding standards, not a replacement of them.

To summarise, always add a ChangeLog message whenever your commit a change. Make sure your patch, if possible, improves the operation of the library, not just fixes things - i.e. there are many places where things are just hacked together from long ago and really aren't correct. It's better to rewrite the whole thing correctly, then just make some temporary fix.

Some particular pieces of code which may seem odd or wrong may in fact be there for particular and obscure, but necessary reasons. If you have questions, ask on bug-gnustep@gnu.org or gnustep-dev@gnu.org.

ChangeLog Entries
=================

Always include a ChangeLog entry for work that you do. Look for the ChangeLog file in the current directory or look up to any number of parent directories. Typically there is one for each library.

Emacs currently formats the header like this:

     2000-03-11  Adam Fedor  <fedor@gnu.org>

and formats changes to functions/methods like this:

     * Source/NSSlider.m ([NSSlider -initWithFrame:]):

to which you add your own comments on the same line (with word wrapping). Although if you're making similar changes to multiple methods, it's ok to leave out the function/method name.

Important: Changelog entries should state what was changed, not why it was changed. It's more appropriate to put that in the source code, where someone can find it, or in the documentation.

Coding Style
============

The point is not what style is 'better' in the abstract -- it's what style is standard and readily usable by all the people wanting to use/work on GNUstep.  A reasonably good consistent style is better for collaborative work than a collection of styles irrespective of their individual merits.  If you commit changes that don't conform to the project standards, that just means that someone else will have a tedious time making the necessary corrections (or removing your changes).

The GNUstep coding standards are essentially the same as the GNU coding standards (http://www.gnu.org/prep/standards_toc.html), but here is a summary of the essentials.

White space should be used for clarity throughout.  In particular, variable declarations should be separated from code by a blank line and function/method implementations should be separated by a blank line.

Tabs should not be used (use spaces instead). If you do use them (please don't) they really, really, must be for tab-stops at the standard intervals of 8 spaces.

All binary operators should be surrounded by white space with the exception of the comma (only a trailing white space), and the . and -> structure member references (no space).

     x = y + z;
     x += 2;
     x = ptr->field;
     x = record.member;
     x++, y++;

Brackets should have space only before the leading bracket and after the trailing bracket (as in this example), though there are odd occasions where those spaces might be omitted ((eg. when brackets are doubled)). This applies to square brackets too.

Where round brackets are used for type-casts or at the end of a statement, there is normally no space between the closing bracket and the following expression or semicolon-

     a = (int)b;
     - (void) methodWithArg1: (int)arg1 andArg2: (float)arg2;
     a = foo (ax, y, z);

The placement of curly brackets is part of the indentation rules. the correct GNU style is

       if (...)
         {
           ...
         }

For function implementations, the function names must begin on column zero (types on the preceding line).  For function predeclaration, the types and the name should appear on the same line if possible.

     static int myFunction(int a, int b);

     static int
     myFunction(int a, int b)
     {
       return a + b;
     }

The curly brackets enclosing function and method implementations should be based in column 0.  Indentation is in steps of two spaces.

     int
     myMax(int a, int b)
     {
       if (a < b)
         {
           return b;
         }
       return a;
     }

Lines longer than 80 columns must be split up, if possible with the line wrap occurring immediately before an operator.  The wrapped lines are indented by two spaces form the original.

       if ((conditionalTestVariable1 > conditionaltestVariable2)
         && (conditionalTestvariable3 > conditionalTestvariable4))
         {
           // Do something here.
         }

Some things the standards seem to think are 'should' rather than 'must':

Multiline comments should use /* ... */ while single line comments may use //.

In a C/ObjC variable declaration, the * refers to the variable, not to the type, so you write

       char	*foo;

not

       char*	foo;

Using the latter approach encourages newbie programmers to thing they can declare two pointer variables by writing

       char*	foo,bar;

when of course they need

       char	*foo, *bar;

or (in my opinion better)

       char	*foo;
       char	*bar;

An exception to the indentation rules for Objective-C: We normally don't break long methods by indenting subsequent lines by two spaces, but make the parts of the method line up instead. The way to do this is indent so the colons line up.

       [receiver doSomethingWith: firstArg
                             and: secondArg
                            also: thirdArg];

That's the style used mostly in the GNUstep code - and therefore the one I try to keep to, however, the standard two space indentation is also acceptable (and sometimes necessary to prevent the text exceeding the 80 character line length limit).

       [receiver doSomethingWith: firstArg
         and: secondArg
         also: thirdArg];

My own preference (not part of the standard in any way) is to generally use curly brackets for control constructs, event where only one line of code is involved

       if (a)
         {
           x = y;
         }

Where using conditional compilation you should comment the #else and #endif with the condition expression used in the #if line, to make it easy to find the matching lines.

     #if condition
     // some code here
     #else /* not condition */
     #endif /* condition */

Memory Management
=================

In anticipation of the day when we can make the use of a Garbage Collector possible for all GNUstep apps (it's almost-usable/usable-with-care for non-gui apps now), the normal use of retain/release/autorelease is deprecated.

You should always use the macros RETAIN(), RELEASE() and AUTORELEASE() (defined in NSObject.h) instead.

There are also some extra macros that may be of use -

  * ASSIGN(object,value) to assign an object variable, performing the appropriate retain/release as necessary.

  * ASSIGNCOPY(object,value) to copy the value and assign it to the object.

  * DESTROY(object) to release an object variable and set it to nil.

  * TEST_RETAIN(object) to retain an object if it is non-nil

  * TEST_RELEASE(object) to release an object if it is non-nil

  * TEST_AUTORELEASE(object) to autorelease an object if it is non-nil

  * CREATE_AUTORELEASE_POOL(name) to create an autorelease pool with the specified name.

  * IF_NO_GC(X) compile the code 'X' only if GarbageCollection is not in use.

Error Handling
==============

Initialisation methods (e.g. -init) should, upon failure to initialise the class, release itself and return nil. This may mean in certain cases, that it should catch exceptions, since the calling method will be expecting a nil object rather than an exception on failure. However, init methods should endeavour to provide some information, via NSLog, on the failure.

All other methods should cause an exception on failure*, unless returning nil is a valid response (e.g. [dictionary objectForKey: nil]) or if documented otherwise.

Failure here is a relative term. I'd interpret failure to occur when either system resources have been exceeded, an operation was performed on invalid data, or a required precondition was not met. On the other hand, passing a nil object as a parameter (as in [(NSMutableData *)data appendData: nil]), or other "unusual" requests should succeed in a reasonable manner (or return nil, if appropriate) and/or reasonable default values could be used.

If an error is recoverable or it does not damage the internal state of an object, it's ok not to raise an error. At the very least, though, a message should be printed through NSLog.

Special care should be taken in methods that create resources like allocate memory or open files or obtain general system resources (locks, shared memory etc.) from the kernel. If an exception is generated between the allocation of the resource and its disposal, the resource will be simply lost without any possibility to release. The code should check for exceptions and if something bad occurs it should release all the allocated resources and re-raise the exception.

Unfortunately there is no nice way to do this automatically in OpenStep. Java has the "finally" block which is specifically designed for this task. A similar mechanism exists in libFoundation with the CLEANUP and FINALLY blocks.

Variable Declaration
====================

All variables should be declared at the beginning of a block. The new C99 standard (and gcc 3.X) allow variables to be declared anywhere in a block, including after executable code. However, in order to be compatible with older compilers, all GNUstep programs should keep the old behaviour.

Certainly we would consider it a bug to introduce code into the GNUstep libraries which stopped them compiling with one of the commonly used compilers.

Naming Conventions
==================

The convention for naming items in GNUstep differs from the GNU standard as it needs to be compatible with OpenStep/MacOS-X.

Public classes, variables, functions and constants begin with the NS prefix if they are part of the OpenStep or MacOS-X APIs, and begin with GS if they are GNUstep extensions.  GNUstep extensions must not use the NS prefix.

Class, public function, and global variable names have the first letter of each word in the name capitalised (underscores are not used).

     @class	NSRunLoop;
     GSSetUserName();
     NSGenericException;

Method and instance variable names are similarly capitalised, except that the first letter of the first word is usually not capitalised (there are a few exceptions to this where the first word is an acronym and all the letters in it are capitals).  Underscores are not used in these names except to indicate that the method/variable is private, in which case the name begins with an underscore.

     {
       int	publicInstanceVariable;
       int	_privateInstanceVariable;
     }
     - (void) publicMethod;
     - (void) _privateMethod;

The names of accessor methods (methods used to set or get the value of an instance variable) must mirror the names of the instance variables. The name of a setter method is of the form 'setVar' where 'Var' is the instance variable name with any leading underscore removed and with the first letter converted to uppercase. The name of the getter method is the same as the instance variable name (with any leading underscore removed).

     {
       int	_amplitude;
       int	frequency;
     }
     - (int) amplitude;
     - (int) frequency;
     - (void) setAmplitude: (int)anAmplitude;
     - (void) setFrequencey: (int)aFrequency;

Object Persistence
==================

The standard method of saving and restoring object information in GNUstep is through the use of the -encodeWithCoder: and -initWithCoder: methods. Any object which requires persistence implements these methods. They are used, for instance by Gorm, to save GUI interface elements. It is important that all changes to these methods be backward compatible with previously stored archives (for instance, those created by Gorm). The easiest way to do this is to use class version numbers to indicate which archive configuration should be read.

Documentation
=============

Document every method you change or add!  This makes it easier to fix our lack of documentation and keep up to date with changes. Make sure you do not copy either the OpenStep or Cocoa documentation. Some methods are so simple you might have to intentionally reword the documentation so it is different.

Currently there is a difference of opinion on whether to document in the header or in the source file, although we generally lean towards the header currently. Make sure you are consistent with the current method of documentation in the source file you are changing.

Before You Commit
=================

  * Make sure you have a ChangeLog entry

  * Make sure everything still compiles

  * Make sure you've tested the change as much as is reasonable.

  * If you have added a class, add the class to Foundation/Foundation.h or Appkit/Appkit.h if appropriate.

  * Documentation the methods you have changed or added.

  * If you have updated and configure checks, be sure to run both autoconf and autoheader.

Contributing
============

Contributing code is not difficult. Here are some general guidelines:

  * We maintain the right to accept or reject potential contributions. Generally, the only reasons for rejecting contributions are cases where they duplicate existing or nearly-released code, contain unremovable specific machine dependencies, or are somehow incompatible with the rest of the library.

  * Acceptance of contributions means that the code is accepted for adaptation into GNUstep.  We reserve the right to make various editorial changes in code. Very often, this merely entails formatting, maintenance of various conventions, etc. Contributors are always given authorship credit and shown the final version for approval.

  * Contributors must assign their copyright to FSF via a form sent out upon acceptance. Assigning copyright to FSF ensures that the code may be freely distributed.

  * Assistance in providing documentation, test files, and debugging support is strongly encouraged.

Extensions, comments, and suggested modifications of existing GNUstep features are also very welcome.