gcc/libobjc

GNU Objective C notes
*********************

This document is to explain what has been done, and a little about how
specific features differ from other implementations. The runtime has
been completely rewritten in gcc 2.4. The earlier runtime had several
severe bugs and was rather incomplete. The compiler has had several
new features added as well.

This is not documentation for Objective C, it is usable to someone
who knows Objective C from somewhere else.


Runtime API functions
=====================

The runtime is modeled after the NeXT Objective C runtime. That is,
most functions have semantics as it is known from the NeXT. The
names, however, have changed. All runtime API functions have names
of lowercase letters and underscores as opposed to the
`traditional' mixed case names.
The runtime api functions are not documented as of now.
Someone offered to write it, and did it, but we were not allowed to
use it by his university (Very sad story). We have started writing
the documentation over again. This will be announced in appropriate
places when it becomes available.


Protocols
=========

Protocols are now fully supported. The semantics is exactly as on the
NeXT. There is a flag to specify how protocols should be typechecked
when adopted to classes. The normal typechecker requires that all
methods in a given protocol must be implemented in the class that
adopts it -- it is not enough to inherit them. The flag
`-Wno-protocol' causes it to allow inherited methods, while
`-Wprotocols' is the default which requires them defined.


+load
===========
This method, if defined, is called for each class and category
implementation when the class is loaded into the runtime. This method
is not inherited, and is thus not called for a subclass that doesn't
define it itself. Thus, each +load method is called exactly once by
the runtime. The runtime invocation of this method is thread safe.


+initialize
===========

This method, if defined, is called before any other instance or class
methods of that particular class. For the GNU runtime, this method is
not inherited, and is thus not called as initializer for a subclass that
doesn't define it itself. Thus, each +initialize method is called exactly
once by the runtime (or never if no methods of that particular class is
never called). It is wise to guard against multiple invocations anyway
to remain portable with the NeXT runtime. The runtime invocation of
this method is thread safe.


Passivation/Activation/Typedstreams
===================================

This is supported in the style of NeXT TypedStream's. Consult the
headerfile Typedstreams.h for api functions. I (Kresten) have
rewritten it in Objective C, but this implementation is not part of
2.4, it is available from the GNU Objective C prerelease archive.
There is one difference worth noting concerning objects stored with
objc_write_object_reference (aka NXWriteObjectReference). When these
are read back in, their object is not guaranteed to be available until
the `-awake' method is called in the object that requests that object.
To objc_read_object you must pass a pointer to an id, which is valid
after exit from the function calling it (like e.g. an instance
variable). In general, you should not use objects read in until the
-awake method is called.


Acknowledgements
================

The GNU Objective C team: Geoffrey Knauth (manager),
Tom Wood (compiler) and Kresten Krab Thorup
(runtime) would like to thank a some people for
participating in the development of the present GNU Objective C.

Paul Burchard and Andrew McCallum
has been very helpful debugging the
runtime. Eric Herring has been very helpful
cleaning up after the documentation-copyright disaster and is now
helping with the new documentation.

Steve Naroff and Richard Stallman
has been very helpful with implementation details
in the compiler.


Bug Reports
===========

Please read the section `Submitting Bugreports' of the gcc manual
before you submit any bugs.