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jeremybenn |
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GNU Objective C notes
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*********************
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This document is to explain what has been done, and a little about how
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specific features differ from other implementations. The runtime has
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been completely rewritten in gcc 2.4. The earlier runtime had several
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severe bugs and was rather incomplete. The compiler has had several
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new features added as well.
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This is not documentation for Objective C, it is usable to someone
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who knows Objective C from somewhere else.
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Runtime API functions
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=====================
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The runtime is modeled after the NeXT Objective C runtime. That is,
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most functions have semantics as it is known from the NeXT. The
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names, however, have changed. All runtime API functions have names
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of lowercase letters and underscores as opposed to the
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`traditional' mixed case names.
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The runtime api functions are not documented as of now.
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Someone offered to write it, and did it, but we were not allowed to
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use it by his university (Very sad story). We have started writing
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the documentation over again. This will be announced in appropriate
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places when it becomes available.
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Protocols
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=========
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Protocols are now fully supported. The semantics is exactly as on the
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NeXT. There is a flag to specify how protocols should be typechecked
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when adopted to classes. The normal typechecker requires that all
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methods in a given protocol must be implemented in the class that
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adopts it -- it is not enough to inherit them. The flag
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`-Wno-protocol' causes it to allow inherited methods, while
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`-Wprotocols' is the default which requires them defined.
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+load
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===========
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This method, if defined, is called for each class and category
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implementation when the class is loaded into the runtime. This method
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is not inherited, and is thus not called for a subclass that doesn't
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define it itself. Thus, each +load method is called exactly once by
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the runtime. The runtime invocation of this method is thread safe.
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+initialize
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===========
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This method, if defined, is called before any other instance or class
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methods of that particular class. For the GNU runtime, this method is
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not inherited, and is thus not called as initializer for a subclass that
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doesn't define it itself. Thus, each +initialize method is called exactly
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once by the runtime (or never if no methods of that particular class is
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never called). It is wise to guard against multiple invocations anyway
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to remain portable with the NeXT runtime. The runtime invocation of
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this method is thread safe.
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Passivation/Activation/Typedstreams
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===================================
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This is supported in the style of NeXT TypedStream's. Consult the
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headerfile Typedstreams.h for api functions. I (Kresten) have
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rewritten it in Objective C, but this implementation is not part of
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2.4, it is available from the GNU Objective C prerelease archive.
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There is one difference worth noting concerning objects stored with
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objc_write_object_reference (aka NXWriteObjectReference). When these
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are read back in, their object is not guaranteed to be available until
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the `-awake' method is called in the object that requests that object.
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To objc_read_object you must pass a pointer to an id, which is valid
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after exit from the function calling it (like e.g. an instance
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variable). In general, you should not use objects read in until the
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-awake method is called.
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Acknowledgements
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================
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The GNU Objective C team: Geoffrey Knauth (manager),
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Tom Wood (compiler) and Kresten Krab Thorup
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(runtime) would like to thank a some people for
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participating in the development of the present GNU Objective C.
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Paul Burchard and Andrew McCallum
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has been very helpful debugging the
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runtime. Eric Herring has been very helpful
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cleaning up after the documentation-copyright disaster and is now
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helping with the new documentation.
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Steve Naroff and Richard Stallman
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has been very helpful with implementation details
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in the compiler.
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Bug Reports
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===========
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Please read the section `Submitting Bugreports' of the gcc manual
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before you submit any bugs.
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