Re: [boost] Re: MultiMethod/Visitor Interest

From: Lassi A.Tuura (lassi.tuura_at_[hidden])
Date: 2004-09-20 13:25:47

You might also want to take a look at a multi-method implementation I  
have written several years ago for comparison.  I would definitely love  
to see a (de facto or real) standard multi-method implementation.  I  
never had the energy to pursue a full proposal to the C++ language  
committee.  I wrote my implementation to prove that this could be done  
in a compiler and thus could be integrated into the language.  It is  
definitely not optimal to do it in a library as everyone probably  
knows.

Our implementation is not completely in the template coding style of  
boost, but as far as multi-methods go, it has been sufficiently capable  
for our needs; the main client has been a visualisation system I can  
describe in more detail if necessary.  The front-end interface to  
developers uses a couple of macros, not templates.  The macros are  
designed to give a feel of normal function definitions, but there are  
syntax limitations.  Macros were chosen because a "nice" interface  
using templates simply didn't work across a variety of compilers --  
both compile-time and run-time issues cropped up.  Templates might work  
today fine; the macros do make fair use of templates internally  
already.  There might still be issues with dynamically unloading  
templated code though.

Back to the core matter.  Features:
1) practically everything is at run-time, no compile-time limits or  
restrictions
2) no dispatch-time memory allocation (except if the method family has  
changed, see point 6)
3) any polymorphic class can be used as such for dispatching, no need  
to touch the participating classes in any way
4) handles real polymorphic calls, no need to have an exact match
5) supports arbitrary number of polymorphic arguments, including just  
one (the macro and template sugar on top is more restricted); arguments  
used in dispatch must be pointers to polymorphic types, and can't be  
null pointers at call time
6) supports arbitrary number of non-polymorphic arguments (actually  
limitation of the top syntax sugar layer, the dispatch engine only sees  
arguments used in dispatch); arguments not used in dispatch can be of  
any type, including scalars
7) handles dynamic changes the method family at run-time: it's ok to  
dynamically load and unload members of the family at any time
8) multi-methods can be globals or class (static) members; they are  
objects with operator() overload, not functions
9) the member functions of a multi-method can be either free-floating  
(global) functions or class (static) member functions, independent of  
whether the multi-method itself was a global or class-static (modulo  
privacy rules)
10) handles run-time ambiguity using the same rules as compile-time  
overload resolution, and refuses to make the call
11) supports large class hierarchies with several polymorphic arguments  
and several multimethods: doesn't compute the full dispatch matrix
12) decent dispatch time (= has never showed up in any profiling we've  
done)

Currently it doesn't support invoking the next dispatch method, and it  
isn't thread-safe, but both are quite simple to add and will be done  
when it becomes an issue.  In fact, making it thread-safe will  
naturally add support for next method dispatch.

Documentation:
http://seal.web.cern.ch/seal/snapshot/apiref/ 
classseal_1_1MultiMethod.html

API and implementation:
http://lcgapp.cern.ch/lxr/SEAL/source/Foundation/SealBase/SealBase/ 
MultiMethod.h
http://lcgapp.cern.ch/lxr/SEAL/source/Foundation/SealBase/src/ 
MultiMethod.cpp

Tests:
http://lcgapp.cern.ch/lxr/SEAL/source/Foundation/SealBase/tests/ 
MultiMethod01.cpp
http://lcgapp.cern.ch/lxr/SEAL/source/Foundation/SealBase/tests/ 
MultiMethod02.cpp
http://lcgapp.cern.ch/lxr/SEAL/source/Foundation/SealBase/tests/ 
MultiMethod03.cpp

Depends on:
http://lcgapp.cern.ch/lxr/SEAL/source/Foundation/SealBase/SealBase/ 
XTypeInfo.h
http://lcgapp.cern.ch/lxr/SEAL/source/Foundation/SealBase/src/ 
XTypeInfo.cpp

Non-test examples of use:
http://lxcms73.cern.ch/lxr/IGUANACMS/ident?i=MMM_DEFUN_FUNC

Lassi
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