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Subject: [boost] [multimethods] Proposal: Open Multi-Methods
From: Jean-Louis Leroy (jl_at_[hidden])
Date: 2013-07-27 07:23:00


I am working on library that pretty much implements what Pirkelbauer,
Solodkyy and Stroustrup describe in their paper "Open Multi-Methods for
C++" - although there are some divergences. I have posted it on
GitHub: Please examine the
files in the "examples" directory to get an idea of how to use it.

Salient features are:

* syntax: is relatively uncluttered. There are no limitations on the
   number of virtual arguments. Virtual and non-virtual arguments can
   be arbitrarily mixed. Multiple inheritance is supported, with some
   limitations - see below.

* speed: close to a virtual function call when the hierarchies
   involved in the virtual arguments collaborate with the
   library. Calling a method that does nothing, with a single virtual
   argument in a single inheritance hierarchy is 33% slower than the
   equivalent virtual function call. The difference becomes unnoticeable
   if the functions perform a few simple maths operations. See

* size: the dispatch table is constructed in terms of class
   groups. This results, most of the time, in a table devoid of
   redundancies. The size is bounded by the product of the number of
   specializers - not classes - in each dimension.

* support for "foreign" class hierarchies: the library can be used
   without modifications to existing classes, at the cost of lower
   performance. Collaborating and foreign arguments can be freely
   mixed. Performance is still quite good, see the benchmarks.

* next: a pointer to the next most specialized method is available
   inside method specializations - see examples/next.cpp.
   Alternatively, it is possible to call a specialization directly.

Current limitations are:

* restrictions on hierarchy: the classes involved in a method call
   must have a single root. Repeated inheritance is not
   supported. Multiple inheritance is supported, provided that the
   two conditions above are respected.

* no overloading: multi-methods are implemented as constexpr function
   objects. This does not allow for overloading. This restriction can
   be easily side-stepped by wrapping multi-methods with different
   names in a set of overloaded functions. I know how to make
   multi-methods functions instead of objects, at the cost of a
   slightly less nice syntax for specializers. This is open to

Besides, the library differs from Stroustrup et al's
paper on the following points:

* a specializer specializes only one multi-method: what would "next"
   mean otherwise? What would its static type be?

* pure multi-methods are allowed: if called, an exception is
   thrown. This seems a better choice given that most of the time the
   declared virtual arguments will be abstract classes anyway.

The library is essentially feature complete (if anything ever is). I am
now going to
work on the documentation, a build system, some clean up and compatibility
(I tested it only with g++ 4.7.2 only so far).

Is there interest in the Boost community for this stuff? If yes, I
will adapt it to the guidelines and submit it for formal review. Of
course, suggestions are welcome.

Thank you for your time,
Jean-Louis Leroy

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