There seems to be some interest in my multimethod library so I thought I'd post a message describing the design in more detail. In this message I'll focus on the API and ignore the implementation except when it affects the API. I may post another message later focusing on the implementation. The vault is still full, so if you want to look at the code go to <https://sourceforge.net/projects/multimethods/>. The draft proposal to add multimethods as a core feature is also interesting: <http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1529.html#1.1.1>. Also <http://www.op59.net/accu-2003-multimethods.html> has some info on why multimethods are a Good Thing. The design is actually quite simple. Function objects (aka methods) may be added to a generic function object. Then the generic function can be called with a set of actual arguments. It will then dispatch to the most specialized method using the dynamic type of the actual arguments. One way to think of this is as a form of runtime overloading. The API for generic functions looks like this (using an arity 2 function as an example): template <typename Result, typename Formal1, typename Formal2> class generic_function2 : public detail::base_generic_function< Result > { When this is instantiated it creates a function object. Currently the arity is encoded in the class name, but presumably boost::preprocessor could be used to allow users to instantiate the classes in the way that boost:;function does. base_generic_function is a helper class that does all of the heavy lifting. Method formal arguments and actual argument types must be compatible with Formal1 and Formal2 (this is checked at compile time). The formal argument types may also be any_type which means that the formal argument in the methods for that position may be of any type. The API is as follows. First there are some standard typedefs: typedef Result result_type; typedef typename Formal1 first_argument_type; typedef typename Formal2 second_argument_type; A default ctor (and compiler provided copy ctor/assignment op): generic_function2(); Members to add methods: template <typename R, typename T1, typename T2> generic_function2& operator<<(R (*method)(T1, T2)); template <typename Functor> generic_function2& operator<<(Functor method); If the method has the same arguments as an existing method a std::runtime_error exception is thrown. If the Functor version is used the functor must have result_type, first_argument_type, and second_argument_type typedefs. A member used to invoke the generic function: template <typename T1, typename T2> RESULT operator()(const T1& arg1, const T2& arg2) const; This works by building a list of the applicable methods and then sorting this list from most to least specific using the dynamic type of the actual arguments. A method A is more specific than another method B if, for a given set of actual argument, for each argument, A has a better match for the actual argument than B in at least one slot and the other arguments are no worse than B's (this is similar to the rule used for overloaded functions). Argument matching follows the usual C++ rules so derived types are better matches than base types and promotions are better matches than conversions. If no applicable methods or a most specific method cannot be found a runtime_error exception is thrown. When a method is called it may call the next most specific method using one of the following methods: RESULT next_method(); template <typename T1, typename T2> RESULT next_method(const T1& arg1, const T2& arg2); The second overload calls the next method using a new set of actual arguments, but the types of those arguments don't affect which method is called. The first overload calls the next method with the last set of actual arguments that were used. If there are no more methods or the next method is ambiguous a runtime_error exception is thrown. The only missing piece is that clients of the library must register polymorphic types if they want them to be treated polymorphically. This is done with a macro that looks like this: #define DECLARE_SUBTYPE(type, base) This seems like a simple yet flexible API, but there's probably room for improvement. For example: 0) The names "generic function" and "method" seem potentially confusing in a C++ context (they're from Dylan and (I think) CLOS). 1) There's no support for removing methods. This seems like a good idea, but it's not possible to compare arbitrary function objects so we'd probably have to add an add_method member which returned a cookie that a remove_method member would take to determine which method to remove (similar to the way boost::signal works). 2) Policy decisions like type compatibility and ordering are hardwired into the code. It might make sense to refactor these into policy classes although I'm not sure what they'd be used for. Maybe special casing smart pointers. 3) The library allows lossy conversions like floats to chars. This was done so that it world work as much like normal C++ code as possible, but it's a pretty evil thing. Maybe an option could be added that would reject such conversions. 4) boost::bind is a bit annoying because it doesn't provide the typedefs that the library requires. This is a known issue with bind and, as far as I know, isn't fixable. The workaround is to assign the result of boost:;bind to a boost:;function and add that to the generic function. 5) Method formal arguments can't be non-const reference types. The problem here is that typeid(const T&) == typeid(T&) == typeid(T). It might be possible to work around this, but the type code is already full of hacks working around typeid and type_info limitations... 6) It might be a good idea to add a member that will return a function pointer or boost:;function to the most specific method for a given set of types. This would allow users to skip the overhead of dispatching when using multimethods inside loops in some cases. -- Jesse