----- Original Message ----- From: "Marco Costalba" <mcostalba@gmail.com> To: <boost@lists.boost.org> Sent: Wednesday, March 05, 2008 1:53 PM Subject: Re: [boost] [announce] Multi-signature overload of any function orfunction object
On Wed, Mar 5, 2008 at 12:19 PM, Neal Becker <ndbecker2@gmail.com> wrote:
Do you have any design doc I can read?
A struct overload it's a hierarchy of many overload_function template classes.
Each overload_function template it's instantiated on given signature. One overload_function per signature.
The overload_function template defines one operator() that forwards its calling arguments to a function wrapper called 'functor'.
This functor it's a very simple home made replacement of boost.function.
Each overload_function defines also a couple of set() member functions that are used to link one of our simplified 'functor' with a specific function/function object the user passes in as the set() argument.
At calling time, according to the passed in arguments the correct overload_function::operator() will be linked by the compiler, but how to invoke the correct overload_function::set() among all the ones that are instantiated by our struct overload ?
To achieve this a SFINAE tecnique is used on an (hidden) second argument of set().
Indeed one signature of set() is:
template<typename Fun> void set(Fun* fun, typename enable_if<is_compatible<Fun, Sig>::value>::type = 0);
When, as example, a pointer to a function 'fun' is passed in the signature Fun of the pointer is tested against Sig that is the signature on which this overload_function is instantiated.
Value of is_compatible<Fun, Sig>::value is true only for types F that are functions with signature Sig or functors whose operator() has signature Sig. So, given that all the signatures on which is defined an overload struct are unique, only one of the set() member functions will be enabled.
More or less that's all.
Marco
Hello, I have take a lock to the code. Could you tell us if there is a deep raison to not use boost::function, boost::enable_if, boost::mpl::list ...? I have no doubt of the usefulness of such a overload class. What happens if we forget to initialize one of the signatures? crash ? What can you do to improbe this aspect. Do you think that is possible to have a version ensuring a default function for each signature? Somting like overload_ns::overload < signatures<int() , int(char), , double(int, char), , char(string), , string(string, string, string) >, default_functions<foo1, &foo2, foo3, &foo4, foo5_ptr> > f; or overload_ns::overload < fct<int(),foo1>, , fct<int(char), &foo2>, , fct<double(int, char), foo3>, , fct<char(string), &foo4>, , fct<string(string, string, string), foo5_ptr> > f; Could you give a practical example? Which is the context you have used it? Regards _____________________ Vicente Juan Botet Escriba