Re: [Boost-users] Auto dispatch metaprogramming tricks

Steven Watanabe schrieb:

AMDG

Alexander Lamaison wrote:

...

I would like to dispatch messages to a class based simply on the *presence* of a message handler method.

Currently, the classes maintain an MPL vector of the messages they handle and must implement a on(message) method for each one. The dispatcher uses this compile-time list to build the dispatching code. This means the information is maintained twice and may fall out of sync (e.g. adding the handler but forgetting to update the message vector.

Are there any template metaprogramming tricks I can employ to dispatch the message to a handler method if it exists and the default handler otherwise? All this information is available at compile time. The question is are templates are powerful enough to make use of it?

Hi Alexander, here is a class that i use to check the existence of a member function. It works on MSVC 9. But as indicated by strasser ( sorry, cant find your name in the email) it might be not portable.

template<typename T> struct HasSizeMethod { private: template struct SFINAE {}; typedef char yes_type; typedef char(&no_type)[2]; template<typename U> static no_type test_HasSizeMethod(...); template<typename U> static yes_type test_HasSizeMethod(SFINAE*); public: typedef bool value_type; BOOST_STATIC_CONSTANT(value_type , value = sizeof(yes_type) == sizeof(test_HasSizeMethod<T>(0) ) ); typedef mpl::bool_<value> type; };

Yes. The code looks something like this:

typedef char no; struct yes { char dummy[2]; };

struct has_on_result { has_on_result operator,(int); };

no check_on_result(const has_on_result&); yes check_on_result(...);

template<class T> struct has_on_impl : T { using T::on; has_on_result on(...); };

template struct has_on : boost::mpl::bool_< sizeof(check_on_result(((has_on_impl<T>*)0)->on(*((M*)0)) , 0)) != sizeof(no)

...

{};

Hi Steve, as indicated by strasser i have also difficulties to compile your code. But your code seems to be more generic, since you dont need to provide the result_type of the member function. Can you provide a working example? # include # include # include typedef char no; struct yes { char dummy[2]; }; struct has_on_result { has_on_result operator,(int); }; no check_on_result(const has_on_result&); yes check_on_result(...); template<class T> struct has_on_impl : T { using T::on; has_on_result on(...); }; template struct has_on : boost::mpl::bool_< sizeof(check_on_result(((has_on_impl<T>*)0)->on(*((M*)0)) , 0)) != sizeof(no)

{};

struct has_on_struct { int on(int ) const { return 1; } }; struct arbitrary_struct { int no(int) const { return 1; } }; int main() { BOOST_MPL_ASSERT((has_on )); return boost::exit_success; } There is also a more generic working example in the ticket. See https://svn.boost.org/trac/boost/ticket/3783

In Christ, Steven Watanabe

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