Re: [Boost-users] [bind] Need Help - Function Composition

Subject: Re: [Boost-users] [bind] Need Help - Function Composition
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-12-18 12:38:10

AMDG

loadcom wrote:
> <snip>
>
> // helper function
> template<typename F>
> less_by_func<typename F::argument_type, typename F::result_type> make_less_by_func(F func)
> {
>     return less_by_func<typename F::argument_type, typename F::result_type>(func);
> }
>
> <snip>
>
>     int min_abs = std::min_element(a_list.begin(), a_list.end(),
>                 min2max::make_less_by_func(
>                     boost::function<int(A)>(
>                         boost::bind<int>(
>                             std::ptr_fun(&std::abs<int>), boost::bind(&A::i_, _1)
>                         )
>                     )
>                 ) )->i_;
>     std::cout << min_abs << std::endl;
>
> This code works fine, what I don't understand is why
> I couldn't remove the explicit construction of a
> boost::function<int(A)> object and let compiler do the
> implicit conversion(construction) silently. That means
> the following code doesn't work:
>
>     int min_abs = std::min_element(a_list.begin(), a_list.end(),
>                 min2max::make_less_by_func(
>                     boost::bind<int>(
>                         std::ptr_fun(&std::abs<int>), boost::bind(&A::i_, _1)
>                     )
>                 ) )->i_;
>   

The function objects created by Boost.Bind are plolymorphic. Thus, since
they can be called with multiple different argument types, there is no way
for them to provide an argument_type typedef. This means that in

template<typename F>
less_by_func<typename F::argument_type, typename F::result_type> 
make_less_by_func(F func);

F is deduced as the result of the bind call, and SFINAE prevents this 
template from matching.

The following definition of less_by_func should work better (untested)

template<class F>
struct less_by_func {
typedef bool result_type;
F f;
template<class T>
bool operator()(const T& t1, const T& t2) {
return(f(t1) < f(t2));
}
};

template<class F>
less_by_func<F> make_less_by_func(F f) {
less_by_func<F> result = { f };
return(result);
}

In Christ,
Steven Watanabe