[boost] [Proposal] A little reasonable modification of boost::bind_t

Subject: [boost] [Proposal] A little reasonable modification of boost::bind_t
From: Arthur Athrun (athrunarthur_at_[hidden])
Date: 2013-05-11 02:52:12

Hi, everyone,

I'd like to propose a little modification of boost:: bind to expose more
information in boost:: _bi:: bind_t.

Here is my motivation problem. Suppose this is a function which takes boost:
:_bi::bind_t as a parameter. But at the same time, I'd like to enumerate
all binded parameters and do something for specific types, like the
following pseudo code shows.

template< class F>
void foo(F & f)
{
   foreach(T& arg, f.arguments())
   {
        if(boost::is_same<T, ID_t>::value) //here ID_t is a user specific
type
              //do something for arg.
   }
}

I found that all binded arguments are stored in a listN object. But
unfortunately, the list object (L _l in boost::_bi::bind_t) is set to
private access and there is no method to boost::_bi::bind_t to access the
listN object and the type of listN.

I don't know why you developers didn't provide a method to access the
listN object.
But it seems reasonable to provide a public method in
<boost\bind\bind_template.hpp>, like this

        typedef L argu_type;

        const L & arguments() const {return l_;}

Also, the class "template<class T> class value" in <boost\bind.hpp>
doesn't provide any access to T. It will be reasonable to add access
of type T, like this

public:
        typedef T val_type;

With such modifications, we can access all binded arguments in the
boost::_bi::bind_t. Here is a showcase.

template<class T, class VT>void Test(VT t, T * p = 0)
{
        //This parameter is not binded, so ignore it!
}template<class T, class VT>void Test(VT t, typename T::val_type * p = 0)
{
        //T::val_type is the parameter's type，
        //and can use boost::is_same<T::val_type, ID_t>::value to check it!
        std::cout<<"great! we get a value here!"<<t.get()<<std::endl;
}

template <class R, class F, class A1, class A2>void
retrive_args(boost::_bi::bind_t<R, F, boost::_bi::list2<A1, A2> > &
f)
{
        Test<A1>(f.arguments()[_1]);
        Test<A2>(f.arguments()[_2]);
}

int f(int a, double b)
{
        std::cout<<a<<std::endl;
        return a;
}

int main(int argc, char * argv[])
{
        double t = 1.0;
        retrive_args(boost::bind(f, _1, 0.2)); //Here will retrive the second
parameters only.
        return 0;
}

I don't know if I get the right way to implement it. Please let me
know if you have another implementation. Any comments or ideas will be
appreciated. Thanks.

-- 
Athrun Arthur
Cluster and Grid Computing Lab (CGCL)
Services Computing Technology and System Lab (SCTS)
School of Computer Science and Technology (CS)
Huazhong University of Science and Technology (HUST)
Wuhan, 430074, China
EMail:athrunarthur_at_[hidden]