kprateek88@yahoo.com (Prateek R Karandikar) wrote in message
> ... In contrast a real pointer
> (to an object) can be used as an iterator, because the size_type, etc
> typedefs aren't used directly( In::size_type), but through
> iterator_traits (iterator_traits<In>::size_type)...

Sorry, I meant value_type, difference_type, etc , not size_type.

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We can't pass a pointer to function to a function (that expects a
function object) that uses the result_type, argument_type, etc
typedefs, instead we have to use ptr_fun. In contrast a real pointer
(to an object) can be used as an iterator, because the size_type, etc
typedefs aren't used directly( In::size_type), but through
iterator_traits(iterator_traits<In>::size_type). An analogous approach
can be used for function objects:

template <typename Op>
struct func_obj1_traits  //unary function
{
 typedef typename Op::argument_type argument_type;
 typedef typename Op::result_type result_type;
};

template <typename Arg, typename Res>
struct func_obj1_traits< Res(*)(Arg) >   //specialisation for pointer
to unary function
{
 typedef Arg argument_type;
 typedef Res result_type;
};


template <typename BinOp>
struct func_obj2_traits  //binary function
{
 typedef typename BinOp::first_argument_type first_argument_type;
 typedef typename BinOp::second_argument_type second_argument_type;
 typedef typename BinOp::result_type result_type;
};

template <typename Arg1, typename Arg2, typename Res>
struct func_obj2_traits< Res(*)(Arg1, Arg2) >   //specialisation for
pointer to binary function
{
 typedef Arg1 first_argument_type;
 typedef Arg2 second_argument_type;
 typedef Res result_type;
};

In a function using function objects, instead of using
Op::result_type, func_obj1_traits<Op>::result_type would be used.
Similarly for the other typedefs.

This would save the user from the burden of using ptr_fun and would be
as efficient.

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