Re: [Boost-users] [boost.function] evaluating needed arguments only

From: Olaf Krzikalla (krzikalla_at_[hidden])
Date: 2008-06-02 08:15:39

Steven Watanabe wrote:
> Boost.Function doesn't provide such a utility.  I can't see a way to
> get this information out of boost::bind using the public interface, either.
> visit_each skips placeholders. 
IMHO it should be a feature of boost::function.

> If there is a way you would need 
> something like this
> 
> #include <boost/function.hpp>
> #include <boost/bind.hpp>
> 
> struct bar {};
> 
> class tSlot {
> public:
>     template<class T>
>     tSlot(const T& t) : uses_placeholder1(false), 
> uses_placeholder2(false), f(t) {
>         // do some magic to find which arguments are used.
>     }
But what is the magic here?
An approach could be the introduction of a class function_invoker 
leaving boost::function untouched:

----
struct function_invoker
{
   template<class T>
   function_invoker(function<T>& f);  // throws, if f.empty();
   // true, if a placeholder is used by f (should the index i
   // be zero-based?):
   bool uses_placeholder(int i) const;
   // binds arg to a placeholder
   template<class T>
   void bind_placeholder(int i, T& arg);
   template<class T>
   void bind_placeholder(int i, const T& arg);
   // throws, if a placeholder used by f is not bound to an argument:
   fn_result call();
};
----
I'm aware that the implementation won't be that easy, esp. the static 
computing of the type of fn_result could become difficult.
However it would solve my problem without bloating boost::function:
void foo(const tSlot& s)
{
   function_invoker i(s);
   s.bind_placeholder(1, 1);
   bar b;
   if (i.uses_placeholder(2))  // index is one-based)
   {
     expensive_calculation(b);
     s.bind_placeholder(2, b);
   }
   s.call();
}
I'm wondering if Douglas Gregor is lurking and what his opinion is.
Best regards
Olaf Krzikalla