Re: [boost] Overloading a function taking a boost::function

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-05-12 11:19:48

On Sunday 12 May 2002 07:52 am, you wrote:
> > function object traits to work with.
> >
> > If we _did_ have a working set of function traits, it would be possible
> > to make Boost.Function objects implicitly constructed only from function
>
> objects
>
> > that are callable by the Boost.Function object.
>
> This has nothing to do with my original problem, but I'm curious what this
> Boost.Function constructor would look like if there was a working
> boost::arity_traits class. I'm relatively new to generic programming using
> C++ templates and I'd like to learn more.
>
> Thanks in advance for teaching me a new trick. :)
>
> Dirk Gerrits

I shouldn't have said "it would be possible" but been a little more cautious 
and said "it might be possible". There are a few tricks that might be usable, 
e.g., the has_member idea (it's in the archives) could probably be applied 
like this:

template<bool Cond> struct yes
{
  typedef void* type;
};

template<> struct yes<false>
{
  // no type typedef
};

template<typename R, typename T1, ..., typename TN>
class function {
public:
  template<typename F> 
  function(F f, 
           typename yes<is_callable<F, R, T1, ..., TN>::value>::type = 0);

  // ...
};

The idea being that if F is callable, yes<true>::type will be void* and 
everything is okay. However, if F is not callable then yes<true>::type does 
not even exist so the constructor cannot be instantiated. 

This worked in a little proof-of-concept test. 

The next step is to work on is_callable. I know that the following cases can 
be handled:
  - function pointers
  - member function pointers
  - anything derived from std::unary_function or std::binary_function
  - anything that has function object traits and is not partially bound

I don't know if is_callable can be written to handle arbitrary function 
objects.

        Doug