Re: [Boost-users] [Boost.TypeTraits] Adding specialisations

From: Kirit Sælensminde (kirit.saelensminde_at_[hidden])
Date: 2007-10-03 23:39:27

John Maddock wrote:
> Kirit Sælensminde wrote:
>> John Maddock wrote:
>>> Kirit Sælensminde wrote:
>>>> I think I have a misunderstanding of what the boost::function_traits
>>>> is doing. From your explanation it seems that boost::function_traits
>>>> is there to examine only function signatures where a function type
>>>> is of the form R( A1, A2, etc. ),
>>> Correct.
>> What is the reason for the restriction?
> 
> Well I didn't write that trait.... but the general idea was that the type 
> traits library should encapsulate the C++ type system only, and that higher 
> level functionality would be provided later.
> 
>> BTW boost::result_of does help with the verbiage if include the
>> remove_pointer, but it still doesn't work with the result of a
>> boost::bind.
> 
> :-(
> 
> The trouble is, I'm not sure that there's any way to actually implement that 
> in the general sense without full introspection in C++, and/or a working 
> typeof operator.  However, I would have expected Boost.Bind to play nice and 
> work with result_of.
> 
> Sorry I can't be more helpful at present.

Thanks a lot for your time and explanations. Now that I understand more 
what boost::function_traits is trying to do it makes a lot more sense.

The documentation of function_traits (at least at 
http://www.boost.org/doc/html/boost_typetraits/reference.html#boost_typetraits.function_traits 
) has no indication of what it is meant to do. Maybe something along the 
lines of this could be added to the documentation (the function pointer 
restriction is not very visible as the last example - it's only now that 
I've looked at the documentation a lot more closely that I've noticed it 
was there at all):

"function_traits is intended to introspect only C++ functions of the 
form R (), R( A1 ), R ( A1, ... etc. ) and not function pointers or 
class member functions. To convert a function pointer type to a suitable 
type use boost::remove_pointer."

I hesitate to add something about using boost::mem_fn to convert a 
member function to a suitable type as I've not been able to get it to do 
it yet. I'm guessing that boost::mem_fn is intended to do one of the 
following two type transformations, but again it isn't clear from the 
documentation.

R (C::*)( ... ) const -> R ( const C *, ... )
R (C::*)( ... ) -> R ( C *, ... )

But maybe it is this:

R (C::*)( ... ) const -> R (*)( const C *, ... )
R (C::*)( ... ) -> R (*)( C *, ... )

Or one of the above with a C& instead of a C*. Maybe a note on which 
type transformation it does should be in the documentation?

K