Re: [Boost-users] [tuple] no easy access to get function

22 Jan 2006


      ----- Mensaje original -----
De: François Duranleau 
Fecha: Domingo, Enero 22, 2006 6:08 am
Asunto: Re: [Boost-users] [tuple] no easy access to get function
...
On Sat, 21 Jan 2006, JOAQUIN LOPEZ MU?Z wrote:
...
----- Mensaje original -----
De: Frank Birbacher 
Fecha: Sábado, Enero 21, 2006 2:29 am
Asunto: [Boost-users] [tuple] no easy access to get function
...
Today I wanted to std::transform a range of tuples into a 
sequence of
one element. I thought I could just take the address of some
tuple::get<N> function. Maybe also specify the Tuple type as 
template>> parameter [...]
I'd rather want to take the address of boost::tuple::get,
but the template parameters won't match.
Maybe you forgot the final s in "tuples" when
writing
boost::tuples::get
There is a thin wrapper declared in the boost namespace as well. 
The 
probleme is elsewhere. The argument to the get function is not a 
boost::tuple<>, but some base class template. Thus the template 
arguments 
of the function are not and an integer and a tuple, but an integer 
and 
something else (actually, a cons if you look in the code). 
To take 
the address of such a function, you need to bind the integer, as 
well as 
HT and TT, not a tuple.
Yep, I see. You're right. Well, you can refer to get<> as follows

boost::tuples::get

Note that this is not relying on undocumented implementation,
so called "const lists" are explained and made public at

http://boost.org/libs/tuple/doc/tuple_advanced_interface.html

So I'd say you can use the former reference to get safely.
...
However, I am wondering if there would be a problem to declare the 
thin 
wrapper simply like this (example with the const version):
template< int N ,
          typename Tuple >
inline
typename tuples::access_traits<
             typename tuples::element< N , Tuples >::type
         >::const_type
get( const Tuple& t )
{
    return tuples::get< N >( c ) ;
}
?
That would also work, but my proposal above is simpler.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

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