[boost] Re: [mpl] easy access to inherit_linearly elements

From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-01-17 23:32:33

On 01/17/2005 07:35 PM, David Abrahams wrote:
> Larry Evans <cppljevans_at_[hidden]> writes:
> 
> 
>>But the aforementioned
>>
>>   t.get_inherit_side<...>:::type::left_type::value;
>>
>>does compile?  OK, now I see.  Instead of >::head_type, there
>>should have been >::type::head_type.  Does it make sense now?
> 
> 
> Nope.  I'm still looking for a member whose name is get_tail<...> and
> not seeing any such thing.
> 
The lastest upload, get_ith_head_test.cpp has a member function,
get_ith_head, which does this.

The code:

    t.get_inherit_side<...>:::type::left_type::value;

is simply a qualified access to the member variable of a supertype of t.
There's no need for a member method, and all get_ith_head does is
simply use what amounts to the above code to access the proper
superclass member variable.  As mentioned previously, this is no
different than:

    t.sk::sk+1::sn::value;

where t is of type T, and:

    s0 is supertype of T
    s1 is supertype of s0
    ...
    sk is supertype of sk-1
    ...
    sn is supertype of sn-1

and all get_tail<T,Index> (and it's successor, get_ith_tail) does is
create a typedef for the Index-th supertype of T.  There's obviously
a 1-to-1 mapping between the values and qualifiers:

   sk::sk+1::...::sn

where:

   for k in 0..n

hence, there's no problem with having the same member variable
name since it's prefixed by it's unique qualifier.  Isn't this
what tuple_base.hpp's element template does, except it just
keeps following the tail_type typedefs for k times?