Re: Defect Report: tuple::tail() should be in the public interface

From:
cbarron413@adelphia.net (Carl Barron)
Newsgroups:
comp.std.c++
Date:
Mon, 21 May 2007 22:51:58 GMT
Message-ID:
<210520071724256667%cbarron413@adelphia.net>
In article <46517ea1$0$15148$4c368faf@roadrunner.com>, Joe Gottman
<jgottman@carolina.rr.com> wrote:
\

A general approach is to construct an mpl sequence from tuple_size<> and
tuple_element<> and creating a boost::variant from that sequence. You
could convert the tuple to an stl::container of the variant type and
just use ordinary algorithms, providing a functor that handles all types
passed. or just do above and pass get<N>(t) to the functor/function
like above.


    I agree that this is will work, but this is a heck of a lot of
machinery to require just to print out a tuple. Also, it is not
reusable, so clients would have to repeat the pattern to do something
else with the entire tuple. And as for copying, in every implementation
of tuple I have seen a tuple<X, Y, Z> contains an object of tuple<Y, Z>
internally, either by inheritance or as a member element, so there
should not be any copying necessary in the implementation of tail().


  TR1 and n2134.pdf do not require a lisp like structure for
std::tr1::tuple<T1,...TN>. As such I do not see a defect.

The example is not written to be a library component. It is possible to
write the unit classes to have a functor template parameter and call
that functor with the result of tr1::get<N>(), in a similiar approach
further its not that complicated an algorithm.

for about 100 lines of code you can provide a tuple_view of the tuple
using boost::mpl then mpl::for_each() can handle any functor that
handles all the distinct types of your tuple.

boost::fusion is a possible replacement, but it is not widely known or
used at this time.

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