Re: [Boost-Users] Problems with mpl::push_back

From: Aleksey Gurtovoy (alexy_at_[hidden])
Date: 2003-05-06 06:58:31

Joaquín M López Muñoz wrote:
>
> #include <boost/mpl/list.hpp>
> #include <boost/mpl/push_back.hpp>
>
> int main()
> {
>   typedef boost::mpl::list<> type0;
>   typedef boost::mpl::push_back<type0,int>::type type1;
>   typedef boost::mpl::push_back<type1,char>::type type2;
>
>   return 0;
> }
>
> This fails in MSVC 6.5 and gcc 3.2. The error seems to indicate
> that push_back does not provide a type named 'type'. If I replace
> push_back with push_front, eveything works wonderfully.
> Am I doing any gross error, is the documentation wrong, ideas,
> clues? Thanx in advance.

Hi Joaquín,

The code above doesn't compile because 'list', although being an
extensible sequence, does not support O(1) insertion/removal of
elements at the end - only at the beginning. Please see
http://www.boost.org/libs/mpl/doc/ref/Reference/list.html.

Having said that, I should also mention that there is a mismatch
between the docs and the code specifically related to 'push_back' -
namely, if in the above example you replace 'list' with 'vector', it
still won't compile, contrary to what is said on the vector's
reference page.

This is a known issue. Basically, currently 'vector's characteristics
_as an extensible sequence_ are the same as these of the 'list'; of
course, it's still a random access sequence - the docs don't lie
here :). It's going to be fixed in the next release; the new 'vector'
implementation is going to provide constant time insertion at both
ends, giving you both 'push_front' _and_ 'push_back'.

Meanwhile, please bear with us :).

HTH,
Aleksey