Re: [Boost-users] [MPL] Front of a set of types

Subject: Re: [Boost-users] [MPL] Front of a set of types
From: OvermindDL1 (overminddl1_at_[hidden])
Date: 2009-08-31 04:30:43

On Mon, Aug 31, 2009 at 2:20 AM, Marco Guazzone<marco.guazzone_at_[hidden]> wrote:
> Dear all,
>
> Consider this:
>
> ---[code]---
> #include <boost/mpl/front.hpp>
> #include <boost/mpl/set.hpp>
> #include <boost/type_traits/is_same.hpp>
>
> #define BOOST_TEST_MODULE "boost::mpl::set tests"
> #define BOOST_TEST_DYN_LINK
> #define BOOST_TEST_MAIN
> #include <boost/test/unit_test.hpp>
>
> BOOST_AUTO_TEST_CASE( mpl_set )
> {
>  typedef boost::mpl::set<int,long,char> s;
>  BOOST_CHECK( (boost::is_same<int,
> boost::mpl::front<s>::type>::value) );     // Test #1
>  BOOST_CHECK( (boost::is_same<long,
> boost::mpl::front<s>::type>::value) );  // Test #2
>  BOOST_CHECK( (boost::is_same<char,
> boost::mpl::front<s>::type>::value) );  // Test #3
> }
> ---[/code]---
>
> Reading the docs
>  http://www.boost.org/doc/libs/1_40_0/libs/mpl/doc/refmanual/set.html
> the "front" of a set<t1,t2,...,tn> return the first element type in s
> which for me is t1
> So what I expect is the test to fail on tests #2 and #3 and hence to
> succeed on test #1
>
> Instead the test fails on tests #1 and #2 and succeeds on test #3.
>
> Looking inside the code I find out this is OK since a setN<T1,..,TN>
> is defined from TN and set{N-1}<T1,...,T{N-1}>, resulting in TN be the
> very front element
>
> Anyway, in my opinion this is counterintuitive.
>
> Maybe a note in the document would be helpful.
>
> What do you think?
>
> Thank you very much!

I do not know of set being ordered or any implementation I use
(considering that runtime sets are generally hash maps), so I would
not expect that to be ordered personally.  I would use vector if I
want something ordered.