Re: [Boost-users] boost::assign::list_of and C++0x

Subject: Re: [Boost-users] boost::assign::list_of and C++0x
From: asif saeed (asif.lse2_at_[hidden])
Date: 2011-06-03 01:04:19

Hi, Boost::assign::list_of is absolutely wonderful. I used it in my last
project. I was not aware of C++0x initializer lists. Is this something
something similar to Assign::List of? Could you please give me a link to it?
Thanks,Asif

On Fri, Jun 3, 2011 at 9:48 AM, Travis Gockel <travis_at_[hidden]> wrote:

> I'm moving a fairly large project over to C++0x, since it is about to be
> published (crosses fingers) and we need to stay up with the times.  In this
> project, we use the wonderful boost::assign::list_of to assign collections
> (since initializer lists don't exist yet).  We are using g++ 4.5.1 and Boost
> 1.46.1.
>
> When switching to C++0x, bits of code like this break:
>
>     std::vector<int> vec;
>     vec = boost::assign::list_of(1)(2)(3);
>
> with the following error:
>
> break_list_of.cpp:7:41: error: ambiguous overload for ‘operator=’ in ‘vec =
> ((boost::assign_detail::generic_list<int>*)boost::assign::list_of(const T&)
> [with T = int]().boost::assign_detail::generic_list<T>::operator() [with T =
> int, boost::assign_detail::generic_list<T> =
> boost::assign_detail::generic_list<int>,
> boost::assign_detail::generic_list<T>::Ty = int](((const
> boost::assign_detail::generic_list<int>::Ty&)((const
> boost::assign_detail::generic_list<int>::Ty*)(&2)))))->boost::assign_detail::generic_list<T>::operator()
> [with T = int, boost::assign_detail::generic_list<T> =
> boost::assign_detail::generic_list<int>,
> boost::assign_detail::generic_list<T>::Ty = int](((const
> boost::assign_detail::generic_list<int>::Ty&)((const
> boost::assign_detail::generic_list<int>::Ty*)(&3))))’
> /usr/include/c++/4.5/bits/vector.tcc:156:5: note: candidates are:
> std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(const
> std::vector<_Tp, _Alloc>&) [with _Tp = int, _Alloc = std::allocator<int>]
> /usr/include/c++/4.5/bits/stl_vector.h:336:7: note:
> std::vector<_Tp, _Alloc>& std::vector<_Tp,
> _Alloc>::operator=(std::vector<_Tp, _Alloc>&&) [with _Tp = int, _Alloc =
> std::allocator<int>, std::vector<_Tp, _Alloc> = std::vector<int>]
> /usr/include/c++/4.5/bits/stl_vector.h:357:7: note:
> std::vector<_Tp, _Alloc>& std::vector<_Tp,
> _Alloc>::operator=(std::initializer_list<_CharT>) [with _Tp = int, _Alloc =
> std::allocator<int>, std::vector<_Tp, _Alloc> = std::vector<int>]
>
> The problem is that there are now multiple ways to assign to vectors (and
> all the other collection types).  Ideally, I would replace all usage of
> boost::assign::list_of with C++0x initializer lists, but, alas, I am lazy.
> Furthermore, I think I know how to fix this (see example below) and think
> the Boost community would benefit from having boost::assign::list_of work in
> C++0x and C++.
>
> template <typename T>
> struct is_initializer_list : boost::false_type { };
>
> template <typename T>
> struct is_initializer_list<std::initializer_list<T> > : boost::true_type {
> };
>
> template <typename T>
> struct wrapped_type
> {
>     wrapped_type(const T& val) :
>             _val(val)
>     { }
>
>     template <typename TOut,
>               // use new support for default template arguments in
> functions
>               typename TPhony = typename
> boost::disable_if<is_initializer_list<TOut> >::type
>              >
>     operator TOut() const
>     {
>         return _val;
>     }
>
> private:
>     T _val;
> };
>
> I was wondering if anybody has solved this problem (or a similar one)
> already.  I would patch it myself, but I only have g++ and clang to test it
> on.  Also, there is a high probability that my solution is flawed in some
> way.
>
> Thanks for the help!  And, as always, thanks for the wonderful library.
>
> --
> - Travis Gockel
>
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