Re: [Boost-users] [fusion] more trouble with transform

Subject: Re: [Boost-users] [fusion] more trouble with transform
From: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2011-11-12 06:18:57

Gennadiy Rozental <rogeeff_at_[hidden]> writes:

> Hi,
>
> Now I am trying to perform transform which mutates:
>
> ------
> #include <boost/fusion/container/list/list.hpp>
> #include <boost/fusion/container/list/convert.hpp>
> #include <boost/fusion/container/generation/make_list.hpp>
> #include <boost/fusion/algorithm/transformation/transform.hpp>
> namespace fusion=boost::fusion;
>
> struct C {};
>
> struct B {
>     C   make_c() { return C(); }
> };
>
> struct make_c {
>     template<typename Sig> struct result;
>
>     template <typename T>
>     struct  result<make_c(T&)>
>     {
>         typedef C type;
>     };
>     template <typename T>
>     struct  result<make_c(T)>
>     {
>         typedef C type;
>     };
>
>     template <typename T>
>     C
>     operator()(T& t) const { return t.make_c(); }
> };
>
> int
> main()
> {
>     B b1, b2;
>     auto z1 = fusion::make_list( b1, b2 );
>
>     auto z2 = fusion::as_list( fusion::transform( z1, make_c() ) );
>
>     return 0;
> }
> ----
>
> Both gcc and msvc complain like this:
>
> test.cpp:29:46: error: passing ‘const B’ as ‘this’ argument of ‘C B::make_c()’
> discards qualifiers.
>
> What am I missing now?
>
> Gennadiy

fusion::transform only accepts cref-qualified sequences.  That is why
the resulting transform view will be store a reference to z1 const and
thus the arguments passed to make_c are const-qualified.

https://svn.boost.org/trac/boost/ticket/3446

Construct the transform view by hand and your example should work fine.

        Christopher