Re: [Boost-users] [fusion] Compile Error using BOOST_FUSION_DEFINE_STRUCT_INLINE with VC10 and GCC 4.2.1

24 Oct 2012

      Hi,

thanks for the reply!

On 2012-10-24 04:45, Nathan Ridge wrote:
...
For MSVC, this is caused by a bug that seems to be triggered when a 
triply
nested template class (i.e. a nested template class inside a nested
template class inside a template class) is partially specialized. I 
reported
the bug to Microsoft [1], but haven't heard back from them.
So this bug is known. Could a warning be added to the documentation 
until this is fixed? (if it is possible at all?). At last for the 
compilers which are officially supported by boost.
...
GCC 4.2 seems to be suffering from a similar bug. Your example code 
works
with GCC 4.5 and higher, and fails to compile with GCC 4.4 and lower. 
There's
not much to do in terms of reporting this bug, since 4.4 and earlier 
are no
longer maintained.
Good to know. So either I workaround/disable the feature on my side on 
VC or GCC<4.5 or we find a workaround. OK, let's try the latter first, 
since i honestly don't know how to to workaround it on my side ;)
...
If anyone can suggest a workaround for these compiler bugs, I am 
happy to
implement them.
I have tried to understand what the macro generates and maybe we can 
fix it by using fusion/mpl instead of partial template specialization. I 
don't fully understand the macro, but maybe i am not too far off, so 
that my solution might still work.
the need for partial template spcialization arised because basically 
the type/member list (A1, a1)(A2,a2)... is translated into

typedef A1 t1_type;
typedef A2 t2_type;
...
t1_type& t1;//linked to a1
t2_type& t2;//linked to a2
...

or something very similar to it.

thus you need to specialize deref to do something roughly like( i 
simplyfy here because i am too lazy to look up the original solution):

template<int>
struct deref{};
template<>
struct deref<1>{
     typedef t1_type& type;
     type call(Sequence& seq){
         return seq.t1;
     }
};
...

now think about the following:
typedef boost::fusion::vector t_seq;
t_seq t;//tied to a1...an
template<int N>
struct deref{
     typedef result_of::at_c type;
     type call(Sequence& seq){
         return at_c<N>(seq.t);
     }
};

could this work? Of course this is detrimental to the compil times 
since fusion::vector needs to be itnroduced and instantiated. However 
this might just be the solution used for GCC<4.5 and MSVC, in this cas 
"slow" is better than "compiler gives up".

Gretings,
Oswin

Re: [Boost-users] [fusion] Compile Error using BOOST_FUSION_DEFINE_STRUCT_INLINE with VC10 and GCC 4.2.1

Oswin Krause