Re: [Boost-users] sizeof(boost::fusion:map)

Subject: Re: [Boost-users] sizeof(boost::fusion:map)
From: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2010-11-06 08:49:41

Joel de Guzman schrieb:
> On 11/6/2010 7:56 AM, Christopher Schmidt wrote:
[snip]
>> I think gcc is correct. There are two distinct instances of X in B, and
>> those may not have the same address. That's why there is that 4 byte
>> displacement.
>> It is pretty much the same in Fusion. All inbuilt sequences and the
>> vector data storage share a common base class - fusion::sequence_root -
>> that's where the 4 resp. 8 bytes come from.
> 
> Good analysis, Christopher. We should do something about this.
> sequence_root is only there for easy tag detection. There should
> be better way.

I committed a preliminary fix to the trunk. See

https://svn.boost.org/trac/boost/changeset/66411

for more details. It does not break Fusion and Proto on gcc 4.5.1
-std=c++0x, gcc 4.5.1, msvc 10, msvc 9 and gcc 3.4.2 .

I removed fusion::sequence_root and added an implicit conversion
operator to fusion::sequence_base .

https://svn.boost.org/trac/boost/browser/trunk/boost/fusion/support/sequence_base.hpp?rev=66411#L36

Instead of using boost::is_base_of<fusion::sequence_root, Seq> to detect
inbuilt fusion container we can use boost::is_convertible<Seq,
fusion::detail::from_sequence_convertible_type> now.

https://svn.boost.org/trac/boost/browser/trunk/boost/fusion/support/is_sequence.hpp?rev=66411#L34

This solution is not pretty but it does not make the situation worse either.

-Christopher