[boost] Re: Empty member optimization

From: David Abrahams (dave_at_[hidden])
Date: 2004-09-10 06:52:30

"John Maddock" <john_at_[hidden]> writes:

>> Well, compressed_pair applies to a pair and compressed_member
>> applies memberwise. In a situation where you have more than two
>> members compressed_member may compact better.
>>
>> Consider this situation:
>>
>>    struct A {};
>>    struct B {};
>>
>>    struct C
>>    : compressed_member<A>
>>    , compressed_member<A, 1>
>>    , compressed_member<B, 1>
>>    , compressed_member<B, 1>
>>    {
>>    };
>>
>> Here, each A gets its address but the Bs can share those same
>> addresses. I think applying the optimization memberwise may scale
>> better for arbitrary tuples.
>
> Except that most compilers don't implement the empty-base-optimisation
> when there is multiple inheritance taking place, in fact for at least
> one compile (Borland's) this is a serious dis-optimisation, with
> 8-bytes per base class getting added!
>
> The compressed_pair way would be:
>
> struct C
> {
> private:
>     boost::compressed_pair<
>         empty_type1,
>         boost::compressed_pair<
>             empty_type2,
>             non_empty_type> > m_data;
> };
>
> and of course this way struct C doesn't have any "accidental" base
> classes, not even private ones.

But then C can't itself be an empty class, IIUC.  Am I wrong about
that?

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com