[boost] Re: Empty member optimization

From: David Abrahams (dave_at_[hidden])
Date: 2004-09-09 08:30:30

Joao Abecasis <jpabecasis_at_[hidden]> writes:

> John Maddock wrote:
>
>  > How does this differ from boost::compressed_pair ?
>
> 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.

I think the question is whether it's ever desirable to break the
current C++ invariant that no two objects of the same type will ever
share an address **in generic code** -- that is, when you don't know
anything about the assumptions that may be made by that type.

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