Re: [boost] Empty member optimization

From: John Maddock (john_at_[hidden])
Date: 2004-09-10 05:03:50

> 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.

John.