Re: [boost] Re: Empty member optimization

From: Joao Abecasis (jpabecasis_at_[hidden])
Date: 2004-09-11 05:43:08

David Abrahams wrote:

[snip lots of embarassing comments :-) ]

>>The invariant is violated only with a mix of empty
>>compressed_members and empty member variables.
>>    
>>
>
>Two empty compressed_members used as bases should be sufficient.
>
>    struct X
>      : compressed_member<Base>
>      , compressed_member<Derived>
>    {};
>  
>

Hmm... I completely missed those cases.

>>struct A {};
>>
>>struct B : compressed_member<A>
>>{
>>    int something()
>>    {
>>        A a;
>>    }
>>};
>>
>>is illegal with private inheritance. 
>>    
>>
>
>???  why would private inheritance have an effect on the legality of
>auto variables of any type??
>  
>

In the above example, A refers to a private inaccessible base of B. The 
scoping operator has to be used to instantiate A inside B.

I'll have a new implementation later in the week. I'll have to drop the 
memberwise inheritance and head for something similar to a tuple. 
Possibly following the syntax suggestion of Joaquín López Muñoz.

In the new implementation a structure is built to actually hold the data 
(either through public inheritance or as data members ;-) but it is 
never instantiated. The compiler ensures the invariants are maintained 
for that structure. Then I take its size and pad my 
compressed_tuple-wannabe with it. The same casting hacks, placement new, 
etc, are then used to avoid inheritance, provide a clean interface and 
maintain the initialization order.

Only this new tuple will not be empty ever. And we'll have to live with 
that ;-).

I took a look at Borland 5.5.1 and was able to come up with a few tricks 
that provide some form of EBO. But it's a no win situation because there 
is no way to actually determine it a given type is empty (with this 
compiler) to start with. For instance, empty classes take up the size of 
the base class. If the (final) base class is empty it's size is 8 but if 
it has a single char data member it's size will be 1!

Regards,

João Abecasis