Re: [boost] Re: In-place construction for variant

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2003-10-27 22:23:23

--- David Abrahams <dave_at_[hidden]> wrote:
[..]
> 
> Wha??  Copying the _LHS_ to heap backup??  That can't possibly result
> in the basic guarantee unless I've completely missed something.  

Yeah, we all missed something. :)
The current variant does create a heap copy of the *lhs* and 
switches to it if there is an exception. 

> I
> believe my suggestion has been misunderstood.  The only way to get
> this right is to use heap storage for the new rhs.  In other words:
> 
>    variant<A,B> internally is conceptually a union {A,B,A*,B*};
> 
>    direct construction of a variant with A or B constructs an A or B
>    in-place.  Assignment from the same type assigns in-place.
>    Assignment of the other type allocates the new RHS on the heap,
>    destroys the LHS, and copies the pointer to the heap-allocated RHS
>    copy into the union.
> This was exactly my understanding

There should be a special case if the variant has a non-throw
copy-constructable type. Other than that this was my
understanding too, till I looked at the variant's codes.

If the variant doesn't have any guarantees about 
the storage location of the controlled object,
it seems that it is possible to create a heap
copy of the *lhs* not *rhs* and try to allocate
*rhs* in-place. If it is true, it seems that 
the current implementation is more efficient 
in terms of memory fragmentation (create a temp. *lhs* 
and if everything is fine, delete it).  For example
provided that there were no exceptions,
an array of variants will occupy a continuous 
chunk of storage.

Also, I mentioned it to Eric that the current 
implementation doesn't actually provide basic guarantees. 
He was going to look into that.

Eugene

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