Re: [boost] Proposal: Monotonic Containers

Subject: Re: [boost] Proposal: Monotonic Containers
From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2009-06-11 10:25:34

Christopher Jefferson skrev:
> 
> On 11 Jun 2009, at 13:35, Thorsten Ottosen wrote:

>>>> That doesn't do exactly what I want for two reasons, but most might 
>>>> be fixable.
>>>>
>>>> 1) I want run-time variable amounts of space (this does involve some 
>>>> alloca, and probably macro, hackery).

>>>>
>>>> For (1) I use a macro something like:
>>>>
>>>> #define GET_ALLOCA_MEM(type, x) box<type>(x, alloca(x * sizeof(type))
>>>>
>>>> Because obviously you can't call alloca inside the auto_buffer 
>>>> constructor.
>>> Replying to myself, passing an external buffer into auto_buffer isn't 
>>> completely trivial, because you would also have to disable to copy 
>>> constructor (at least, I can't think of anything else sensible to do).
>>
>> As far as I know, alloca() is not portable. Am I wrong?
> 
> It isn't standard I believe, but it exists on at least Mac OS X, 
> FreeBSD, linux and windows, and software I write uses it successfully on 
> all these platforms.
>>
>> You might use variable lenght arrays (C99), but see "Imperfect C++" by 
>> Matthew Wilson on why they suck. His analysis is the reason he 
>> implements stlsoft::auto_buffer instead.
> 
> stlsoft::auto_buffer seems to have exactly the same restriction as your 
> auto_buffer, you must declare the amount of space (or at least an upper 
> bound) as a compile-time constant. I haven't got a copy of Imperfect 
> C++, but I have to say my personal experience is alloca and VLA (I 
> experimented with using a VLA of chars instead of alloca, the two are 
> basically implemented the same underneath) work fine.

In his book he describes how differently the VLA implemetations behave, 
and how they have serious problems with recursion. I don't know if 
implementations have improved snice then.

Anyway, I don't see an easy way to get rid of the restruction.

Maybe it is possible to use a custom dummy allocator with auto_buffer.
Then pass an allocator instance (which has been initialized somehow with 
alloca()) to the constructor of auto_buffer.

Then you can say

boost::alloca_allocator<T> alloc( n, alloca(sizeof(T)*n ) );
boost::auto_buffer<T,boost::store_n_objects<0>,boost::bounded_grow_policy,boost::alloca_allocator<T>> 
buffer( alloc );

I wish we had template alias :-)

As for disabling the copy-constructor, then it would be nice, but I 
think you can smiply make the allocator non-copyable to do that.

-Thorsten