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