Christopher Jefferson skrev:
On 10 Jun 2009, at 22:28, Christopher Jefferson wrote:
On 10 Jun 2009, at 22:08, Thorsten Ottosen wrote:
Christopher Jefferson skrev:
Very interesting. I have been working on a similar, but much more limited, proposal which just does this for vector. I decided I had to implement a new class, because building on top of the 'allocator' framework wasn't sensible, because you waste too much buffer space. For example, I believe your example won't work. I'd be interested to see what you think. Consider your example, in g++ (which I know how it operates internally) boost::monotonic::inline_storage<100*sizeof(Object)> storage; // create local storage on the stack boost::monotonic::vector<Object> deathrow(storage); // create a std::vector that uses this storage foreach (object in world) { if (IsDead(object)) deathrow.push_back(object); // allocation is just advancing a pointer } This will build a vector which will continuously expand, through sizes 1,2,4,8,16,32. At each step, it will allocate a new block of memory and free the old one, making the storage size 1,3,7,15,31,63. When we try to push_back beyond 32 elements, the next allocation will overflow the buffer. Am I missing something?
Maybe
http://www.cs.aau.dk/~nesotto/boost/trunk/libs/auto_buffer/doc/html/ http://www.cs.aau.dk/~nesotto/boost/trunk/boost/auto_buffer/
is what you are looking for?
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). 2) I want it to be an error if it is necessary to go to the heap.
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? 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. For 2), if you want to get an error when the stacl buffer is exhausted, you may write a custom GrowPolicy that gies an error inside GrowPolicy::new_capacity(). -Thorsten