Thorsten Ottosen wrote:
Mathias Gaunard skrev:
Thorsten Ottosen wrote:
Well, AFAIK, they are very similar. You set the capacity once when you initialize am object:
boost::auto_buffer<T> buffer( get_max_capacity_from_somewhere() );
isn't that exactly what you do with VLAs?
With VLAs, the buffer is on the stack whatever the capacity. With auto_buffer, the buffer is on the stack only if the capacity is smaller than N (which you define to be 256 by default).
Yes. I'm talking about the conceptual behavior of the class.
A VLA is a fixed-size array whose size is determined at runtime. It is conceptually more like scoped_array than auto_buffer. Basically, we have all these array type possibilities (fixed capacity implies dynamic size): - fixed compile-time size: C/C++ arrays or boost::array - fixed runtime size: VLAs or scoped_array (we need better, something without pointers in the interface and with copy semantics) - fixed compile-time capacity: nothing (something would be useful) - fixed runtime capacity: auto_buffer - variable capacity (necessarily at runtime): std::vector All of these situations have different strategies for memory management: - allocate everything within the object itself, that is to say on the execution stack (only possible for fixed compile-time size or capacity) - allocate everything in an area external to the object, such as was is colloquially known as the heap, and reference that area (possible for every situation, and may be a better strategy even if the first is possible) - allocate the buffer in the object itself if the buffer is smaller than a certain fixed compile-time size, and use an external memory source otherwise. That technique is typically known as Small Buffer Optimization (possible for every situation) So auto_buffer is a fixed runtime capacity array which uses the third strategy. I suggest all array type possibilities be implemented with all memory management strategies. Which means obviously making some kind of policy-based design, with SBO something independent that you can plug into auto_buffer. I have worked on some kind of allocator interface that allows SBO, if there is interest.