stack-based std::containers
I have functions like say a quartic equation solver that can return a variable number of solutions. It would be clean IMO to return or pass by reference a std::vector or like container that store the solutions. However heap allocation is fairly expensive here so the usual solution is to have a local array with max solutions elements and pass that by reference into the solver and return the number of solutions. This is ugly to use and read. Is there a library for stack-based std-like containers? Alternatively is it possible to make an compatible stack-based allocator for std::vector using alloca? Thanks, Michael Marcin
Michael Marcin wrote:
I have functions like say a quartic equation solver that can return a variable number of solutions. It would be clean IMO to return or pass by reference a std::vector or like container that store the solutions. However heap allocation is fairly expensive here so the usual solution is to have a local array with max solutions elements and pass that by reference into the solver and return the number of solutions. This is ugly to use and read. Is there a library for stack-based std-like containers? Alternatively is it possible to make an compatible stack-based allocator for std::vector using alloca?
You can definitely make a stack-based version of allocators. The allocator would store a pointer to the local array. It's still fairly ugly though. I suggest that you could look at STLSoft's auto_buffer: http://synesis.com.au/software/stlsoft/help/classstlsoft_1_1auto__buffer.htm... I suggested the construction of a similar class in boost for SoC last year. -Thorsten
On Mar 1, 2007, at 2:11 AM, Thorsten Ottosen wrote:
Michael Marcin wrote:
I have functions like say a quartic equation solver that can return a variable number of solutions. It would be clean IMO to return or pass by reference a std::vector or like container that store the solutions. However heap allocation is fairly expensive here so the usual solution is to have a local array with max solutions elements and pass that by reference into the solver and return the number of solutions. This is ugly to use and read. Is there a library for stack-based std-like containers? Alternatively is it possible to make an compatible stack-based allocator for std::vector using alloca?
You can definitely make a stack-based version of allocators. The allocator would store a pointer to the local array. It's still fairly ugly though.
Ugly but really efficient. The question is how to tune it (choose the capacity). If you know there aren't going to be more than N instances at any given time, then it's a real good solution. For instance e.g. if you have no more than one temporary at any given time, per thread, and you have an upper limit on the number of threads. You would need to make double sure your code can handle std::bad_alloc, though, just in case. However, as you say, the usual way to handle this kind of stack allocation is to pass in the buffer to use to return the solution. I don't know why you say this is ugly: this is the design used by the STL. Just pass in an OutputIterator "result" (your buffer's begin) and return an OutputIterator (the end of the buffer holding the solution). Make that a template parameter and you can handle any situation, including dynamic allocation by passing an std::insert_iterator<...> or even an iterator adapter that will throw an exception if you overflow your buffer (is there such an iterator adaptor in boost???) Am I missing something? -- Herve
participants (3)
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Hervé Brönnimann -
Michael Marcin -
Thorsten Ottosen