Extensible 'resource' loading design critique

Alan Woodland <ajw05@aber.ac.uk>
Mon, 21 Jan 2008 16:23:59 +0000

I'm currently trying to devise a flexible way for loading a variety of
different resources from a variety of different sources. I've sketched
out the following design in C++, and would appreciate some comments on
the C++ side of things.

- Is my auto_ptr use reasonable? Exception safety is fairly important
  here. Any other exception safety related comments?
- Should/Could I have used anything from <algorithm>?
- Is there a better way of doing the copying with the istream in load()?
  It feels like a hack like that.

The main aims are to provide a mechanism for allowing 'LoaderModules' to
indicate if they could load something from a given stream. The important
thing that was worrying me though was that one module could interfere
with the istream, and break another module, hence the copying with
stringstreams to isolate them.

Loader and LoaderModule both have one template parameter - P, the
product they load, e.g. Texture/Mesh. In practice the product will be an
interface also, but I've not included any examples here since they're
not really relevant to the question and probably OT anyway.

Eventually I might decide to add other things like caching/sharing etc.,
but for the time being that isn't important so I'll be waiting for it to
flag in a profiler.

#include <string>
#include <istream>
#include <sstream>
#include <fstream>
#include <vector>
#include <exception>
#include <memory>

template <typename P>
class Loader;

template <typename P>
class LoaderModule {
   virtual bool accept(std::istream& i) = 0;
   virtual std::auto_ptr<P> load(std::istream& i) = 0;

   virtual ~LoaderModule() { }

   LoaderModule() {

   void reg() {

template <typename P>
class Loader {
   friend class LoaderModule<P>;
   static Loader<P>& getInstance() {
      static Loader<P> *inst = NULL;

      if (!inst)
         inst = new Loader<P>;

      return *inst;

   std::auto_ptr<P> load(const std::string& uri) const {
      //in practice I'll make this more flexible here than just files.
      std::ifstream in(fn.c_str());
      return load(in);

   std::auto_ptr<P> load(std::istream& s) const {
      std::ostringstream tmpstream;

      if (!(tmpstream << s.rdbuf())) {
         // in final version will be more sensible with exceptions here.
         throw std::exception();

      const std::string& copy = tmpstream.str();
      for (typename modules_t::const_iterator it = modules.begin(); it
!= modules.end(); ++it) {
         std::istringstream i(copy);

     if ((*it)->accept(i)) {
       std::auto_ptr<P> result = (*it)->load(i);
            return result;

      // in final version will also be more sensible with the exception
      throw std::exception();

   void addModule(LoaderModule<P> *m) {

   typedef std::vector<LoaderModule<P>*> modules_t;
   modules_t modules;

   Loader() { }
   ~Loader() {
      for(typename modules_t::iterator it = modules.begin(); it !=
modules.end(); ++it) {
         delete *it;


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