Automatically Instantiating over a bunch of types in C++

In our library we have a number of "plugins", which are implemented in their own cpp files. Each plugin defines a template function, and should instantiate this function over a whole bunch of types. The number of types can be quite large, 30-100 of them, and can change depending on some compile time options. Each instance really have to be compiled and optimized individually, the performance improves by 10-100 times. The question is what is the best way to instantiate all of these functions.

Each plugin is written by a scientist who does not really know C++, so the code inside each plugin must be hidden inside macros or some simple construct. I have a half-baked solution based on a "database" of instances:

template<int plugin_id, class T>
struct S
{
   typedef T (*ftype)(T);
   ftype fp; 
};
// By default we don't have any instances
template<int plugin_id, class T> S::ftype S::fp = 0;

Now a user that wants to use a plugin can check the value of

S<SOME_PLUGIN,double>::fp

to see if there is a version of this plugin for the double type. The template instantiation of fp will generate a weak reference, so the linker will use the "real" instance if we define it in a plugin implementation file. Inside the implementation of SOME_PLUGIN we will have an instantiation

template<> S<SOME_PLUGIN,double>::ftype S<SOME_PLUGIN,double>::fp = 
       some_plugin_implementation;

This seems to work. The question is if there is some way to automatically repeat this last statement for all types of interest. The types can be stored in a template class or generated by a template loop. I would prefer something that can be hidden by a macro. Of course this can be solved by an external code generator, but it's hard to do this portably and it interfers with the build systems of the people that use the library. Putting all th开发者_StackOverflow中文版e plugins in header files solves the problem, but makes the compiler explode (needing many gigabytes of memory and a very long compilation time).

---

I've used http://www.boost.org/doc/libs/1_44_0/libs/preprocessor/doc/index.html for such magic, in particular SEQ_FOR_EACH.

---

You could use a type list from Boost.MPL and then create a class template that recursively eats that list and instantiates every type. This would however make them all nested structs of that class template.

---

Hmm, I don't think I understand your problem correctly, so apologies if this answer is way off the mark, but could you not have a static member of S, which has a static instance of ftype, and return a reference to that, this way, you don't need to explicitly have an instance defined in your implementation files... i.e.

template<int plugin_id, class T>
struct S
{
   typedef T (*ftype)(T);
   static ftype& instance()
   {
     static ftype _fp = T::create();
     return _fp;
   }
};

and instead of accessing S<SOME_PLUGIN,double>::fp, you'd do S<SOME_PLUGIN,double>::instance(). To instantiate, at some point you have to call S<>::instance(). Do you need this to happen automagically as well?

EDIT: just noticed that you have a copy constructor, for ftype, changed the above code.. now you have to define a factory method in T called create() to really create the instance.

EDIT: Okay, I can't think of a clean way of doing this automatically, i.e. I don't believe there is a way to (at compile time) build a list of types, and then instantiate. However you could do it using a mix... Hopefully the example below will give you some ideas...

#include <iostream>
#include <typeinfo>
#include <boost/fusion/include/vector.hpp>
#include <boost/fusion/algorithm.hpp>


using namespace std;

// This simply calls the static instantiate function
struct instantiate
{
  template <typename T>
  void operator()(T const& x) const
  {
    T::instance();
  }
};

// Shared header, presumably all plugin developers will use this header?
template<int plugin_id, class T>
struct S
{
  typedef T (*ftype)(T);

  static ftype& instance()
  {
    cout << "S: " << typeid(S<plugin_id, T>).name() << endl;
    static ftype _fp; // = T::create();
    return _fp;
  }
};

// This is an additional struct, each plugin developer will have to implement
// one of these...
template <int plugin_id>
struct S_Types
{
  // All they have to do is add the types that they will support to this vector
  static void instance()
  {
    boost::fusion::vector<
      S<plugin_id, double>,
      S<plugin_id, int>,
      S<plugin_id, char>
    > supported_types;
    boost::fusion::for_each(supported_types, instantiate());
  }
};

// This is a global register, so once a plugin has been developed,
// add it to this list. 
struct S_Register
{
  S_Register()
  {
    // Add each plugin here, you'll only have to do this when a new plugin
    // is created, unfortunately you have to do it manually, can't
    // think of a way of adding a type at compile time...
    boost::fusion::vector<
      S_Types<0>,
      S_Types<1>,
      S_Types<2>
    > plugins;
    boost::fusion::for_each(plugins, instantiate());
  }
};

int main(void)
{
  // single instance of the register, defining this here, effectively
  // triggers calls to instanc() of all the plugins and supported types...
  S_Register reg; 

  return 0;
}

Basically uses a fusion vector to define all the possible instances that could exist. It will take a little bit of work from you and the developers, as I've outlined in the code... hopefully it'll give you an idea...