Question

Suppose i want something simple like the following:

I have an core-algorithm, which randomly selects one of the specialized algorithms (specialized at compile-time) and process this algorithm. These specialized algorithms are implemented trough functors.

The question is now: how to implement a container, which is build at compile-time, where the core-algorithm can first check the size of this container (“i got 4 algorithms -> need to randomly select algorithm 0-3”) and can then execute the functor in this container (“randomly chosen 2 -> process the third functor in container”).

How would one implement it as simple as possible? I suppose it is possible.

Is there any connection to the curiously recurring template idiom? (wiki link)
Is there a simple way with the use of Boost::Fusion? (official doc)

Edit: All the algorithms will be used in the core-algorithm. The use pattern (random-numbers) is a runtime-decision (so i don’t need compile-time-rands). The algorithm just has to know the container of functors and the size of this container for safe access.

Answer 1

If you want your core-algorithm to execute a specialized algorithm, there should be some kind of contract between the core-algorithm and the specialized algorithm.

If you define this contract as an interface, your container is simply a container containing pointers to these interfaces, e.g.:

class IAlgorithm
   {
   public:
      virtual double operator()(double d) = 0;
   };

typedef std::vector<IAlgorithm *> Algorithms;

Calling a random algorithm is then simply taking the size of the vector, taking a random value between zero and the size of the list (0..size-1), taking the entry at that position and calling the interface.

Alternatively, you can also use the new C++0x std::function construction, like this:

#include <functional>
typedef std::function<double(double)> Algorithm;
typedef std::vector<Algorithm> Algorithms;

Taking an algorithm is similar, you should be able to call an algorithm like this:

Algorithms myAlgorithms;
...
double myresult = myAlgorithms[2](mydouble);

This approach has the advantage that you can also use lambda’s.

EDIT: This is an example that uses lambda’s. It compiles and works as expected with Visual Studio 2010 (just tested this myself):

#include <iostream>
#include <vector>
#include <functional> 
typedef std::function<double(double)> Algorithm; 
typedef std::vector<Algorithm> Algorithms; 

int main()
{
Algorithms algorithms;
algorithms.push_back([](double d)->double{return d+d;});
algorithms.push_back([](double d)->double{return d*d;});

std::cout << algorithms[0](5) << std::endl;
std::cout << algorithms[1](5) << std::endl;
}