Int-To-Type idiom

#include <iostream>
using namespace std;

template <int N>
class Array
{
    enum class InsertOp : int { NONE, INSERTION, QUICK };           // Three insert method to choose.

    template <InsertOp N>
    class IntToType {};                                             // Differenciate type by this class. 

    static constexpr enum InsertOp sc_op = N == 0 || N == 1 ? NONE :    // Here deduce which insert method should be used.
                                           N < 100 ? INSERTION : QUICK;

    // other members...
public:
    // Interface.
    void sort()    { sort(IntToType<sc_op>()); }                    // Call different insert method by type.

private:
    // logic.
    void sort(IntToType<NONE>) { cout <<"no-op." <<endl; }
    void sort(IntToType<INSERTION>) { cout <<"insertion sort." <<endl; }
    void sort(IntToType<QUICK>) { cout <<"quick sort." <<endl; }
};



int main(int argc, char *argv[])
{
    Array<1> t0;
    t1.sort();                                                       // no-op.

    Array<10> t10;
    t10.sort();                                                      // insertion sort.

    Array<100> t100;
    t100.sort();                                                     // quick sort.
    
    return 0;
}

 

　　'class Array' is a fixed size array. We want to use sort algorithm on it, deduced by the number of element.

　　Every 'Array<N>' with unique 'N' will create a different class, whose object will use "the best" sort method, that's to say: if number of element is zero or one, there is no reason to swap element, so no operation will occur; insertion sort will be used if between 2 and 100 sincing the better performance for little amount of element; for large quantity, quick insertion is the best chosen.

　　The key of this trick is to use 'sc_op' to classify insertion into three method, and use 'class IntToType' to create different class by template and differenciate function call at compile-time. That's really cool.