compile_time_prime_in_range.cpp

#include <iostream>
#define N 1000
/*
  This is quite a good example of Template Meta Programming. Especially, in hackathons that accept executables or does not have
  time limit in compilation, computation-heavy tasks as prime num discovery in a range can be hacked in compile time. And the result
  can be expressed in O(1) or O(n) with some reasonable constant factor.
  
  Reference: https://en.wikibooks.org/wiki/C%2B%2B_Programming/Templates/Template_Meta-Programming
*/

template <int p,int i>
class is_prime {
public:
	enum {prim = ((p%i) && is_prime<p,i-1>::prim)};
};

template<int p>
class is_prime<p,1>{
public:
	enum {prim = 1};
};

template <int i>
class Prime_print {
public:
	Prime_print<i-1> a;
	enum { prim = is_prime<i,i-1>::prim};
	void f(){
		a.f();
		if(prim)
			std::cout<<"prime number: "<<i<<std::endl;
	}
};
template<>
class Prime_print<1> {
public:
	enum {prim = 0};
	void f(){}
};

int main(){
  Prime_print<N> a;
	a.f();
}