Question

The below program calculates 2 raised to the power n without using any loop,runtime recursion or library function[pow].
It uses TEMPLATE METAPROGRAMMING technique.

#include <iostream>
using namespace std;

template<int n> struct funStruct
{
    enum { val = 2*funStruct<n-1>::val };
};

template<> struct funStruct<0>
{
    enum { val = 1 };
};

int main()
{
    cout << funStruct<8>::val << endl;
    return 0;
}

I was wandering that can i use function in place of structure?

Answer 1

The obvious solution, as @tdammes points out is just a non-iterative non-recursive approach:

constexpr int pow2( unsigned pwr ) {
   return 1 << per;
}

By using constexpr the compiler will compute the result at compile time and yield a compile time constant. If you still want to use recursion, you can:

constexpr int pow2( unsigned pwr ) {
   return pwr==0? 1 : 2*pow2(pwr-1);
}

Which is basically the same compile time recursion than your metaprogramming trick in a slightly more concise and easy to read manner. The use of constexpr, of course, requires C++11, so if you don’t have it, you can always use either the original metaprogramming trick, or @tdammers approach adapted:

template <unsigned int N>
struct pow2 {
   static const unsigned int value = 1 << N;
};