primary template to compute sqrt(N) via iteration and partial specialization to end the iteration : template parameter « template « C++ Tutorial

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/* The following code example is taken from the book
 * "C++ Templates - The Complete Guide"
 * by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
 *
 * (C) Copyright David Vandevoorde and Nicolai M. Josuttis 2002.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */
#include <iostream>


// primary template to compute sqrt(N) via iteration
template <int N, int I=1>
class Sqrt {
  public:
    enum { result = (I*I<N) ? Sqrt<N,I+1>::result
                            : I };
};

// partial specialization to end the iteration
template<int N>
class Sqrt<N,N> {
  public:
    enum { result = N };
};


int main()
{
    std::cout << "Sqrt<16>::result = " << Sqrt<16>::result << std::endl;
    std::cout << "Sqrt<25>::result = " << Sqrt<25>::result << std::endl;
    std::cout << "Sqrt<42>::result = " << Sqrt<42>::result << std::endl;
    std::cout << "Sqrt<1>::result =  " << Sqrt<1>::result << std::endl;
}

Sqrt<16>::result = 4
Sqrt<25>::result = 5
Sqrt<42>::result = 7
Sqrt<1>::result =  1