Sqrt: primary template: yield second or third argument depending on first argument : 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: yield second or third argument depending on first argument
template<bool C, typename Ta, typename Tb>
class IfThenElse;

// partial specialization: true yields second argument
template<typename Ta, typename Tb>
class IfThenElse<true, Ta, Tb> {
  public:
    typedef Ta ResultT;
};

// partial specialization: false yields third argument
template<typename Ta, typename Tb>
class IfThenElse<false, Ta, Tb> {
  public:
    typedef Tb ResultT;
};



// template to yield template argument as result
template<int N>
class Value {
  public:
    enum { result = N };
};

// template to compute sqrt(N) via iteration
template <int N, int I=1>
class Sqrt {
  public:
    // instantiate next step or result type as branch
    typedef typename IfThenElse<(I*I<N),
                                Sqrt<N,I+1>,
                                Value<I>
                               >::ResultT
            SubT;

    // use the result of branch type
    enum { result = SubT::result };
};

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