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;
};



// primary template for main recursive step
template<int N, int LO=1, int HI=N>
class Sqrt {
  public:
    // compute the midpoint, rounded up
    enum { mid = (LO+HI+1)/2 };

    // search a not too large value in a halved interval
    typedef typename IfThenElse<(N<mid*mid),
                                Sqrt<N,LO,mid-1>,
                                Sqrt<N,mid,HI> >::ResultT
            SubT;
    enum { result = SubT::result };
};

// partial specialization for end of recursion criterion
template<int N, int S>
class Sqrt<N, S, S> {
  public:
    enum { result = S };
};


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

Sqrt<16>::result = 4
Sqrt<25>::result = 5
Sqrt<42>::result = 6
Sqrt<1>::result =  1