Functor compose 2 : Functor « Development « 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>

#include <cmath>
#include <cstdlib>

class Abs {
  public:
    // ''function call'':
    double operator() (double v) const {
        return std::abs(v);
    }
};

class Sine {
  public:
    // ''function call'':
    double operator() (double a) const {
        return std::sin(a);
    }
};



template <typename FO1, typename FO2>
class Composer {
  private:
    FO1 fo1;  // first/inner function object to call
    FO2 fo2;  // second/outer function object to call
  public:
    // constructor: initialize function objects
    Composer (FO1 f1, FO2 f2)
     : fo1(f1), fo2(f2) {
    }

    // ''function call'': nested call of function objects
    double operator() (double v) {
        return fo2(fo1(v));
    }
};


template <typename FO1, typename FO2>
inline
Composer<FO1,FO2> compose (FO1 f1, FO2 f2) {
    return Composer<FO1,FO2> (f1, f2);
}


template<typename FO>
void print_values (FO fo)
{
    for (int i=-2; i<3; ++i) {
        std::cout << "f(" << i*0.1
                  << ") = " << fo(i*0.1)
                  << "\n";
    }
}

int main()
{
    // print sin(abs(-0.5))
    std::cout << compose(Abs(),Sine())(0.5) << "\n\n";

    // print abs() of some values
    print_values(Abs());
    std::cout << '\n';

    // print sin() of some values
    print_values(Sine());
    std::cout << '\n';

    // print sin(abs()) of some values
    print_values(compose(Abs(),Sine()));
    std::cout << '\n';

    // print abs(sin()) of some values
    print_values(compose(Sine(),Abs()));
}

0.479426

f(-0.2) = 0.2
f(-0.1) = 0.1
f(0) = 0
f(0.1) = 0.1
f(0.2) = 0.2

f(-0.2) = -0.198669
f(-0.1) = -0.0998334
f(0) = 0
f(0.1) = 0.0998334
f(0.2) = 0.198669

f(-0.2) = 0.198669
f(-0.1) = 0.0998334
f(0) = 0
f(0.1) = 0.0998334
f(0.2) = 0.198669

f(-0.2) = 0.198669
f(-0.1) = 0.0998334
f(0) = 0
f(0.1) = 0.0998334
f(0.2) = 0.198669