#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 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 << Composer<Abs,Sine>(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(Composer<Abs, Sine>(Abs(), Sine()));
    std::cout << '\n';

    // print abs(sin()) of some values
    print_values(Composer<Sine, Abs>(Sine(), Abs()));
}