PracticeDev/study_cpp/cpp_primer_source_code/Chapter 18/somedefs.h

// somedefs.h
#include <iostream>

template <typename T, typename F>
  T use_f(T v, F f)
  {
      static int count = 0;
      count++;
      std::cout << "use_f count = " << count
		        << ", &count = " << &count << endl;
      return f(v);
  }

  class Fp
  {
  private:
      double z_;
  public:
      Fp(double z = 1.0) : z_(z) {}
      double operator()(double p) { return z_*p; }
  };

  class Fq
  {
  private:
      double z_;
  public:
      Fq(double z = 1.0) : z_(z) {}
      double operator()(double q) { return z_+ q; }
  };