Re: What is the meaning of these lines?

red floyd wrote:


If you are passing a function then you are trying to do functional
programming. C-style callbacks are one example of functional programming in
C/C++.

The safer C++ equivalent is a design pattern where you inherit from an ABC
that has abstract callbacks and fill them in with the function you want to
pass.

For example, the following C++ ABC exposes a function "f" that applies the
function "g" twice:

  template<typename T>
  class ABC {
  public:
    virtual T g(T) = 0;
    T f(T x) { return g(g(x)); }
  };

we can derive from it, implementing a function that doubles its argument:

  template<typename T>
  class D : public ABC<T> {
    T g(T x) { return x * 2; }
  };

  #include <iostream>

  int main() {
    std::cout << D<int>().f(3) << std::endl;
    return 0;
  }

The functional equivalent is more elegant:

  let f g x = g(g x);;
  let g x = 2 * x;;
  Printf.printf "%d\n" (f g 3);;

If you're only writing toy programs then C or C++ are fine. But if you're
writing serious programs and you're interested in performance, reliability
and clarity then a functional programming language is clearly preferable.

--
Dr Jon D Harrop, Flying Frog Consultancy
The F#.NET Journal
http://www.ffconsultancy.com/products/fsharp_journal/?u6