Re: Is this class design correct? A better way?

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Sat, 17 Feb 2007 00:01:03 +0100
Message-ID:
<53mrdgF1tchf9U1@mid.individual.net>
* nw:

I was wondering if someone would be able to give me some comments on
the following class structure, it feels to me as if there must be a
better way, but I'm unsure what it is, perhaps I should be using
multiple inheritance?


You should be using /inheritance/ and-or /templating/ instead of a type
switch.

For inheritance, you can represent the integration algorithm directly as
a derived class override of a Body virtual member function, or you can
represent it as an object passed to the Body constructor(s).

[snip]

Compilable example code implementing this design follows. Apologies if
I've been overly verbose.

Any help greatly appreciated!


OK...

#include <iostream>


Formally you also need <ostream>.

using namespace std;

class Body {

  public:
  double x, y, z;
  double a;


Public member variables = ungood.

Name 'a' = ungood.

  static const int BODY_INTEGRATE_EULER=1;
  static const int BODY_INTEGRATE_VERLET=2;
  static const int BODY_INTEGRATE_LEAPFROG=3;


All uppercase for non-macros = ungood.

  int integration_type;


'int' to represent something with limited number of possible values =
ungood.

Representing type as value = in general, and in this case, ungood.

  virtual bool calculate_force(Body &b) = 0;

  Body() {
    integration_type=BODY_INTEGRATE_EULER;
  }


See the FAQ item titled 'Should my constructors use "initialization
lists" or "assignment"?', currently available at <url:
http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.6>.

  bool integrate() {
    if(integration_type == BODY_INTEGRATE_EULER) {
      // .. do euler
      cout << "Euler integration" << endl;
      return true;
    } else
    if(integration_type == BODY_INTEGRATE_VERLET) {
      // .. do verlet
      cout << "Verlet integration" << endl;
      return true;
    } else
    if(integration_type == BODY_INTEGRATE_LEAPFROG) {
      // .. do leapfrog
      cout << "Leapfrog integration" << endl;
      return true;
    }
  }


This type selection is what you need to avoid.

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