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>

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?

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!

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;

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;

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;

}

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;

}

}

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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