From: deepblack AT geocities DOT com (Luis Coelho)
Newsgroups: comp.lang.c++,comp.os.msdos.djgpp
Subject: Re: How to implement "They are all Vectors, but different realities?"
Date: Mon, 18 Jan 1999 22:08:27 GMT
Organization: Universidade Tecnica de Lisboa
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References: <36A68B87 DOT FFBA110C AT ohriki DOT t DOT u-tokyo DOT ac DOT jp>
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On 21 Jan 1999 02:05:28 GMT, Shue-Cheng CHEN
<scchen AT ohriki DOT t DOT u-tokyo DOT ac DOT jp> uttered the following words:

>Hi!
>
>     We know "Force" and "Velocity" are both "Vectors" in mathematics, so
>I defined them to own all behaviors and data of Vector (has been defined),
>without duplicating the code, as follows,
>
>        class Force    : public Vector {...};
>        class Velocity : public Vector {...};
>
>Therefore, the operators of Vector, for example, operator+(Vector&, Vector&)
>can be used for Force and Velocity, like
>
>        Force    + Force      // Ok
>        Velocity + Velocity   // Ok
>
>    But "Force" and "Velocity" are different physical realities, so
>
>        Force + Velocity   // Wrong
>
>I would like to deploit the common implementation of them to avoid
>duplicating their code, but how should I do to prevent from Force +
>Vector meaningless operation?
>
>    Thanks a lot!
>
>Best Regards,
>
>Shue-Cheng CHEN
>


It seems to me as what you are doing is trying to reuse (is this
spelt/spelled right?) the Vector implementation (the code) and
*NOT* its interface (what clients should do with it) as you don't
want people adding different types of Vectors.

So, this leaves you with two possibilities:

1- 

class Force, private Vector // private inhearince
{
 public:
 //...
 Force operator+(const Force&) const;
 //...
};

or
2-

class Force
{
 Vector m_vec; // having a Vector as a member function
 //...
};

They both do more or less the same, and you dispach the
operations to your Vector, be it a base class or a member. But
what you're doing is probably going to work very badly, consider

<code>
class Force, public Vector { /*...*/ };
class Velocity, public Vector { /*....*/ };

void f(Vector& a, Vector& b)
{
 a = a + b;
 return;
}

int main()
{
 Force kraft(0,1);
 Velocity v(1,0);
 f(kraft,v); // this line changes kraft
 return 0; 
}
</code>

if you have any problems with this, feel free to email me
privately,
Regards,
Luis Coelho.
C++ Programming Language, 3rd Ed. by B. Stroustrup. My exercise answers at:
http://www.geocities.com/SiliconValley/Way/3972/index.html