Has a inheritance, not is a

Question

I know that when creating derived classes in C++ a structure like this:

class A {
    ...
}
class B: public A {
    ...
}

Would mean that there is a relation ship between A and B such that B "is a" A. I am wondering what if I wanted an A-B relationship such that B has a A not that B is a A.

My objective is to write a code that computes wave-function and corresponding properties of that wave-function for a particular molecule. I want my molecule to have a wave-function object. Can I i just create the molecule and wave-function classes separately, and then call the wave-function constructor in the molecule constructor?

class Molecule {
      Molecule(...);

      Molecule::Molecule(...)
      {
         wavefunction WF(...);
      }
};

IF i can do this is it the correct way to create this has-a relationship? How would I access the wave-function object? Like this:

mol.WF

assuming I had created a molecule with the name mol. This is my first whack at a program with more than one custom class and I am confusing myself very fast.

I think what you want to use is [composition](http://www.learncpp.com/cpp-tutorial/102-composition/) — Mariano D'Ascanio, Jul 09 '14 at 19:14
Not use inheritance for this. Inheritance is only for `is as`, and is frowned upon because imposes tighter coupling between classes. Use composition instead (check for "inheritance vs composition" for a larger and better explanation). — SJuan76, Jul 09 '14 at 19:15
You may want to pick up an [introductory C++ book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) so you can get a handle on how various programming idioms are expressed in C++ and how you can use them to your advantage. — In silico, Jul 09 '14 at 19:19
@Insilico I pickd up a copy of _C++_ _Primer_ from a coworker thank you for linking that post. I see that the post has been put on hold for being unclear. I was trying to ask if composition was possible, but I didn't know what that was So i had a hard time getting my point across. I marked cameron's post as the answer because It came with a detailed example that was very helpful for understanding the concept. — Ajay, Jul 10 '14 at 15:18
@Deduplicator I was not sure if private inheritance was the best way to go here because there are elements of molecule that would not apply to wave-function. If I was to use private inheritance the wave-function would still inherit all of the molecule's elements, no? — Ajay, Jul 10 '14 at 15:22

Baptiste Wicht · Answer 1 · 2014-07-09T19:23:33.583

4

I'm not sure I understand, but why don't simply declare a variable of type wavefunction in Molecule ?

class Molecule {
public:
      wavefunction wf;

      //...
};

Then, you can init it in constructor and manipulate it as a variable of Molecule. You can use it from outside the cass with mol.wf

This is called composition and this is what is used for "has a" relationship.

Edit: At the VERY last, you could use private inheritance, but composition by adding a field is much more practical and should be used instead almost always. And it would not be possible to use the wf fields outside of Molecule.

edited Jul 09 '14 at 19:23

answered Jul 09 '14 at 19:14

Baptiste Wicht

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I think the OP's question is how to express a "has-a" relationship instead of a "is-a" relationship in C++. As you've answered, this is done by declaring a member variable in the class. – In silico Jul 09 '14 at 19:16

score 3 · Accepted Answer · answered Jul 09 '14 at 19:21

Yes, this is possible. It looks like all you're missing is a simple example:

class Molecule {
public:
    wavefunction WF;    // This is a member variable; each Molecule will
                        // "have a" wavefunction named WF

    // When the constructor is called, the constructor of all member
    // variables is called too. We can explicitly specify arguments to
    // them using the 'initializer-list' (the part after ':')
    Molecule()
        : WF(/* constructor arguments... */)
    {
        // Here inside the constructor (and all methods of this class)
        // you can refer to WF directly or via 'this' (the implicit pointer
        // to the current molecule's instance):
        WF.foo();
        this->WF.foo();
    }
};

You can then refer to the WF through its Molecule's instance like so:

Molecule m;
m.WF.foo();

score 2 · Answer 3 · answered Jul 09 '14 at 19:17

2

You can implement a has a relationship by making the type a member of the type that has it.

class A {
    ...
}
class B {
    A a;
    ...
}

answered Jul 09 '14 at 19:17

Jonathan Wood

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Or a `unique_ptr` if optional. Or using private inheritance... – Deduplicator Jul 09 '14 at 19:19

Has a inheritance, not is a

3 Answers3