accessing the parts of a composite class

Question

I am somehow confused with the concept of composition and encapsulation.

Here is the example which helps my bottleneck : I have a class A, which has parts of types B and C. B has also parts of types D and E.

class A
{
   B itsB;
   C itsC;
}

class B
{
   D itsD;
   E itsE;
}

class D
{
   int mAlpha;
   int getAlpha();
   void setAplha(int);
}

A itsA;

For this case, how do you read-only access mAlpha ?

This way ? itsA->getItsB()->getItsD()->getAlpha()

or this way ? itsA->getAlpha(), where A::getAlpha() accesses itsB->getItsD()->getAlpha()

What about the way to write access mAlpha ?

This way ? itsA->getItsB()->getItsD()->setAlpha(10.0)

or this way ? itsA->getItsB()->setAlpha(10.0), where A::getAlpha() accesses itsB->getItsD()->setAlpha(10.0)

So the question is ; to access/modify the parts of a composite class, while obeying the encapsulation principle, should you access only via the composite class' interface ?

Otherwise, it seems that you access the part, modify it, without the knowledge/permission of the owner composite.

Please help me to get rid of this confusion

Thanks.

Answer 1

So the question is ; to access/modify the parts of a composite class, while obeying the encapsulation principle, should you access only via the composite class' interface ?

According to the encapsulation principle, the composite's parts should be hidden. Assuming that mAlpha is logically a property of A, but implemented in D, you should add accessor methods in A that look up/change the mAlpha member in its D part. Otherwise, you'd be leaking implementation details of A (that it is a composite of a D and some other stuff).