Polymorphism object creation - diamond inheritance hierarchy

Question

Language : C/C++
Problem : Taking the common example, where

A is the parent class.
B and C both inherited from class A.
D is inherited from both B and C

and we want to access A's function through an object of D

Note: it's virtual inheritance! So we know there will be only one object of A.

The question is : Whose object will be first created B's or C's ?

Keeping the hierarchy in mind

   A
 /   \
B     C
 \   /
   D

score 0 · Answer 1 · answered Aug 03 '12 at 11:52

0

In short, there is no guarantee in OO itself regarding this.

If you're talking specific implementations or languages (but the question does not) there may be guarantees regarding this.

answered Aug 03 '12 at 11:52

Joachim Isaksson

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@AliShahzeb In C++ it depends on the order you're listing the classes you're inheriting, for example, `class D : public B, public C` would construct B before C. – Joachim Isaksson Aug 03 '12 at 12:01

score 0 · Answer 2 · answered Aug 03 '12 at 11:54

0

Yeah, this problem is sometimes also apostrophed as diamond of death.

Unfortunately, your question cannot be answered: it is based on the current language you are programming in.

For instance, in Scala (where multiple inheritance is allowed for traits), there is a quite rigirous rule for instantiation along the linearization algorithm, there you can find the proper details.

I suppose you're playing with C++. If that's right, take a look on the proper section of the C++ FAQ (the problem is also mentioned there).

answered Aug 03 '12 at 11:54

rlegendi

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thnx for answering :) i owe you big one :) – Ali Shahzeb Aug 03 '12 at 11:58
No prob, but you could add the label of the language you are coding in, and also edit the question text just to make it more informative for others. – rlegendi Aug 03 '12 at 12:00

score 0 · Answer 3 · answered Aug 03 '12 at 12:02

If you are talking C++ and the order of instantion of the classes, then it will depend on the inheritance order in the declaration.

So, in

class D :  public B,public C

B will be instantiated first then C.

class D :  public C,public B

C will be instantiated first then B.

Polymorphism object creation - diamond inheritance hierarchy

3 Answers3