Value of member variable in multiple inheritance

Question

Hi

Please refer class D which is inherited from B and C

As D's constructor gets called last post all base classes, 3 is expected to get for a in case D has constructor

Question is about value of a if D constructor is commented
I was expecting output 2 but it prints 0..why?

B constructor sets a as 1 and then c should set to 2 but why output is 0?

If D has been inherited like D : public C, public B

Output should be 1.? Is this correct ?
https://code.sololearn.com/cKkgvSg6agzo/?ref=app

Answer 1

This should help you:

https://en.wikipedia.org/wiki/Virtual_inheritance

Answer 2

Yeah I am aware about diamond problem and virtual inheritance..

My concern is about constructor calls.... If object d is not calling constructors if B and C, isn't it a bad thing ? What if these two classes manages resources via constructor

Answer 3

I have different perspective to look at this as below :

C is derived from A and hence variable a of A is accessible to C. Variable is still in A and just available to C object as well due to inheritance

Now as soon as C constructor gets called , variable of struct A is assigned to 2...

I hope inheritance of D from C forces D' default constructor to call C's default constructor which should set value of a to 2

I feel that value should be 2. Is it not true ?

Answer 4

Also if I just remove virtual from struct c declaration and D just gets inherited from C alone, I got 2 as output as expected without D constructor uncommented

Does this means due to virtual inheritance, C and B constructors don't get called ? Dimaond problem forced us to use virtual inheritance but it is having flaw of not calling inheritance heircrchy constructors

Isn't it ?

Answer 5

Basic question : why constructor not called for struct C even though D obj is created

Rest all are the work around and not much interested... Also value of a is not main concern for me.

I just want to understand that c constructor should get caled or bit for object of D ?

Answer 6

I just updated a line into c struct constructor...

It seems c constructor getting called and prints hi as well

Now another question: why a is not modified and prints value 0?