base class constructs five times

Question

This "base" class constructs for b , medium1 and medium2. When i use "top" class, it construct 2 times more. Totally constructing 5 times(writing to screen 5 times).

Question: What can i change in the "base" class to make it construct only once and for all.?

#include "stdafx.h"
#include<stdlib.h> 

class base
{
public:
    base(){x=5;printf(" %i ",x);}
    int x;
}b;

class medium1:public base{}m1;

class medium2:public base{}m2;

class top:public medium1,medium2{};


int main() {
    top ten;
    getchar();
    return 0;
}

what are pitfalls of virtual base class? Thanks for answering my trivial question.

1- `m1` , 2 - `m2` , 3 - `b`, 4,5 - `ten` due to inheritance from `medium1,medium2` — Mr.Anubis, Aug 14 '12 at 21:28
Virtual base classes won't get the constructor count down to 1. There are four objects whose types are `base` or a type derived from `base`, so the least you can get is four constructions. — Pete Becker, Aug 14 '12 at 21:29
@tuğrul büyükışık: Your question has no meaningful answer. It is not possible to have it constructed only once. If you meant something else from what you actually asked, please, clarify your question. — AnT, Aug 14 '12 at 21:29
I want all of them to use only the original constructed base — huseyin tugrul buyukisik, Aug 14 '12 at 21:33
If you want all those objects to use the same member `x`, just make `x` a `static` member. — aschepler, Aug 14 '12 at 21:33
For example: base gets a timer data. All classes uses that same time data. Not the different times — huseyin tugrul buyukisik, Aug 14 '12 at 21:33
@tuğrul büyükışık: Independent objects cannot share one common base subobject. The language has no such feature. So, as long as you implement the concept of "inheritance" through language feature known as "inheritance", you will have at least 4 separate bases (since you have 4 independent objects). — AnT, Aug 14 '12 at 21:35
@tuğrul büyükışık: One way to go is to make timer data `static` in the case class. You will still have many bases (4 or 5), but all these bases will share the same timer data. — AnT, Aug 14 '12 at 21:37

AnT · Accepted Answer · 2012-08-14T21:31:41.947

6

It is not possible to have base construct only once instead of all five original constructions. You have four complete objects in your program that contain base subobjects. This immediately means that regardless of what you do, you will have at least four base constructions.

Additionally, the suggestion to use virtual inheritance mentioned in other answers will actually require making changes to other classes, not to base class. Meanwhile your question insists that changes shall be made to base.

edited Aug 14 '12 at 21:31

answered Aug 14 '12 at 21:26

AnT

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i made medium 1 and medium2 virtual and decreased to 4 constructions. yes you are right – huseyin tugrul buyukisik Aug 14 '12 at 21:31

score 5 · Answer 2 · answered Aug 14 '12 at 21:23

This will construct base only once when you create a top class.

class medium1:virtual public base{};
class medium2:virtual public base{};

The rest come from the fact that you create objects b, m1 and m2. You can't really prevent calling the destructor of the base class because, well, that's what you tell the compiler to do. If you don't want to create base objects, don't derive from it.

score 2 · Answer 3 · edited May 23 '17 at 12:04

2

You can make it a virtual base class, but that comes with a giant set of pitfalls.

edited May 23 '17 at 12:04

Community

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answered Aug 14 '12 at 21:23

Jon

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what are pitfalls of virtual base class? – huseyin tugrul buyukisik Aug 14 '12 at 21:24

score 1 · Answer 4 · answered Aug 14 '12 at 21:23

1

You can have a single base class object if you use virtual inheritance:

class medium1:virtual public base{}m1;

class medium2: virtual public base{}m2;

answered Aug 14 '12 at 21:23

juanchopanza

216,937
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score 1 · Answer 5 · answered Aug 14 '12 at 21:34

This gets you down to two calls, but I think since you have double inheritance that's the best you'll get. If you eliminate the variable b in your base definition then you're there. http://codepad.org/pn016D7b

#include<stdlib.h> 

class base
{
public:
    base(){x=5;printf(" %i ",x);}
    int x;
}b;

class medium1:virtual public base 
{
public:
    medium1():base(){};
};

class medium2:virtual public base{};

class top:virtual public medium1,medium2{};


int main() {
    top ten;
    getchar();
    return 0;
}

base class constructs five times

5 Answers5