why derived class member is giving garbage value

Question

    #include<iostream>

    using namespace std;

    class base
    {
    int i;
    public:
    base()
    {
    i=10;
    cout<<"Base Constructor\n";
    }
    base(base &a)
    {
    cout<<"base Copy Constructor\n";
    a.i=i;

    }

    ~base()
    {
    cout<<"base Destructor\n";
    cout<<i<<endl;
    }

    };

    class derived:public base
    {
    int j;
    public:
    derived()
    {
    j=20;
    cout<<"derived Constructor\n";
    }
    derived(derived &a)
    {
    cout<<"derived Copy Constructor\n";
    a.j=j;
    cout<<j<<endl;
    }

    ~derived()
    {
    cout<<"derived Destructor\n";
    }

    };

    main()
    {
    base obj;
    base obj1=obj;

    derived ob;
    derived ob1=ob;

    }

i am a beginner in cpp i was trying to understand single inheritance how it behaves how its const,dest behaves but got a problem in the derived class derived member giving garbage value.. can somebody explain me.

[Slicing](http://stackoverflow.com/questions/274626/what-is-the-slicing-problem-in-c) maybe? — πάντα ῥεῖ, May 14 '14 at 19:30

score 1 · Accepted Answer · answered May 14 '14 at 19:32

1

Your copy constructors are wrong. You are copying to the source rather than destination. Try replacing:

base(base &a)
{
  cout<<"base Copy Constructor\n";
  a.i=i;
}

With:

base(base &a)
{
  cout<<"base Copy Constructor\n";
  i = a.i;
}

AND

derived(derived &a)
{
  cout<<"derived Copy Constructor\n";
  a.j=j;
  cout<<j<<endl;
}

with

derived(derived &a)
{
  cout<<"derived Copy Constructor\n";
  j=a.j;
  cout<<j<<endl;
}

answered May 14 '14 at 19:32

Harry

1,362
12
19

or even better, use initialiser lists - since then it's impossible to get the assignment the wrong way round. – Richard Hodges May 14 '14 at 19:33
@RichardHodges - Agreed. One step at a time though... – Harry May 14 '14 at 19:35
i got that copy constructor thing but why it is giving garbage value?? – user3207191 May 14 '14 at 19:38
You are then printing the value of j (`cout< – Harry May 14 '14 at 19:40
thanks that was the problem in copy constructor.. but i was printing j. which was initialize in constructor – user3207191 May 14 '14 at 19:41
Copy-constructors take a reference to `const`. – David G May 14 '14 at 19:43
@Harry oh yes, I see... – Richard Hodges May 14 '14 at 19:45
@user3207191 If you call the copy constructor your default constructor is **not** called – Harry May 14 '14 at 19:49

score 0 · Answer 2 · answered May 14 '14 at 19:41

0

Your copy constructors are implemented incorrectly.

You are modifying the object being passed in, not the object you are constructing. Try this with:

i=a.i;

and

j=a.j;

in your copy constructors, instead of the other way around.

This should (probably) do what it looks like you're trying to do, and you should be able to see why the way you are currently doing it is resulting in uninitialised member variables.

answered May 14 '14 at 19:41

quercus32

116
1
1
5

thank u i got it now.. :) – user3207191 May 14 '14 at 19:43

why derived class member is giving garbage value

2 Answers2