parameterized constructor with dynamic array of object

Question

In the below program i have used two classes , and i am trying to relate the with aggregation, i have declare class A as private in class B, and with the help of constructor i am initizing base address of class A object to private member of class B object that is (A object). i am trying to pass the class A values to class B usng parametrized constructor, but i am getting garbage values like,

#include <iostream>
using namespace std;

void create (B * &obj2, int siz)
{
    std::cout << obj2[0].get_nn ();     //this will run fine
    for (int i = 0; i < siz; i++)
        obj2[i] = B (10, "pranjal");    //this will also run fine

    std::cout << obj2[0].get_nn ();
}
// same line printing again, this will not give output
// *************************************** 

void display ()
{
    std::cout << object.get_data () << object.get_stringdata ();
}

// giving garbage values
// why is it happening
// *********************************program
// ************************************** enter code here
// Online C++ compiler to run C++ program online

class A {
    int rool;
    string name;
  public:
    A () { };
    A (int a, string name);
    int get_data () {
        return rool;
    }
    string get_stringdata () {
        return this->name;
    }
};

A::A (int a, string name)
{
    this->rool = a;
    this->name = name;
}

void getdetails (A * obj)
{
    for (int i = 0; i < 3; i++)
        obj[i] = A (20, "pranjal");
}

class B {

    int bbb;
    string name;
    A object;
  public:
    B () {};
    B (A s) {
        object = s;
    }
    string get_nn () {
        return object.get_stringdata ();
    }
    B (int a, string b);

    void display () {
        std::cout << object.get_data () << object.get_stringdata ();
    }
};

void dis (B * obj2)
{
    for (int i = 0; i < 2; i++) {
        obj2[i].display ();
    }
}

void create (B * &obj2, int siz)
{
    std::cout << obj2[0].get_nn ();
    for (int i = 0; i < siz; i++)
        obj2[i] = B (10, "pranjal");

    std::cout << obj2[0].get_nn () << "sd";
}

B::B (int a, string b)
{
    bbb = a;
    name = b;
}

int main ()
{
    A *obj = new A[3];
    getdetails (obj);
    
    B *obj2 = new B[3];
    
    for (int i = 0; i < 3; i++) {
        obj2[i] = B (obj[i]);
    }

    create (obj2, 3);
    dis (obj2);
    obj2->display ();

    return 0;
}

Your garbage values appear to be code. Is this correct or a cut and paste mistake? — user4581301, May 05 '21 at 06:21
In `create`, I don't think `delete[] obj2;` is a good idea. It seems counter productive. — user4581301, May 05 '21 at 06:25
ohh sorry i was just trying the changes in program, if icomment that delete[] obj2, then too i am getting garbage value — pranjal, May 05 '21 at 06:38
pranjal sd 846122816 846122816 846122816 ai am getting this output, instead it should print pranjal , plz help — pranjal, May 05 '21 at 06:42
Please review and [edit] your question to remove the comments from the code (or make them proper comments). See also our [formatting help](https://stackoverflow.com/help/formatting). — Lukas-T, May 05 '21 at 06:50
When this assignment takes place `obj2[i]=B(obj[i]);` the values are indeterminate. — David C. Rankin, May 05 '21 at 06:51
this is working fine actually, but second parameterized constructor is giving problem, by assiggning values to obj2[i]=B(10,"pranjal"); my A object variable data, which i assiged to obj2[j] is gettting vanished and giving garbage, plz suggest me what do i do? — pranjal, May 05 '21 at 06:55
Side note: https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice (this actually applies for *all* namespaces...). — Aconcagua, May 05 '21 at 07:02
Side note 2: You should get used to use constructor's initialiser list (not to be confused with `std::initializer_list`), it should look like `A::A(int a, string name) : rool(a), name(name) { }` (analogously for those of B), and your default constructor leaves `rool` uninitialised. Be aware that there are types that *only* can be initialised that way (references, `const` members, non-assignable types, ...). — Aconcagua, May 05 '21 at 07:05
Side note 3: No need for reference to pointer as parameter for `create` function: You get one indirection by the pointer already, and you do not intend to modify the pointer that is *passed* to the function. Thus: `void create(B* obj, size_t size)` – the correct type to specify sizes is `size_t`, by the way... — Aconcagua, May 05 '21 at 07:21

Aconcagua · Answer 1 · 2021-05-05T07:49:01.637

void create(B *&obj2,int siz)
{
    std::cout<<obj2[0].get_nn();      //this will run fine
    for(int i=0;i<siz;i++)
        obj2[i]=B(10,"pranjal");
    std::cout<<obj2[0].get_nn(); // same line printing again, this will not give output
}

Of course it won't: B(int, std::string) does not call a constructor of A explicitly, so A's default constructor is called. This implicitly calls name's default constructor, too, which will create an empty string.

For a similar reason you get garbage values: Your default constructor does not assign a value to rool, thus leaves it uninitialised; it will remain at the value that was there in memory before, which ever it was, which is what you consider 'garbage'.

This occurs because with obj2[i] = B(10,"pranjal") a new object is created and then copied into the target object (with optimisations, can be created directly in place, but obviously you cannot rely on).

Fix your constructor of A to assign a value to rool and the problem should go away. You might chose a non-zero value to have a more clearly visible effect:

A() : rool(1977) { }

Alternatively you can provide provide a default value to rool (requires C++11):

int rool = 1977;

That can be convenient especially if you have a larger number of constructors.

In general: You should not leave primitive types of any of your objects uninitialised to avoid similar problems.

score 0 · Accepted Answer · answered May 05 '21 at 07:57

In create, you fully replace the previous B objects with brand new ones using the 2 parameters constructor... which just leaves the A object member default initialized.

What could be done to fix?

Pass the parameters of B to A ctor:
```
 B::B (int a, string b): object(a, b)
 {
     bbb = a;
     name = b;
 }
```
That way you initialize the A object member with the same parameters as its parent

Reset the A object member to its previous value:

 void create (B * &obj2, int siz)
 {
     std::cout << obj2[0].get_nn ();     //this will run fine
     for (int i = 0; i < siz; i++) {
         A old = obj2[i].object;
         obj2[i] = B (10, "pranjal");    //this will also run fine
         obj2[i].object = old;
     }
     std::cout << obj2[0].get_nn ();
 }

But as it requires accesses to private members, create should be declared friend in B class:

 class B {
     ...
     friend void create (B * &obj2, int siz);
 };

parameterized constructor with dynamic array of object

2 Answers2