parameterized constructor with dynamic array of object - c++

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;
}

Answer 1

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);
};

Answer 2

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.