Dynamically adding an object?

Question

#include <iostream>
#include <vector>
#include <iomanip>

using namespace std;

static int c;

class DAI   //DynamicArrayInput
{
public :
    void getCountry()
    {
    cout << "Country : ";
    cin >> Country;
    }

    void putCountry()
    {
    cout << Country << endl;
    }
    static int putCount()
    {
    return count;
    }
private :
    static int count;
    char Country[30];
};



int main()
{
vector< DAI > A;
DAI B[3];
//DAI * B;

cout << "Enter name of countries\n";
  for(int i=0; i < 3; i++)
  {
        //THIS DOESN'T WORK
    /*
    c++;   //c is a static variable declared at top
    B = new DAI[sizeof(DAI) * c/sizeof(DAI)];
    GIVES WRONG OUTPUT :
    Enter name of countries
    Country : a
    Country : b
    Country : c
    Printing name of countries


    c
    size of vector A is 3
    vector after initialisation:

    */  
    A.push_back(B[i]);
    B[i].getCountry(); 
  }
  cout << "Printing name of countries\n";
  for(int i=0; i < 3; i++)
  {
    B[i].putCountry();
  }

cout << "size of vector A is " << A.size() << "\
    \nvector after initialisation:" << endl;

return 0;
}


**/*
CORRECT OUTPUT WITH LIMITED ARRAY SIZE = 3:
*Enter name of countries
 Country : a
 Country : b
 Country : c
 Printing name of countries
 a
 b
 c
 size of vector A is 3
 vector after initialisation:*
*/**

I am just learning and this is not my homework, I dont want to be limited to any array size such as above but when i try to use the staements in the first for loop as mentioned it does not work i mean this does not works

c++;   //c is a static variable declared at top
B = new DAI[sizeof(DAI) * c/sizeof(DAI)];  //B is a pointer of type class DAI

My aim is to use arrays of objects i like dynamically so that i can input any number of countries I don't want to use something like B[3] with something known array size but i am unable to do.

Please guide me, help me.

Use an std::vector or std::deque, which are auto-resizing containers. I see you are holding arrays in a vector - you could already have std::vector<std::vector<DAI> >. — wkl
– wkl, Commented Apr 27, 2011 at 15:27
Well I have used it in the program but Unable to proceed with it see first line of main — mukesh
– mukesh, Commented Apr 27, 2011 at 15:33

Bo Persson · Accepted Answer · 2011-04-27 16:15:46Z

2

c++; //c is a static variable declared at top
B = new DAI[sizeof(DAI) * c/sizeof(DAI)];

Here you are confusing new with malloc. To allocate c elements of type DAI, just do

B = new DAI[c];

However, your best bet is to stay with std::vector, which handles size and capacity itself.

for (int i = 0; i != 10; ++i)
    A.push_back(DAI());

adds 10 DAIs to the vector A.

Edit
You can also create the vector with a number of DAIs from the start:

std::vector<DAI> A(10);

This also creates a vector with 10 elements, all at once. Unlike B[3], which always contains 3 elements, the vector can grow or shrink as needed without you having to handle the allocations.

edited Apr 27, 2011 at 16:15

answered Apr 27, 2011 at 15:38

Bo Persson

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4 Comments

mukesh Over a year ago

very nice! Thanks, Bo I will try B = new DAI[c] I dont get the second one DAI()

mukesh Over a year ago

yes, your second idea works ...thanks a lot but whats DAI() is it a constructor call how does this works

Bo Persson Over a year ago

Yes, it is a constructor creating new DAIs to be added to the vector. I also added a new example to my answer on how to create the vector with elements, without using a loop.

mukesh Over a year ago

Thanks,I will accept your answer because I can accept only one at a time even though I haven't tried to implement the answer given below by Julio his answer also seems reasonable to me.

Julio Guerra · Accepted Answer · 2011-04-27 15:42:37Z

0

Your array is static DAI[3] v (v is in the process' stack). To be able to add an element dynamically, you need to change it with DAI[INITIAL_SZ] v = new DAI[INITIAL_SZ] (now v is in the heap) so that now you can realloc it when adding/deleting elements.

answered Apr 27, 2011 at 15:42

Julio Guerra

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