Dynamically allocating memory

Hello,

I am trying to allocate an array, store some int values in it, and then print them out.

Code:

---

#include <iostream>
using namespace std;
int main()
{
       
        int size;
        cout<<"\nEnter no. of variables:";
        cin>>size;
        int *ptr=new int[size];
        for(int i=0;i<size;i++,ptr++)
        {
                cout<<endl<<"Value of ptr["<<i<<"]=";
            cin>>*ptr;
        } 
        ptr-=size;
        for(int i=0;i<size;i++,ptr++)
            cout<<endl<<"ptr["<<i<<"]="<<*ptr;
               
        delete[] ptr;
        return 0;
}

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It is giving output like this:

Code:

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Enter no. of variables:3
value of ptr[0]=56
value of ptr[1]=89
value of ptr[2]=4

ptr[0]=56
ptr[1]=89
segmentation fault (core dumped)


-----------------------------------------------
Program exited with code:139
Press return to continue

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Please point out what's wrong in the code and what is segmentation fault ?

Thank you.

The seg fault could be because you attempt to delete[] a pointer that points to a non-existent object. The problem is that you are modifying ptr when it is supposed to point to the first element of the array.

For a solution, I suggest that you just use array indexing:

Code:

---

#include <iostream>
using namespace std;
int main()
{
    unsigned int size;
    cout << "\nEnter no. of variables:";
    cin >> size;
    int* ptr = new int[size];
    for (unsigned int i = 0; i < size; ++i)
    {
        cout << endl << "Value of ptr[" << i << "]=";
        cin >> ptr[i];
    }

    for (unsigned int i = 0; i < size; ++i)
        cout << endl << "ptr[" << i << "]=" << ptr[i];

    delete[] ptr;
    return 0;
}

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Oh yes, why are you not using a std::vector instead?

yes thanks. array indexing is easier.

I also got my original code running by adding

Code:

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ptr-=size

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before

Code:

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delete[] ptr

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Quote:

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Oh yes, why are you not using a std::vector instead?

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I don't know what vectors are. I couldn't find vectors in the tutorials I am following.(?)

maybe it is time to replace your tutorial?

sure I will, please suggest..

http://www.cprogramming.com/tutorial/stl/vector.html

Quote:

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I also got my original code running by adding

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I did not want to suggest that as it is rather fragile. With such a technique, not only must you carefully match your new[] with a delete[], but you must keep track of where exactly ptr is pointing so you can reset it as needed.

Quote:

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maybe it is time to replace your tutorial?

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Indeed. You would do better to learn how to use a std::vector before you learn how to use dynamically allocated arrays. I suggest borrowing Accelerated C++ by Koenig and Moo from your local library.

The variable size should be a std::size_t (defined in <cstddef>) instead of an unsigned int, since that's the proper type for an array index (though it might happen to be the same type on your platform).

Quote:

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The variable size should be a std::size_t (defined in <cstddef>) instead of an unsigned int, since that's the proper type for an array index (though it might happen to be the same type on your platform).

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That's true, though for such small values as expected from user input it would make no difference besides a possible compiler warning.