Creating an Array of Structures on the Heap in C++

Question

I need to declare an array of structures on the heap, then transfer data from parallel arrays on the stack and from calculations into each structure. I declared

struct Grades
{
    string  studentName;
    int     scores[4];
    double  average;
};

....

Grades *art1301 = new Grades;

....

(art1301 + i)->studentName = names[i];

for((int i = 0 ; i < 5 ; i++ )
(art1301 + i)->scores[j] = exams[i][j];

(art1301 + i)->average = average;

My program accesses the first record, but it crashes after it accesses the first field of the second record. I don't understand why it works for the first record, but dies in the middle of the second? Am I accessing the structure correctly?

Thank you.

Here's a hint: This is wrong (for your purposes): Grades *art1301 = new Grades; — Qaz
– Qaz, Commented Apr 28, 2012 at 22:10
You are declaring only one Grades struct on the heap. You do not have enough memory allocated for more than one, so you probably get a writing violation consequentially. To be able to work one more than one Grades on the heap, then this change should be made Grades* art1301 = new Grades[i+1], where I assume i + 1 is the number of structs you want allocated. — user1114055
– user1114055, Commented Apr 28, 2012 at 22:10
Please don't use pointer arithmetic to index an array. Use the indexing operator []. — David Heffernan
– David Heffernan, Commented Apr 28, 2012 at 22:19
Thank you, Alex. I thought that was the wrong way to declare the structure, but every way I tried to edit it, the compiler rejected. — Opcode
– Opcode, Commented Apr 28, 2012 at 22:21

Kerrek SB · Accepted Answer · 2012-04-28 22:16:43Z

2

To allocate an array, you need the array form of new, with the square brackets:

Grades *art1301 = new Grades[200];
//                          ^^^^^

The array size can be a dynamically determined quantity.

answered Apr 28, 2012 at 22:16

Kerrek SB

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1 Comment

Opcode Over a year ago

Thanks. Adding the array size put the entire program working. My assignment is now complete.

vguzmanp · Accepted Answer · 2012-04-28 22:17:31Z

0

You aren't allocating memory for an array, you are allocating only for one element.

As someone said in the comments, the key is in the new Grades instruction

In addition, unless you have another i variable declared before (which is a bad practice), that code doesn't compile because (art1301 + i)->studentName = names[i]; will not find variable i

edited Apr 28, 2012 at 22:17

answered Apr 28, 2012 at 22:11

vguzmanp

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