Array of structs with Array in the struct

Question

I am completely stuck on this problem... The code structure that is given is as follows:

typedef struct _myvar{
   uint32_t someInt;
   uint32_t *ptr;
} myvar;
...
myvar **var;
..
var = new myvar*[x]; // where x is an int
for(int i = 0; i < y; i++){ // y is usually 2 or 4
   var[i] = new myvar[z]; //create dynamic 2d array
   for(int j = 0; j < z; j++){
      var[i][j].ptr = new int[b]; //where b is another int
   }
}

What happens is I want to create a 2d structure that has one part of it being 3d i.e. the ptr is just a pointer to an array of ints.

My code compiles but I get seg faults in trying to allocate memory so ptr can point to it. I tried working on this for about an hour and figure it was time for some help.

Thanks!

EDIT1: Fixed code issue in regards to comment on code not compiling.. Secondly... I cannot use vectors as much as I would like to... The data structure that I have there is what I have to use.

EDIT2: b is dynamic and set at the command line. For testing purposes use 16.

Use std::vector wherever possible because it maintains copies of items and manages dynamic memory allocation so you don't have to. — Thomas Matthews
– Thomas Matthews, Commented Jan 30, 2015 at 0:38
That code won't compile. var = new myvar[z] will not work on myvar **var. — danielschemmel
– danielschemmel, Commented Jan 30, 2015 at 0:38
just did and I still get a seg fault when trying to access it... so i.e. cout << myvar[i][j].ptr[k]. where i, j, k should be within the array bounds that were previously used to declare it. — Sharki
– Sharki, Commented Jan 30, 2015 at 0:47

Fabio says Reinstate Monica · Accepted Answer · 2015-01-30 01:12:42Z

1

I think you are getting confused with your indices. In your first loop you use var[i], so i must stop at x, not at y. For the columns I have used j and y. Not sure what z is. Then, as pointed out by others, you shouldn't mix int and uint32_t. Try this:

#include <iostream>
using namespace std;

typedef struct _myvar{
   uint32_t someInt;
   uint32_t *ptr;
} myvar;

int main() {

    myvar **var;

    int x = 3;
    int y = 4;

    var = new myvar*[x]; // where x is an int
    for(int i = 0; i < x; i++) { // i stops at x
        var[i] = new myvar[y]; //create dynamic 2d array
        for(int j = 0; j < y; j++){
            var[i][j].ptr = new uint32_t[16]; // used 16 for testing
        }
    }


    return 0;
}

answered Jan 30, 2015 at 1:12

Fabio says Reinstate Monica

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2 Comments

Sharki Over a year ago

Okay so I have exactly what you have here and I still cannot access any part of ptr. i.e. if I want to set var[0][0].ptr[0] = 0; (numerical 0 not pntr) I get a seg fault. This is were I am confused...

Fabio says Reinstate Monica Over a year ago

@Sharki Could you please post your precise code? I have tried this:

var[0][0].ptr[0] = 0; 	cout << "var[0][0].ptr[0] is " << var[0][0].ptr[0] << endl; 	var[2][3].ptr[15] = 15; 	cout << "var[2][3].ptr[15] is " << var[2][3].ptr[15] << endl;

and it works.

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