Segmentation fault while calling previously assigned values

[arashf]

I wrote this code to add and keep new data into a class.

class sample
{
public:
	int N;
    double** p;
	void concat(sample&,double a[2]);
	sample();
	~sample();
};

#include "test.h"
#include <fstream>

void sample::concat(sample& c, double* a){
	c.p = new double* [c.N+1];
	c.N += 1;
	c.p[c.N-1] = new double [2];
	c.p[c.N-1][0] = a[0];
	c.p[c.N-1][1] = a[1];
}
sample::sample(){
	N=0;
	p = NULL;
}

sample::~sample(){
	delete p;
}

int main(){
sample a;
double s1[] = {1.2,-2};
a.concat(a,s1);
double s2[] = {3,-6.1};
a.concat(a,s2);
printf("%i",a.N);
printf("\n%1.2f",a.p[0][0]);
}

While I call for printf("\n%1.2f",a.p[1][0]); or printf("\n%1.2f",a.p[1][1]); it perfectly gives me values 3 and -6.1. But, when I call for printf("\n%1.2f",a.p[0][0]); or printf("\n%1.2f",a.p[0][1]); it gives segmentation fault.
What goes wrong?

[LB]

Where do you set N?

[arashf]

In the constructor

sample::sample(){
	N=0;
	p = NULL;
}

[LB]

0+1 is 1, which means that the only valid index is 0, because 1 is past the end of the array.

[arashf]

L B,
I don't agree.
N increases each time I call a.concat. The first time I add {1.2,-2} to a and the second time {3,-6.1}. When I call a.p[1][1], there is no segmentation fault. But when I call a.p[0][1], there is.
It looks like it deletes the value of the array when resizing.

[arashf]

Forget about the above codes. The simple code below explains m problem better:

int main (){
	int** a = new int* [1];
	a[0] = new int [2];
	a[0][0]= 1;
	a[0][1] = 11;
	a = new int* [2];
	a[1] = new int [2];
	a[1][0] = 2;
	a[1][1]=22;
	printf("%i,%i",a[0][0],a[0][1]);
}

This gives segmentation fault, while if I call for printing printf("%i,%i",a[1][0],a[1][1]);, every thing goes all right.

[Chervil]

At line 6, there is a memory leak, as the memory previously allocated at lines 2 and 3 is not deleted.

a is assigned a fresh value at line 6, so the previously allocated memory is no longer accessible.

At line 10, a[0] refers to a pointer which has not been initialised.

[arashf]

Dear Chervil
As far as I understand, you are saying that even if I reallocate like

int main (){
	int** a = new int* [1];
	a[0] = new int [2];
	a[0][0]= 1;
	a[0][1] = 11;
	a = new int* [2];
	a[1] = new int [2];
	a[0][0]= 1;
	a[0][1] = 11;
	a[1][0] = 2;
	a[1][1]=22;
	printf("%i,%i",a[0][0],a[0][1]);
}

I still get the same error. I also have to put delete a; between lines 5 and 6.
Did I get it right?

[Chervil]

I'm not saying that putting the delete in the code will fix the segmentation fault. But it will fix the memory leak.
One delete for each new
One delete [] for each new []

If we ignore the memory leak for now, let's simplify the code to focus on the cause of the problem.

Look at this simpler version:

int main ()
{
	int** a;
	a = new int* [2];
	a[1] = new int [2];
	a[1][0] = 2;
	a[1][1] = 22;
	printf("%i,%i",a[0][0],a[0][1]);
}

Now do you see that a[0] is not initialised?
This is effectively the same as the original code, with the same error, a[0] doesn't point to anything.

[arashf]

Chervil,
I got your point about memory leak. On the other hand, the following code which takes care of memory leak without line 5 delete a; still gives segmentation fault.

int main (){
	int** a = new int* [1];
	a[0] = new int [2];
	a[0][0]= 1;
	a[0][1] = 11;
        delete a;
	a = new int* [2];
	a[1] = new int [2];
	a[0][0]= 1;
	a[0][1] = 11;
	a[1][0] = 2;
	a[1][1]=22;
	printf("%i,%i",a[0][0],a[0][1]);
}

What I learned from you is
1- memory leak
2- each new needs a delete
Thanks

[Chervil]

Fixed version:

int main ()
{
    int ** a;
    
    a = new int* [1];
    a[0] = new int [2];
    a[0][0] = 1;
    a[0][1] = 11;
    delete [] a[0];
    delete [] a;

    a = new int* [2];
    a[0] = new int [2];
    a[1] = new int [2];
    a[0][0] = 3;
    a[0][1] = 33;
    a[1][0] = 2;
    a[1][1] = 22;
    printf("%i,%i\n",a[0][0],a[0][1]);
    printf("%i,%i\n",a[1][0],a[1][1]);

    delete [] a[0];
    delete [] a[1];
    delete [] a;
}