Segmentation fault when trying to access an array inside pointer

Question

I got segmentation fault when I start running my project.

I have declared 2 different classes

Class myfirstclass {
int x[4];
};

In the second class

I access the array “x[4]” using the following

myfirstclass  * firstptr;
firstptr -> x[4];

Now when I assigned the “firstptr -> x[4];” to an array to do some computations I got a segmentation fault?

int y[4];
for (int i=0; i<4;i++){
  y[i]= firstptr -> x[i]; -> "This statement what caused the segmentation fault."
}

Can you help me fixing this error, please?

remove the * and indexes start from 0. i.e. myfirstclass firstptr; firstptr.x[3] is the last element. — Adnan Akbar
– Adnan Akbar, Commented Aug 20, 2013 at 8:22

doctorlove · Accepted Answer · 2013-08-20 08:26:32Z

1

If you just do this

myfirstclass  * firstptr;
firstptr -> x[4];

You haven't initialised firstptr. You would need to do something like

myfirstclass  * firstptr = new myfirstclass();

Don't forget to delete firstptr somewhere.
Or just use the stack

myfirstclass  first;

Next, you are using

firstptr -> x[4];

Since you have int x[4]; you have 4 items, so can access x[0], x[1], x[2] and x[3]. There is no x[4]

Note - if you use the stack instead just use . instead of ->

first.x[i];

answered Aug 20, 2013 at 8:26

doctorlove

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2 Comments

redRose Over a year ago

I have missed the point about the allocation and I do it now, but I still got the same segmentation fault on same statement, I mean when I assign this firstptr -> x[i]; to another array in order to do some computation, I got segmentation fault. y[i]= firstptr -> x[i];

doctorlove Over a year ago

@redRose why not just use stack variables instead?

Ivan · Accepted Answer · 2016-07-05 16:32:30Z

1

You have to create object before usage. Something like that:

myfirstclass  * firstptr = new myfirstclass();

Or you should discard using dynamically allocated object

myfirstclass  firstptr;
int y[4];
for (int i=0; i<4;i++){
  y[i]= firstptr.x[i]; -> "This statement what caused the segmentation fault."
}

In order to access x you should make it public:

class myfirstclass {
public:
int x[4];
};

Actually, making data field bublic is not recommended.

edited Jul 5, 2016 at 16:32

answered Aug 20, 2013 at 8:26

Ivan

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2 Comments

redRose Over a year ago

@Ivan I have missed the point about the allocation and I do it now, but I still got the same segmentation fault on same statement, I mean when I assign this firstptr -> x[i]; to another array in order to do some computation, I got segmentation fault. y[i]= firstptr -> x[i];

Ivan Over a year ago

@redRose could you show your code (how does it look with fixes)?

Huygens · Accepted Answer · 2013-08-20 08:24:38Z

0

You need to allocate your class.

Before the for loop do:

firstptr = new myfirstclass;

answered Aug 20, 2013 at 8:24

Huygens

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