Getting segmentation fault (core dumped) error when declaring a int variable

Question

I was trying to create vector like class. Here is my code

#include <bits/stdc++.h>
using namespace std;

template<class t>
class vec{
    t *start;
    int size=0;
    public:
      void add_value(t a){
            *(start+(sizeof(t)*size)) = a;
            cout << (start+(sizeof(t)*size))<< " = "<< *(start+(sizeof(t)*size))<<endl;
            size++;
            // cout << start<< endl;
        }

        void operator [](int i){
            cout << (start+(sizeof(t)*i))<< " = "<< *(start+(sizeof(t)*i))<<endl;
        }
        int length(){
            return size;
        }
};

int main(){
    vec<int> t;
    cout << t.length()<<endl;
    t.add_value(8);
    t.add_value(10);
    cout << t.length()<<endl;
    t[1];
}

This gives me correct output.

0
0x7fff0fe9b5d0 = 8
0x7fff0fe9b5e0 = 10
2
0x7fff0fe9b5e0 = 10

But when declare a int variable in main function like.

int main(){
    int i=0;  //newline
    vec<int> t;
    cout << t.length()<<endl;
    t.add_value(8);
    t.add_value(10);
    cout << t.length()<<endl;
    t[1];
}

output.

0
Segmentation fault (core dumped)

I also tried printing address of start variable and new int variable int those are different.

You invoked *undefined behavior* by dereferencing uninitialized pointer. — MikeCAT, Nov 04 '20 at 15:55
@Sushant Chandla Such an expression like this *(start+(sizeof(t)*size)) = a is incorrect. You need to use *( start + size ) = a — Vlad from Moscow, Nov 04 '20 at 15:57
You're lucky it doesn't seg-fault right away. Initialize `start` and you'll be good. — MartinVeronneau, Nov 04 '20 at 15:57
remove all occurences of `sizeof(t) *`. Pointer arithmetic already takes care of that for you. — Jabberwocky, Nov 04 '20 at 15:58
@VladfromMoscow But OP says "This gives me correct output.". The address is incremented by 16 (0x10) in the output. This is correct according to OP, so the formula `start+(sizeof(t)*size)` should be correct. — MikeCAT, Nov 04 '20 at 15:58
@PaulSanders I agree. It would be better if it returned cash.:) — Vlad from Moscow, Nov 04 '20 at 16:02
Really important note: Where the program crashes is rarely where the bug is. More often the bug wounds the program but it manages to limp on (sometimes for a very, very long time) before falling over and dying of the injury. — user4581301, Nov 04 '20 at 16:08
OT: also read this: https://stackoverflow.com/q/31816095/898348 — Jabberwocky, Nov 04 '20 at 16:16

Jabberwocky · Accepted Answer · 2020-11-04T16:16:44.050

You probably want something like this:

#include <iostream>

using namespace std;

template<class t>
class vec {
  t* start = new t[100];   // initialize the pointer (fixed size of 100, to be improved)
  int size = 0;                                      
public:
  void add_value(t a) {
    *(start + size) = a;
    size++;
  }

  t operator [](int i) {   // should return a t instead of a void
    return *(start + i);
  }

  int length() {
    return size;
  }
};

int main() {
  vec<int> t;
  cout << "t.length() = " << t.length() << endl;

  t.add_value(8);
  t.add_value(10);

  cout << "t.length() = " << t.length() << endl;
  
  // display all values in t
  for (int i = 0; i < t.length(); i++)
  {
    cout << "t[" << i << "] = " << t[i] << endl;
  }
}

All multiplication by sizeof(t) have been removed, because pointer arithmetic already does this for you.

This very poor and minimalist example works as you expect, but the maximum number of elements you can store in the class is 100. I leave the improvement as an exercise to you.

BTW there are many other improvements you need to do, especially you need a destructor and possibly the rule of three

Note:

You should replace all instances of *(start + x) with start[x] which does exactly the same thing but which is more idiomatic and more readable.

Hey, how is the size fixed to 100? I tried it and its working fine even if i replace 100 with 1 and try to add more values. — Sushant Chandla, Nov 04 '20 at 16:23
@SushantChandla you're writing beyond allocated memory which will result in _undefined behaviour_ (google that term) which includes "apparently working fine" — Jabberwocky, Nov 04 '20 at 16:25

Getting segmentation fault (core dumped) error when declaring a int variable

1 Answers1