I get the double free or corruption error , for my class functions

Question

this is my .cpp file content:

#include <iostream>
#include"1.h"
using namespace std;

Stack:: Stack(){
size=20;    
 a=new int[size];   
top=-1;
}

Stack::Stack (int si){

size=si;
a=new int[si];
top =-1;}

Stack::Stack(Stack& s){

a=new int[s.size];
for(int i=0 ; i<s.size; i++)
    a[i]=s.a[i];
size=s.size;
}

Stack::~Stack(){

delete [] a;
}

void Stack::Push(int data){

if(this->isfull())
    cout<<"stack is full!\n";
else
    a[top++]=data;
}

int Stack::Pop(){

if(this->isempty())
    cout<<"stack is empty!\n";
else
    return a[top--];
}

bool Stack::isempty(){

if(top==-1) 
     return true;
 else
    return false ;
}   

bool Stack::isfull(){

if(top==size-1 ) 
    return true ;
else
    return false ;
}

void Stack::Print(){

for(int i=top ; i>-1 ; i--)
    cout<<a[i]<<endl;
}

int main(){

Stack a(3);
a.Push(1);
a.Push(3);
cout<<a.Pop();
a.Push(5);
a.Push(7);
a.Print();  
return 0;
}

And after running the program, i get the following error: Error in `./1': double free or corruption (out): 0x000000000240a010 *** Aborted (core dumped) and I have copy constructor and any thing, what sould I do?

`a[top++]` when top is -1 references the `-1` index of `a` which is UB — Mike Vine, May 10 '17 at 19:53
Possible duplicate of [How to track down a "double free or corruption" error](https://stackoverflow.com/questions/2902064/how-to-track-down-a-double-free-or-corruption-error) — Raedwald, Dec 06 '18 at 13:46

score 2 · Answer 1 · edited May 23 '17 at 12:26

2

The default constructor sets top to -1, so a[top++]=data; in Stack::Push(int) is undefined behavior (attempt to write to a[-1], which is out of the array bounds). This is the corruption the error message refers to, which is only noticed by your standard library when delete [] a is invoked. Change this to a[++top] = data;.
Your copy constructor does not assign to top, leaving its value uninitialized in copies. Reading top from a copy is therefore undefined behavior.
Your copy constructor should be Stack(Stack const &).
You should implement a copy-assignment operator Stack & operator=(Stack const &);.

edited May 23 '17 at 12:26

Community

1
1

answered May 10 '17 at 19:53

cdhowie

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is it really possible to write `Stack(Stack const &)` instead of `Stack(const Stack&)` or is it a typo? – 463035818_is_not_an_ai May 10 '17 at 20:02
not a typo, but for some reason I have never seen it before – 463035818_is_not_an_ai May 10 '17 at 20:04
1

@tobi303 It is possible, and I prefer it because it makes complex pointer declarations more obvious -- the `const` always refers to the thing to the left, so most declarations read naturally right-to-left. `A ** const * const` is a constant pointer to a constant pointer to a pointer to `A`. Likewise, `A const &` is a reference to a constant `A`. `A const &` and `const A &` name the same type. `const A &` is permitted, but I find that having `const` sometimes refer to the thing to the right and sometimes to the left to be a bit silly. I therefore always put it after the thing that's const. – cdhowie May 10 '17 at 20:04
makes sense. I mean it makes no sense to differentiate between reference to const or const reference, but I agree that it helps to be consistent – 463035818_is_not_an_ai May 10 '17 at 20:06
Right, and I find that consistency helps avoid mistakes. Once you get used to writing `A const &` it actually feels quite natural. – cdhowie May 10 '17 at 20:07

score 1 · Answer 2 · answered May 10 '17 at 19:53

1

a[top++] should be a[++top] as you initialize top at -1, else you have out of bound access, so UB.

answered May 10 '17 at 19:53

Jarod42

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I get the double free or corruption error , for my class functions

2 Answers2