Where should I put my delete to make peace with valgrind? - c++

Question

I am trying to write a simple stack with dynamic memory allocation. Must be with new/delete. Sorry, if it's trivial but I would appreciate if someone could take a look. Any advice is appreciated.
It works, but I want to make peace with valgrind --leak-check=full
stack.cpp
#include <assert.h>
#include <stdlib.h>
#include <stdio.h>
#include "stack.h"
#include <algorithm>
#define INITIAL_STACKSIZE 1
stack::stack()
{
top = 0;
size = INITIAL_STACKSIZE;
data = new int[size];
}
stack::~stack()
{
// delete [] ptr; makes it worse "free(): double free detected in tcache 2
// Aborted (core dumped)"
delete [] data;
}
void stack::push(int a)
{
if(top >= size)
{
int new_size = size * 2;
ptr = new int[new_size];
std:: copy(data, data + top, ptr);
data = ptr;
size = new_size;
}
data[top++]=a;
}
int stack::pop()
{
assert(top>0);
return data[--top];
}
stack.h
class stack
{
public:
void push(int a);
int pop();
void clear();
stack();
~stack();
private:
int top;
int size;
int *data;
int *ptr;
};
teststack.cpp
#include <stdio.h>
#include "stack.h"
int main()
{
stack s1;
stack s2;
s1.push(1);
s1.push(2);
s2.push(5);
s2.push(6);
printf("%d %d\n",s1.pop(),s2.pop());
printf("%d\n",s1.pop());
printf("%d\n",s2.pop());
return 0;
}

Answer 1

You need to delete[] data in your stack::push() before you assign ptr to data.
When you assign ptr to data as you do now before freeing the memory stored in data, you lose the pointer to the memory you allocated with new earlier (i.e., you leak memory).
Once you've copied everything out of data with std::copy(), it is safe to free the memory pointed to by data, since the memory has been copied.
So here is what your stack::push() should look like:
void stack::push(int a)
{
if(top >= size)
{
int new_size = size * 2;
ptr = new int[new_size]; // Or if ptr was a local variable,
// this line would be
// int ptr[] = new int[new_size];
std:: copy(data, data + top, ptr);
delete[] data;
data = ptr;
size = new_size;
}
data[top++]=a;
}
I also suggest you remove ptr from being a member variable of the stack class, and instead make it a local variable in the stack::push() method, since it isn't used anywhere else.