realloc(): invalid old size even when malloc() is used to allocate memory

Question

I am trying to implement a Dynamic Stack in c++. i have 3 members in class stack 1.cap is the capacity. 2.top- points to top of stack 3. arr- pointer to an integer.

in the class constrcutor I am allocating memory to stack(malloc). later in the meminc() I am trying to realloc the memory.

I have written a function meminc() to realloc the memory but i get this invalid old size error.

It would be helpful if you let me know what is wrong in this code. I will also appreciate any advice given to me. Thank you.

#include <iostream>

using namespace std;

#define MAXSIZE 5

class stack {
    int cap;
    int top;
    int *arr;

public:
    stack();
    bool push(int x);
    bool full();
    bool pop();
    bool empty();
    bool meminc();
};

stack::stack()
{
    cap = MAXSIZE;
    arr = (int *)malloc(sizeof(int)*MAXSIZE);
    top = -1;
}

bool stack::meminc()
{
    cap = 2 * cap;
    cout << cap << endl;
    this->arr = (int *)realloc(arr, sizeof(int)*cap);
    return(arr ? true : false);
}

bool stack::push(int x)
{
    if (full())
    {
        bool x = meminc();
        if (x)
            cout << "Memory increased" << endl;
        else
            return false;
    }

    arr[top++] = x;
    return true;
}

bool stack::full()
{
    return(top == MAXSIZE - 1 ? true : false);
}

bool stack::pop()
{
    if (empty())
        return false;
    else
    {
        top--;
        return true;
    }
}

bool stack::empty()
{
    return(top == -1 ? true : false);
}

int main()
{
    stack s;
    char y = 'y';
    int choice, x;
    bool check;

    while (y == 'y' || y == 'Y')
    {
        cout << "                 1.push\n                    2.pop\n" << endl;
        cin >> choice;

        switch (choice)
        {
        case 1: cout << "Enter data?" << endl;
            cin >> x;
            check = s.push(x);
            cout << (check ? "              push complete\n" : "              push failed\n");
            break;

        case 2: check = s.pop();
            cout << (check ? "              pop complete\n" : "               pop failed\n");
            break;

        default: cout << "ERROR";
        }
    }
}

Answer 1

To add to john's answer,

the way you're using realloc() is ... flawed.

bool stack::meminc()
{
    cap = 2 * cap;
    cout << cap << endl;
    this->arr = (int *)realloc(arr, sizeof(int)*cap);
    return(arr ? true : false);
}

If realloc() fails it will return nullptr and the only pointer (arr) to the original memory region will be gone. Also, instead of return(arr ? true : false); you should simply use return arr != nullptr;.

The right^tm way to use realloc():

bool stack::meminc()
{
    int *temp = (int*) realloc(arr, sizeof(*temp) * cap * 2);
    if(!temp)
        return false;
    cap *= 2;
    arr = temp;
    return true;
}

Also, where is your copy-ctor, assignment operator and d-tor?