Q:

How do I fix this error? CRT detected that the application wrote to memory after end of heap buffer

I've been trying to make a dynamic array. My main method calls push_back() to add ints to an array 50 times. The push_back() method calls the grow_if_needed() method if the size of the array needs to be doubled. The size() method returns the current capacity of the array. And the toString method is for printing the array as a string. I keep getting this error at the end of the grow_if_needed() method. 
#include <string>
class ArrayList
{
private:
    static int capacity;
    static int count; 
public:
    int& operator [](int i);
    int *arrayPtr = new int[1];
    ArrayList();
    int * push_back(int m);
    int * grow_if_needed();
    int * erase(int m);
    int size();
    std::string toString();
}; 

#include "ArrayList.h"

using namespace std;

int ArrayList::capacity = 1;
int ArrayList::count = 0;

int & ArrayList::operator[](int i)
{
    return arrayPtr[i];
}

ArrayList::ArrayList()
{
    arrayPtr[0] = 0;
}

int * ArrayList::push_back(int m)
{
    if (count == size()) {
        grow_if_needed();
    }
    for (int i = size() - 1; i >= 0; i--) {
        arrayPtr[i + 1] = arrayPtr[i];
    }
    arrayPtr[0] = m;
    count++;

    return arrayPtr;
}

int * ArrayList::grow_if_needed() {
    int k;
    capacity = size() * 2;
    int * tempArray = new int[capacity];
    for (k = 0; k < count; k++) {
        tempArray[k] = arrayPtr[k];
    }
    while (k != capacity) {
        tempArray[k] = 0;
        k++;
    }
    delete [] arrayPtr;
    arrayPtr = tempArray;
        return arrayPtr;
}   

int ArrayList::size()
{
    return capacity;
}

string ArrayList::toString()
{
    string num =  "";
    for (int i = 0; i <= size() - 1; i++) {
        num += std::to_string(arrayPtr[i]) + " ";
    }
    return num;
}

A:

In your push_back function you have this loop:
for (int i = size() - 1; i >= 0; i--) {
    arrayPtr[i + 1] = arrayPtr[i];
}

Here size() - 1 is the top index, so in the first iteration when you do i + 1 you are out of bounds.
Also, this function is called push_back, but it attempts to add to the front? If it added to the end (the "back") you would not need the loop, and your problem would "magically" disappear.

