The code works, but at the end throws "Debug Error", why?

Question

C++ / C#

Task: Write a function that adds another space next to a space (the case where two or more spaces in a row are not taken into account). The function must take a string (an array of characters) as arguments. The function must return the newly formed string.
In general, the program works, but at the end there is such a sign:
CODE:

#include "pch.h"
#include <iostream>
#include <Windows.h>
void add_spaces(char* a) 
{
  int count = 0;
  for (int i = 0; i < strlen(a); i++) 
  {
    if (a[i] == ' ')
      count++;
  }
  char* new_arr = new char[strlen(a) + count];
  for (int i = 0, j = 0; i < strlen(new_arr); i++, j++) 
  {
    if (a[j] == ' ')
    {
      new_arr[i] = ' ';
      new_arr[i + 1] = ' ';
      i++;
    }
    new_arr[i] = a[j];
  }
  for (int i = 0; i < strlen(new_arr); i++) 
  {
    std::cout << new_arr[i];
  }
  delete[]new_arr;
}
int main()
{
  using namespace std;
  SetConsoleCP(1251);
  SetConsoleOutputCP(1251);
  char a[] = " ";
  cout << "Введите строку: ";
  cin.getline(a, 128);
  cout << "\nСтрока после добавления пробелов: ";
  add_spaces(a);
}

Answer 1

After allocating memory, you have garbage in the new_arr array, and you call the strlen(new_arr) function to calculate the length of the string with garbage in memory. This function does not work correctly, it gives a value that does not correspond to reality. Then you fill in the memory you didn't allocate (in new_arr[i]). When you try to delete it using delete[] you get heap corrupt.
PS. using c++ use std::string