Dynamically allocate an array from a textfile and retain its values - c++

Question

After looking at some other similar questions, this is what I've written so far:
#include <iostream>
#include <fstream>
#include <string>
void read_from_file() {
int* arr;
int length;
std::string file_name = "data.txt";
std::ifstream file_data;
file_data.open(file_name.c_str());
if (file_data.is_open())
{
file_data >> length;
arr = new int[length];
for (int i = 0; i < length; i++)
{
file_data >> arr[i];
std::cout << arr[i] << " ";
}
file_data.close();
}
else
std::cerr << "Cannot open the file " << file_name << std::endl;
}
All good here, until I try to manipulate the array's values. For example, I have to check if the array is sorted in ascending order. I don't know how to get back where I read the array and get the values.
bool check_if_sort_asc(int* arr, int length)
{
for (int i = 0; i < length - 1; i++)
if (arr[i] > vect1[i + 1])
return false;
return true;
}
int main()
{
int *arr;
int length;
read_from_file();
if (check_if_sort_asc(arr, length)==true)
std::cout << " The array is sorted in ascending order ";
else
std::cout << "The array is not sorted in ascending order ";
return 0;
}

Answer 1

The immediate fix is this
Change read_from_file to return the data it read
int * read_from_file() {
int *arr = NULL;
then return arr at the end of it
else
std::cerr << "Cannot open the file " << file_name << std::endl;
return arr;
}
and finally use the returned value in main
arr = read_from_file();
and free it when you have finished
delete [] arr;
But your code doesnt compile, because of that vect1 stuff, plus you should use std::vector.
As Bob_ points out you need the length too, so either
use std::vector
make the arr array one bigger and place the length as first entry
use std::vector
pass a length int by reference into read_form_file (see below)
use std::vector
ie
int * read_from_file(int * length) {
int *arr = NULL;
...
file_data >> *length;
But mainly, use std::vector