Need a program for below in C++

Let's have a file containing readings from the measuring apparatus in the range 0 to 100 with an accuracy of 1 - the file contains only space separated integers. The file can be huge (several dozen GB), so it isn't possible to load it in extenso into the computer's memory. Write a program that will sort the numbers from the file in ascending order and save it to a separate file. Also, write unit tests for sample datasets