Programming code for external sorting in c++

I have a research paper about External Sorting and I have to implement it in C++ code.
The External-Merge-Sort algorithm has ready-made code, but it must be implemented in another way, and I must implement a more important algorithm, which is also an external sorting algorithm in C++.

So I need an expert in C++ and it will not be any problem no matter the amount of money.

Thank you very much.