The latest update of Number Duck, the coolest programming library for accessing Excel spreadsheets in C++ and PHP, now allows you to read and write formulas. It's super simple, you just set it just like the string you would type into Excel.
pCell->SetFormula("=SUM(A1:A11)");
Check out the Formula Example and Api Reference for more juicy info.
You can download the latest release for Windows, Linux, C++ and PHP here.
Previous: How to use libPng in a C++ project