This library represents XLSX files writer. The main feature of this library is that it uses C++ standard file streams. On the one hand it results in almost unnoticeable memory and CPU resources consumption while processing (that may be very useful at saving a large data arrays), but on the other hand it makes unfeasible to edit data that were written down once.
Hence, if using this library the structure of the future report should be known enough.
The library is written in C++ with using STL functionality and based on the simple XML writer and ZIP library. Both of them have free licenses.