The xll12 Library
The xll12 add-in library makes it easy to call C/C++ or even Fortran code from from Microsoft Excel. Just write a function and register it with Excel by creating an xll::AddIn object.
The library also let's you create documentation that is integrated with Excel.
The Microsoft Excel C SDK makes it possible for you extend Excel with functions and macros written in native code. The xll12 library provides C++ classes that make Microsoft's SDK much easier to use.
The xll12 library works with all versions of Windows since XP and all versions of Excel since 2007. Supported features include the big grid, wide character strings, plus thread safe and asynchronous functions.
Use this library to create an add-in and help file integrated into Excel documenting how to use your handiwork.