Convert Lat/Long to UTM specifying which zone to use (C++)

I need to convert Latitude/Longitude coordinates into UTM, in C++, specifying which zone to use.

I am working with locations that fall in two zones, and would like to project them all into one zone - since they are not too far from the border, I am hoping the error introduced by doing so won't be too much.

So far I had been using Chuck Gantz's library, which converts LL to UTM, but it does not support conversion from LL to UTM specifying which zone to use.

Answer

You can use MSP GEOTRANS (The UTM class receives a UTM zone override parameter)

or GeographicLib (Use the SetAltZone method).