#include <qrinvoice/fonts/font_manager.hpp>
// ...
qrinvoice::font_manager::set_font_path("c:/path/to/fonts");
See Font Paths first
Instead of setting font path via environment variable, you can initialize the font path using an API call - however make sure to do this only once prior to any other call.
#include <qrinvoice/fonts/font_manager.hpp>
// ...
qrinvoice::font_manager::set_font_path("c:/path/to/fonts");