c++ syntax question

Hi /prog
When working with some other C++ libraries, I often see a class declaration like so:

class LIBRARYNAME_API ClassName
{};

Could someone please explain what the "LIBRARYNAME_API" part actually does?

Probably a macro that expands to __declspec(dllexport)/__attribute__ ((dllexport)) or __declspec(dllimport)/__attribute__ ((dllimport)) or nothing depending on build options (if building a dynamic, if linking to a dynamic lib, or building a static lib)