What is this macro for at the beginning of a class definition?

Question

I\'m looking through the source of a library and many classes are defined using the following form

class THING_API ClassName 
{
...

Jumping

Answer 1

One possible use would be to allow you to conditionally mark the class as callable from other libraries in a cross-platform library.

class ClassName

would be callable on Windows using MSVC but you'd need

class __attribute__ ((visibility("default"))) ClassName

for linux using gcc.

In this case, THING_API might be defined like

#ifdef _WIN32
# define THING_API
#else
# define THING_API __attribute__ ((visibility("default")))
#endif