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

It looks to me very much like export macro, which is required when building a shared library (.dll) on Windows. When compiling with MSVC, You have to put __declspec(export) in that spot when building a library, and __declspec(import) when building its client. This is achieved like so:

#if COMPILING_DLL
    #define THING_API __declspec(dllexport)
#else
    #define THING_API __declspec(dllimport)
#endif

Then you define COMPILING_DLL for the library project, and leave it undefined for all other projects. And if you're not on Windows or compiling a static library, you need to define it blank like it's done in your question.

P. S. Other Windows compilers use their own keywords instead of __declspec(dllimport), but the principle remains.