What is __declspec and when do I need to use it?

Question

I have seen instances of __declspec in the code that I am reading. What is it? And when would I need to use this construct?

Answer 1

I know it's been eight years but I wanted to share this piece of code found in MRuby that shows how __declspec() can bee used at the same level as the export keyword.

/** Declare a public MRuby API function. */
#if defined(MRB_BUILD_AS_DLL)
#if defined(MRB_CORE) || defined(MRB_LIB)
# define MRB_API __declspec(dllexport)
#else
# define MRB_API __declspec(dllimport)
#endif
#else
# define MRB_API extern
#endif