这个额外的关键字在这个 C++ 类声明中是什么意思？

Question

I'm looking at some C++ code which declares a class like this:

class LIBSBML_EXTERN LayoutModelPlugin : public SBasePlugin

What is the LIBSBML_EXTERN keyword for?

Answer 1

It's a macro. I'm not sure exactly what libraries you're using, but likely it's the macro with the definition you can see here: https://github.com/copasi/copasi-dependencies/blob/master/src/libSBML/src/sbml/common/extern.h

#if ( ! defined LIBSBML_STATIC )
/**
 * The following ifdef block is the standard way of creating macros which
 * make exporting from a DLL simpler. All files within this DLL are
 * compiled with the LIBSBML_EXPORTS symbol defined on the command line.
 * This symbol should not be defined on any project that uses this
 * DLL. This way any other project whose source files include this file see
 * LIBSBML_EXTERN functions as being imported from a DLL, wheras this DLL
 * sees symbols defined with this macro as being exported.
 *
 * (From Andrew Finney's sbwdefs.h, with "SBW" replaced by "LIBSBML" :)
 */
#if defined(LIBSBML_EXPORTS)
#  define LIBSBML_EXTERN __declspec(dllexport)
#else
#  define LIBSBML_EXTERN __declspec(dllimport)
#endif

#else
#  define LIBSBML_EXTERN
#endif  /* LIBSBML_STATIC */

It is used to interface the class definition with DLLs, if required.

Answer 2

It is a macro, something that you would define like this:

#define LIBSBML_EXTERN whatever

where whatever is what the compiler puts in place of LIBSBML_EXTERN before compiling.