An example API for an example plugin system in C++.

pluginAPI.h

/**
 * Plugin API header
 */
#ifndef __PLUGIN_API_H__
#define __PLUGIN_API_H__

/*
 * Windows-specific settings for a dynamic library
 */
#ifdef _WIN32
    /* Standard dynamic library extension */
    #ifndef PLUGIN_EXT
        #define PLUGIN_EXT ".dll"
    #endif
    
    /* Calling convention for functions loaded at runtime. */
    #ifndef PLUGIN_CALL
        #define PLUGIN_CALL __stdcall
    #endif
    
    /* Determine if the plugin will be imported at runtime or exported to a library. */
    #ifdef BUILD_PLUGIN
        #define PLUGIN_API __declspec(dllexport)
    #else
        #define PLUGIN_API __declspec(dllimport)
    #endif
    
/*
 * Dynamic library settings for Linux and OSX
 */
#else
    /* Standard dynamic library extension */
    #ifndef PLUGIN_EXT
        #define PLUGIN_EXT ".so"
    #endif
    
    /* Calling convention for functions loaded at runtime. */
    #ifndef PLUGIN_CALL
        #define PLUGIN_CALL __cdecl
    #endif
    
    /* Determine if the plugin will be imported at runtime or exported to a library. */
    #ifndef PLUGIN_API
        #define PLUGIN_API
    #endif
#endif

class PLUGIN_API plugin {
    public:
        virtual ~plugin() = 0;
        
        virtual void printMessage() const = 0;
};

inline plugin::~plugin() {
    // An inline destructor will avoid requiring a .cpp file in each
    // user-created library.
}

extern "C" plugin* PLUGIN_API PLUGIN_CALL createPluginInstance();
typedef plugin* (*pluginFactory_t)();

extern "C" void PLUGIN_API PLUGIN_CALL destroyPluginInstance(plugin* const pPlugin);
typedef void (*pluginDeleter_t)(plugin* const);

#ifndef PLUGIN_FACTORY_NAME
    #define PLUGIN_FACTORY_NAME "createPluginInstance"
#endif

#ifndef PLUGIN_DELETER_NAME
    #define PLUGIN_DELETER_NAME "destroyPluginInstance"
#endif

#endif /* __PLUGIN_API_H__ */