E> Совсем кроспслатформенно никак. Но если ты расскажешь подробности, то многое может оказаться возможным...

class Exporting
{
public:
  virtual int func() = 0;
  virtual float other() = 0;
};

class My : public Exporting
{
public:
  int funct() { return m_str.size(); }
  float other() { return 4.4; } 
private:
  std::string m_str;
};

extern "C"
{
  Exporting * getClass();
}
Exporting * getClass()
{
  return new My();
}

S>class Exporting
S>{
S>public:
S>  virtual int func() = 0;
S>  virtual float other() = 0;
S>};

class Exporting
{
public:
  virtual INT __stdcall func() = 0;
  virtual DOUBLE __stdcall other() = 0;
};

S>class My : public Exporting
S>{
S>public:
S>  int funct() { return m_str.size(); }
S>  float other() { return 4.4; } 
S>private:
S>  std::string m_str;
S>};

S>extern "C"
S>{
S>  Exporting * getClass();
S>}
S>Exporting * getClass()
S>{
S>  return new My();
S>}

S>class Exporting
S>{
S>public:
S>  virtual int func() = 0;
S>  virtual float other() = 0;
S>};
S>

#ifdef WIN32
typedef HINSTANCE engine_handle;
#define ENGINE_HANDLE_FREE(handle) FreeLibrary(handle)
#else
#include <dlfcn.h>
typedef void* engine_handle;
#define ENGINE_HANDLE_FREE(handle) dlclose(handle)
#endif

struct free_library
{
    engine_handle handle;
    free_library(engine_handle handle) : handle(handle){
    }
    void operator()(){
        ENGINE_HANDLE_FREE(handle);
    }
};

Exporting* load_engine(std::string engine, boost::function<void()>& free_lib){
#ifdef  WIN32
    engine += ".dll";
    engine_handle handle =LoadLibrary(engine.c_str());
#else
    engine = "./lib" + engine + ".so";
    engine_handle handle = dlopen(engine.c_str(), RTLD_LAZY);
#endif
    if (!handle)
        throw std::runtime_error((boost::format("load library \"%s\" error") % engine).str());

    typedef Exporting* (getClass)();
    getClass *api = 
#ifdef WIN32
    (getClass*)GetProcAddress(handle, "getClass");
#else
    (getClass*)dlsym(handle, "getClass");
#endif
    if(!api){
        ENGINE_HANDLE_FREE(handle);
        throw std::runtime_error((boost::format("not found getClass() method in library %s") % engine).str());
    }
    free_lib = free_library(handle);
    return api();
}

SomeObject_DoSomething(HANDLE obj, INT param1, FLOAT param2);