factory

//////////
/// switch case method
//////////
enum object_class
{
    object_class_Light,
    object_class_Mesh,
    object_class_COUNT
};
shared_ptr<Object> makeInstance(object_class ClassEnum)
{
    switch (ClassEnum)
    {
        case object_class_Light: return make_shared(new Light());
        case object_class_Mesh: return make_shared(new Mesh());
    }
    return nullptr; // invalid case, might want to assert here
}


//////////
/// x-macro method
//////////
#define ExpandEnum(ClassName) object_class_##ClassName,
#define ExpandCase(ClassName) case object_class_##ClassName: return make_shared(new ClassName());
// to add new classes, just add them to this list by name
#define Classes(ExpandMacro) \
  ExpandMacro(Light) \
  ExpandMacro(Mesh)

enum object_class
{
  Classes(ExpandEnum)
  object_class_COUNT
};
shared_ptr<Object> makeInstance(object_class ClassEnum)
{
  switch (ClassEnum)
  {
    Classes(ExpandCase)
  }
  return nullptr; // invalid case, might want to assert here
}


//////////
/// function pointer table method
//////////
enum object_class
{
  object_class_Light,
  object_class_Mesh,
  object_class_COUNT
};
template<typename type>
shared_ptr<Object> createClassTemplate()
{
  return make_shared<type>();
}
typedef shared_ptr<Object> factory_function();
factory_function CreateClassTable[object_class_COUNT]
{
  createClassTemplate<Light>,
  createClassTemplate<Mesh>,
  // make sure the order matches the enum order - you could use the x-macro trick above to generate this in the correct order
};
shared_ptr<Object> makeInstance(object_class ClassEnum)
{
  return CreateClassTable[ClassEnum]();
}