IDL4CPP — External mapping

The mapping of external in this section is not similar to 7.17.4. In 7.17.4 it is a std::shared_ptr, so a pointer to something which is shared, when copying the struct where the external is a member of the external data will not be copied, just a refcount increment, but in D2.3.4 it is descriibed that when the struct which contains the external member is copied a deep copy is done. Also what is exactly "copy functions", is that the copy constructor or something else? This looks very unspecified, the user doesn't know the exact API and the semantics should be the same as in 7.17.4

Add extra restriction to the mapping of @external

For consistency purposes, this resolution adds an extra restriction to the mapping of @external.