How to declare a table outside of its contract-class?

Question

As we know from the documentation, a table must be initialized as shown below:

class [[eosio::contract]] helloworld: public eosio::contract
{
public:
   helloworld(eosio::name receiver, eosio::name code, eosio::datastream<const char*> ds)
   : contract(receiver, code, ds),
     my_table(receiver, receiver.value) { }

   ///////////////

   struct [[eosio::table]] testtable_j
   {
      name owner;
      std::string data;
      uint64_t primary_key() const { return owner.value; }
   };
   using testtable_t = eosio::multi_index<"testtable"_n, testtable_j>;
   testtable_t my_table;
}

But how can I declare my table outside of class 'helloworld'? For using its type without binding to it (in other .hpp/.cpp files for example). This case doesn't work - precisely speaking, abi not generated. But compilation is ok:

struct [[eosio::table]] testtable_j
{
      name owner;
      std::string data;
      uint64_t primary_key() const { return owner.value; }
};
using testtable_t = eosio::multi_index<"testtable"_n, testtable_j>;

///////////////

class [[eosio::contract]] helloworld: public eosio::contract
{
public:
   helloworld(eosio::name receiver, eosio::name code, eosio::datastream<const char*> ds)
   : contract(receiver, code, ds),
     my_table(receiver, receiver.value) { }
   
   testtable_t my_table;
}

Answer 1

to everyone who is interested - I used a "void*"

.hpp:

void my_method(const name& wax_user, const void* table_itr_ptr);

.cpp:

void my_method(const name& wax_user, const void* table_itr_ptr)
{
   helloworld::testtable_t::const_iterator itr_c = *(static_cast<const helloworld::testtable_t::const_iterator*>(itr_ptr));
   const helloworld::testtable_j& item = *itr_c;
   // ...
   
}

using:

helloworld::testtable_t::const_iterator itr_c = game.get_table_items().require_find(item_id, "error"));
my_method(wax_user, &(*itr_c));

Answer 2

As the answer here is completely irrelevant to the question. I am adding an answer that actually resolves the problem.

What's needed in this case is an extra parameter referring to the contract's name. As follows:

struct [[eosio::table, eosio::contract("helloworld")]] testtable_j {}