How to instantiate a multi index table

Include the eosio.hpp header and declare the eosio namespace usage

#include <eosio/eosio.hpp>
using namespace eosio;

Define the data structure for the multi index table

  struct [[eosio::table]] test_table {
  };

Add to the data structure the fields which define the multi index table

  // the data structure which defines each row of the table
  struct [[eosio::table]] test_table {
+    // this field stores a name for each row of the multi index table
+    name test_primary;
+    // additional data stored in table row, e.g. an uint64_t type data
+    uint64_t datum;
  };

Add definition of the primary index for the multi index table. The primary index type must be uint64_t, it must be unique and it must be named primary_key(), if you don't have this the compiler (eosio-cpp) will generate an error saying it can't find the field to use as the primary key:

  // the data structure which defines each row of the table
  struct [[eosio::table]] test_table {
    // this field stores a name for each row of the multi index table
    name test_primary;
    // additional data stored in table row
    uint64_t datum;
+    // mandatory definition for primary key getter
+    uint64_t primary_key( ) const { return test_primary.value; }
  };

Additional indexes information

Other, secondary, indexes if they will be defined can have duplicates. You can have up to 16 additional indexes and the field types can be uint64_t, uint128_t, uint256_t, double or long double.

For ease of use, define a type alias test_tables based on the multi_index template type, parametarized with a random name "testtaba" and the test_table data structure defined above

  // the data structure which defines each row of the table
  struct [[eosio::table]] test_table {
    // this field stores a name for each row of the multi index table
    name test_primary;
    // additional data stored in table row
    uint64_t datum;
    // mandatory definition for primary key getter
    uint64_t primary_key( ) const { return test_primary.value; }
  };
  
+  typedef eosio::multi_index<"testtaba"_n, test_table> test_tables;

Define the multi index table data member of type test_tables defined in the privious step

  // the data structure which defines each row of the table
  struct [[eosio::table]] test_table {
    // this field stores a name for each row of the multi index table
    name test_primary;
    // additional data stored in table row
    uint64_t datum;
    // mandatory definition for primary key getter
    uint64_t primary_key( ) const { return test_primary.value; }
  };
  
  typedef eosio::multi_index<"testtaba"_n, test_table> test_tables;
+  test_tables testtab;

Instantiate the data member testtab by passing to its constructor the scope (in this case receiver) and the code parameters, these two combined with table name "testtaba" provide access to the partition of the RAM cache used by this multi index table, in this example you will initialize the testtab data member in the smart contract constructor

// contract class constructor
multi_index_example( name receiver, name code, datastream<const char*> ds ) :
   // contract base class contructor
   contract(receiver, code, ds),
   // instantiate multi index instance as data member (find it defined below)
+   testtab(receiver, receiver.value)
   { }

Now you have instantiated the testtab variable as a multi index table which has a primary index defined for its test_primary data member.

Here is how the definition of a multi_index_example contract containing a multi index table could look like after following all the steps above.

multi_index_example.hpp

#include <eosio/eosio.hpp>
using namespace eosio;

// multi index example contract class
class [[eosio::contract]] multi_index_example : public contract {
   public:
      using contract::contract;

      // contract class constructor
      multi_index_example( name receiver, name code, datastream<const char*> ds ) :
         // contract base class contructor
         contract(receiver, code, ds),
         // instantiate multi index instance as data member (find it defined below)
         testtab(receiver, receiver.value)
         { }

      // the row structure of the multi index table, that is, each row of the table
      // will contain an instance of this type of structure
      struct [[eosio::table]] test_table {
        // this field stores a name for each row of the multi index table
        name test_primary;
        // additional data stored in table row
        uint64_t datum;
        // mandatory definition for primary key getter
        uint64_t primary_key( ) const { return test_primary.value; }
      };

      // the multi index type definition, for ease of use define a type alias `test_tables`, 
      // based on the multi_index template type, parametarized with a random name and 
      // the test_table data structure
      typedef eosio::multi_index<"testtaba"_n, test_table> test_tables;

      // the multi index table instance declared as a data member of type test_tables
      test_tables testtab;

      [[eosio::action]] void set( name user );
      [[eosio::action]] void print( name user );

      using set_action = action_wrapper<"set"_n, &multi_index_example::set>;
      using print_action = action_wrapper<"print"_n, &multi_index_example::print>;
};

Full example location

A full example project demonstrating the instantiation and usage of multi index table can be found here.