How to define a primary index

A primary key is required when defining a multi index table structure. See the following example:

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 the definition of the primary index for the multi index table. The primary index type must be uint64_t and must be unique

  // 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; }
  };

Secondary 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 eosio::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;

Declare the multi index table as a data member of type test_tables, as 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;
+  test_tables testtab;

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

Full example location

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