How to define a primary index

Overview

This guide provides instructions to define a primary index for a multi-index table.

Reference

See the following code reference:

• The multi-index class.

Before you begin

Make sure you have the following prerequisites in place:

• An EOSIO development environment, for details consult the Get Started Guide,

Procedure

Complete the following steps to define a primary index for the multi-index table testtab.

1. Preparation And Initialization

Include the eosio.hpp header and use the using directive to access the eosio namespace.

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

2. Define The Table Data Structure

Define the data structure for the multi-index table.

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

Add the data structure data members. Each data member corresponds to a field of the multi-index table. A primary key is required when defining a multi-index table structure, therefore you need to know which is the multi-index table field that is the primary key for your multi-index table. The corresponding data member for the primary key field must store unique values. In this case it is the test_primary data member of type eosio::name.

  // the data structure which defines each row of the table
  struct [[eosio::table]] test_table {
+    // this data member 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;
  };

3. Define The Primary Index

Add the definition of the primary index for the multi-index table. The primary index type must be uint64_t, it must be unique and must be named primary_key(), otherwise the compiler (eosio-cpp) will generate an error saying it can not 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 data member 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; }
  };

• uint64_t
• uint128_t
• uint256_t
• double
• long double

4. Define A Multi-Index Type Alias

For ease of use, define a type alias test_table_t based on the eosio::multi_index template type, parametarized with a random name "testtaba" and the test_table data structure. The names must adhere to EOSIO account name restrictions.

  // the data structure which defines each row of the table
  struct [[eosio::table]] test_table {
    // this data member 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_table_t;

5. Instantiate The Multi-Index Table

Declare the testtab multi-index table as a data member of type test_table_t.

  // the data structure which defines each row of the table
  struct [[eosio::table]] test_table {
    // this data member 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_table_t;
+  test_table_t testtab;

Now you have instantiated a multi-index table, and assigned to testtab variable, which has a primary index defined for its test_primary data member.

Summary

In conclusion, the above instructions show how to define a primary index for a multi-index table.

Next Steps

• You can insert data in the multi-index table.
• You can iterate and retrieve data from the multi-index table.