Insert Data into a Multi-Index Table

Overview

This guide provides instructions for inserting data into a multi-index table.

Prerequisites

• Before proceeding, ensure that you have completed the Getting Started section and that you have followed the Getting Started Documentation Diagram.
• This page assumes you are familiar with Smart Contract Basics.
• A multi-index testtab table instance which stores user objects indexed by the primary key which is of type sysio::name. Consult the section How to instantiate a multi-index table to learn how to set it up.

Steps

Complete the following steps to insert a user object in the testtab multi-index table:

1. Verify If the User Already Exists

Use the multi-index table iterator to find out if the user object already exists. The targeted user is searched based on its account name.

contract.cpp

[[sysio::action]] void multi_index_example::set(name user) {
    // check if the user already exists
    auto itr = testtab.find(user.value);
}

2. Insert the User If Not Found in Table

Use the emplace method to make the insertion if the user object is not already in the multi-index table. Otherwise print an informational message.

contract.cpp

[[sysio::action]] void multi_index_example::set(name user) {
    // check if the user already exists
    auto itr = testtab.find(user.value);
    if (itr == testtab.end()) {
        testtab.emplace(_self, [&](auto& u) {
            u.test_primary = user;
            u.secondary = "second"_n;
            u.datum = 0;
        });
    } else {
        printf("User already exists.");
    }
}

info

A full example project demonstrating the instantiation and usage of multi-index tables can be found in the multi_index example project.

Reference

Relevant classes and methods:

• The multi-index class.
• The multi-index::find(...) method.
• The multi-index::emplace(...) method.

Next Steps

• Iterate and retrieve newly inserted data from the multi-index table.