Ultra.io Docs

Create a Contract ​

Let's start with a simple smart contract that produces the traditional Hello World.

This tutorial introduces the following key concepts:

  • EOSIO Contract Development Toolkit: The toolchain and libraries used to build smart contracts
  • Webassembly (WASM): The virtual machine used to execute a portable binary-code format, hosted in nodeos
  • Application Binary Interfaces (ABI): The interface that defines how data is marshalled to and from the webassembly virtual machine
  • Smart Contracts: The code that defines actions and transactions which may be executed on a blockchain

This tutorial shows how to:

  • Create a simple smart contract with a hi action
  • Compile and deploy the smart contract to an EOSIO blockchain
  • Use the command line to call the hi action of the smart contract

Before you Begin ​

This tutorial requires the following:

  • Knowledge of the C++ programming language
  • A code editor or IDE
  • A fully configured local development environment

Once you complete the tutorial, you should have created a Hello World smart contract and deployed the smart contract on a blockchain.

Procedure to create hello.cpp ​

Follow this procedure to create the Hello World smart contract. Normally you create two files - the header or .hpp file which contains the declarations for the smart contract class and the .cpp file, which contains the implementation of the smart contract actions. In this simple example, you only use a .cpp file.

1. Create a new directory called hello to store your smart contract file ​

sh
mkdir hello

Go to the new directory

sh
cd hello

2. Create a new file, hello.cpp, and open it in your preferred text editor ​

sh
touch hello.cpp

3. Write the smart contract ​

Follow these four steps and add this code to the hello.cpp file.

Step 1. Import the eosio base library with the include directive. ​

Add the line:

cpp
#include <eosio/eosio.hpp>

Step 2. The eosio.hpp contains classes required to write a smart contract, including eosio::contract. Create a standard C++11 class and inherit from the eosio::contract class. Use the [[eosio::contract]] attribute to inform the EOSIO.CDT compiler this is a smart contract. ​

Add the line:

cpp
class [[eosio::contract]] hello : public eosio::contract {};

The EOSIO.CDT compiler automatically generates the main dispatcher and the ABI file. The dispatcher routes action calls to the correct smart contract action. The compiler will create one when using the eosio::contract attribute. Advanced programmers can customize this behavior by defining their own dispatcher.

Step 3. Add a public access specifier and a using-declaration to introduce base class members from eosio::contract. You can now use the default base class constructor. ​

Add these lines:

cpp
public:
	using eosio::contract::contract;

Step 4. Add a hi public action. This action accepts an eosio::name parameter, and prints Hello concatenated with the eosio::name parameter. ​

Add these line:

cpp
	[[eosio::action]] void hi( eosio::name user ) {
		print( "Hello, ", user);
	}

The [[eosio::action]] attribute lets the compiler know this is an action.

The hello.cpp file should now look like this:

cpp
#include <eosio/eosio.hpp>
class [[eosio::contract]] hello : public eosio::contract {
  public:
      using eosio::contract::contract;
      [[eosio::action]] void hi( eosio::name user ) {
         print( "Hello, ", user);
      }
};