Building a Blockchain from Scratch Using Solidity and Ethereum/

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Create Your First Contract

Create Your First Contract

Learn how to create a simple smart contract.

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We’re going to use Solidity to write a simple HelloWorld smart contract. This is our starting point to create more complex logic and functions.

The HelloWorld contract

This contract aims to store a string variable called message on the blockchain. The initial value is set to a "Hello, World!" string, but the contract also provides a public function called setMessage to allow changing the value of the message variable. The basic structure of the contract is:

// SPDX-License-Identifier: MIT



pragma solidity ^0.8.0;



contract HelloWorld {

  

}

  • Line 1: Indicates the license under which the contract is released.

  • Line 3: Specifies the version of the Solidity compiler to be used. The ^ symbol means it is compatible with version 0.8.0 and higher.

  • Line 5: Declares a new smart contract named HelloWorld.

Next, the contract needs a state variable named message. The variable must be accessible to external calls, so it is declared with the public keyword.

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;



contract HelloWorld {    

  string public message;

}

Now, the contract needs the constructor (executed only once when the contract is deployed) and the setMessage() function.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
contract HelloWorld {    
  string public message;    
  
  constructor() {
    message = "Hello, World!";    
  }    
  
  function setMessage(string memory newMessage) public {  
    message = newMessage;
  }
}

The smart contract is now complete.

  • Line 8: The constructor is defined. This is the place where the state variable message is initialized with the "Hello world!" string. ...