Smart contract using Solidity (part 1)

Introduction

Smart contract: Simply a program that runs on the Ethereum blockchain, has specific address on blockchain, not controlled by a user.

Ethereum virtual machine (EVM): It's the environment to Ethereum accounts and smart contracts live.

Solidity: A high-level programming language, use to develop smart contracts.

Remix: An online IDE allows develop, compile, deploy smart contracts for Ethereum.

Here is a simple example of Smart Contract:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.1 <0.9.0;

contract Message {
    string private message;
    
    constructor()  {
        message = "hello world1";
    }
    
    function setMessage(string memory mes) public {
        message = mes;
    }
    
    function getMessage() view public returns(string memory) {
        return message;
    }
}

Smart Contracts life cycle

As shown in the above figure, the smart contract life cycle follows the steps below :

A Smart contract is implemented using a high level language such as Solidity.
Smart contract is compiled to two results that specifically are: Bytecode (which is a stack-based machine language compiled and executable by the EVM) and an ABI - Application Binary Interface (which is a JSON file that describes the distributed contract and the functions of the smart contract). The ABI is the bridge between Web2 and Web3.
After that, the smart contract is deployed. This consists of creating a transaction packing the smart contract bytecode, and sending it to the network. The transaction gets its own contract account which has an address, a balance, a Nonce, and holds the bytecode in the data field.
Finally, a Smart contract is invoked. This consists of external calls to the smart contract.

Special note:

When a contract is created, the constructor function is executed once and that first call is always triggered by an EOA account (Externally Owned Account).
A smart contract is not self-executable. It requires an external call to be executed.
Once it is executed, transactions resulting from the execution are transcribed on the blockchain and eventually smart contracts meta data are updated.

Solidity: Variables and Functions

Variables

The following value types:

Type	Default value	Example
int and uint (all sizes)	0	int number; uint age; // 0
bool	false	bool flag; // false
string	Empty string	string name; // “”
bytes1 to bytes32	All bytes set to 0	bytes3 gender; // 0x000000
Static array	All items set to default value	int[3] numbers; // [0, 0, 0]
Dynamic array	Empty array	int[] numbers; // []
struct	Each element set to default value	struct Student { string name;uint age;bool gender;}
address (bytes20)		address myAddress; // 0x0000000000000000000000000000000000000000
enum		enum status {active, inactive, cancel};
mapping (as hash tables)		mapping(address => unit) balances;

The concept of “undefined” or “null” values does not exist in Solidity.

There are three types of variables:

State Variables: Variables whose values are permanently stored in a contract storage.
Local Variables: Variables whose values are present till function is executing.
Global Variables: Special variables exists in the global namespace used to get information about the blockchain.

Syntax:

<type> <access-modifier> <variable-name> ;

Example:

uint public age;

State Variable Scope:

Name	Describe
Public	Can be accessed internally contract. The compiler automatically creates getter functions for all public state variables.
Internal (default)	Can be accessed internally from the current contract. And contract inheriting from it without using this.
Private	Can be accessed only internally from the current contract.

Functions

Syntax:

function <function-name>(<parameter-list>) <scope> returns(<return-type-list>)  { 
    // block code
 }

Example:

function getSender() public returns (address) {
    return msg.sender;
}

Function Scope:

Name	Describe
Private	Only be called by the main contract itself.
Internal	Only be called by the main contract itself. And contract inheriting from it.
External	Only be called from a third-party.
Public (default)	Externally and internally.

Cryptocurrency example

The following contract implements the simplest form of a cryptocurrency

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.1 <0.9.0;

contract PBCoin {
    address private owner;
    mapping(address => uint) balances;

    event Sent(address from, address to, uint amount);
    
    constructor() {
        owner = msg.sender;
        balances[owner] = 1000000;
    }
    
    function send(address receiver, uint amount) public {
        require(amount <= balances[msg.sender]);
        balances[msg.sender] -= amount;
        balances[receiver] += amount;
        emit Sent(msg.sender, receiver, amount);
    }
    
    function getBalance(address receiver) view public returns(uint) {
        return balances[receiver];
    }
}