#Smart Contract Basics#

Structure of a Contract

Contracts in Solidity are similar to classes in object-oriented languages. Each contract can contain declarations of State Variables, Functions, Function Modifiers, Events, Struct Types and Enum Types. Furthermore, contracts can inherit from other contracts.

There are also special kinds of contracts called libraries and interfaces.

Introduction to Smart Contracts

 A Simple Smart Contract

Storage Example

// SPDX-License-Identifier: GPL-3.0 pragma solidity >=0.4.16 <0.8.0;

contract SimpleStorage { uint storedData;

function set(uint x) public {
    storedData = x;
}

function get() public view returns (uint) {
    return storedData;
}

}

The first line tells you that the source code is licensed under the GPL version 3.0. Machine-readable license specifiers are important in a setting where publishing the source code is the default.

The next line specifies that the source code is written for Solidity version 0.4.16, or a newer version of the language up to, but not including version 0.8.0. This is to ensure that the contract is not compilable with a new (breaking) compiler version, where it could behave differently. Pragmas are common instructions for compilers about how to treat the source code (e.g. pragma once).

A contract in the sense of Solidity is a collection of code (its functions) and data (its state) that resides at a specific address on the Ethereum blockchain. The line uint storedData; declares a state variable called storedData of type uint (unsigned integer of 256 bits). You can think of it as a single slot in a database that you can query and alter by calling functions of the code that manages the database. In this example, the contract defines the functions set and get that can be used to modify or retrieve the value of the variable.

To access a state variable, you do not need the prefix this. as is common in other languages.

This contract does not do much yet apart from (due to the infrastructure built by Ethereum) allowing anyone to store a single number that is accessible by anyone in the world without a (feasible) way to prevent you from publishing this number. Anyone could call set again with a different value and overwrite your number, but the number is still stored in the history of the blockchain. Later, you will see how you can impose access restrictions so that only you can alter the number.