Let's begin with the most basic smart contract example, HelloWorld.sol, shown as follows:
pragma solidity ^0.4.24;
contract HelloWorld {
string public greeting;
constructor() public {
greeting = 'Hello World';
}
function setNewGreeting (string _newGreeting) public {
greeting = _newGreeting;
}
}Solidity's file extension is .sol. It is similar to .js for JavaScript files, and .html for HTML templates.
A solidity source file is typically composed of the following constructs: pragma, comments, and import.
The first line containing the keyword pragma simply says that the source code file will not compile with a compiler earlier than version 0.4.24. Anything newer does not break functionality. The ^ symbol implies another condition—the source file will not work either on compilers beyond version 0.5.0.