State Variables and Data Types

A state variable is permanently stored on the blockchain as part of the contract's data, every write to one costs real money (gas), which shapes how Solidity code is written more than almost anything else.

contract Wallet {
    uint256 public balance;          // unsigned integer, 256 bits
    address public owner;             // an Ethereum address
    bool public isLocked;
    string public label = "Savings";

    constructor() {
        owner = msg.sender;   // the account that deployed the contract
        balance = 0;
    }
}

Core types

Solidity has no floating-point type at all, financial logic is done in whole-number units (like storing cents instead of dollars) to avoid the rounding errors that plague floating-point money math in other languages.

msg.sender

msg.sender is a special, always-available variable holding the address that called the current function, it's the foundation almost every access-control check in Solidity is built on, "is the caller allowed to do this?"

function withdraw() public {
    require(msg.sender == owner, "Not the owner");
    // ...
}