## Writing Smart Contracts in Solidity Getting our hands dirty with some Solidity --- Be sure to test out all this code for yourself in [Remix IDE](https://remix.ethereum.org/). --- ## Essential Syntax --- ### License Identifier (optional) ```solidity // SPDX-License-Identifier: <license-identifier-here> // ex. SPDX-License-Identifier: MIT // ex. SPDX-License-Identifier: GPL-3.0-or-later // ex. SPDX-License-Identifier: UNLICENSED ``` --- ### Solidity Language Version ```solidity pragma solidity ^0.8.4; ``` [Version selection syntax](https://docs.npmjs.com/cli/v6/using-npm/semver) --- ### Contract Declaration ```solidity contract MyContractName { // ... } ``` --- ### Data Types `bool`, `[u]int[8-256]`, `address [payable]`, `bytes[1-32]`, `string`, `mapping`, enums, contracts, arrays, structs _no undefined or null values_ _no floating-point numbers_ --- ### Access Modifiers `public`, `private`, `internal`, `external` --- ### View Modifier Indicates the function does not modify state --- ### Pure Modifier Indicates the function does not access or modify state --- ### Globals `msg.sender`, `msg.value`, `tx.origin`, `require(...)`, `keccak256(...)`, `sha256(...)`, `block.timestamp`, `block.number`, `blockhash(...)` [More](https://docs.soliditylang.org/en/v0.8.4/cheatsheet.html#global-variables) --- ### Gas The cost of running code on the blockchain --- Regular ETH transfer costs 21,000 gas --- If the gas price is 66 Gwei (0.000000066 ETH) 21,000 &times; 0.000000066 ETH = 0.001386 ETH <span>$</span>3.82 @ <span>$</span>2,756.35/ETH --- #### Keeping gas usage low - Only compute what you absolutely have to - Storage I/O is _expensive_, memory less so - Be _very careful_ when writing looping or recursive logic --- _\[exercises\]_ --- ### Events Consume little gas, make it easy to add hooks for off-chain logic --- ### Payable Functions Send ether to a smart contract function Value appears in `msg.value` in units of _wei_ (smallest, indivisible unit of Ethereum) --- #### Attaching ETH to a message call ```solidity myContract.myFunction{ value: 1 ether }(/* ... */); ``` --- Pay addresses ```solidity (bool success,) = payableAddress.call{ value: 1 ether }(""); ``` Use of `send` and `transfer` is somewhat discouraged due to fixed gas allocation --- Change gas limit ```solidity address.functionCall{ gas: 21000 }(/* ... */); ``` Again, fixed/hardcoded gas limits are discouraged --- _\[exercises\]_ --- ## Best Practices Design patterns & security considerations --- Constants prevent reading from storage ```solidity uint256 constant minimumBuy = 1 gwei; ``` --- Function modifiers increase modularity and readability ```solidity modifier onlyOwner() { require(owner() == msg.sender, "Caller is not the owner"); _; } ``` ```solidity function mint () external onlyOwner { // ... } ``` --- ### Receive & Fallback ```solidity // No function keyword receive () external payable { // Called when the contract address is paid directly // Just like any other payable function // Keep in mind the gas limit could be as low as 2300 } ``` ```solidity // Optionally payable fallback () external payable { // Called when no function signature matches } ``` --- ### Re-entrancy Vulnerability 1. _Contract A_ calls _Contract B_ before finishing setting its own state 2. _Contract B_ calls _Contract A_, taking advantage of _Contract A_'s invalid state --- Use the **checks-effects-interactions** order of operations to prevent other contract calls from taking advantage of an invalid intermediate state. --- _\[exercises\]_ --- ## Project Status Update Begin writing your smart contract(s)! --- That's it!