Solidity Cheatsheet and Best practices

Motivation

This document is a cheatsheet for Solidity that you can use to write Smart Contracts for Ethereum based blockchain.

This guide is not intended to teach you Solidity from the ground up, but to help developers with basic knowledge who may struggle to get familiar with Smart Contracts and Blockchain because of the Solidity concepts used.

Older versions of this cheatsheet can be found in Git tags.

Table of contents

Version pragma

pragma solidity ^0.8.34; will compile with a compiler version >= 0.8.34 and < 0.9.0.

Import files

import "filename";

import * as symbolName from "filename"; or import "filename" as symbolName;

import {symbol1 as alias, symbol2} from "filename";

Types

Boolean

bool : true or false

Operators:

Integer

Unsigned : uint8 | uint16 | uint32 | uint64 | uint128 | uint256(uint)

Signed : int8 | int16 | int32 | int64 | int128 | int256(int)

Operators:

Arithmetic overflow and underflow checks are enabled by default in Solidity 0.8.x. Use unchecked { ... } only when you intentionally want wrapping behavior.

Unchecked arithmetic

Use unchecked to disable the default overflow and underflow checks inside a specific block.

function add(uint256 a, uint256 b) public pure returns (uint256) {
    unchecked {
        return a + b;
    }
}

Address

address: Holds an Ethereum address (20 byte value).

address payable: Same as address, but is used for addresses that can receive Ether.

Operators:

Methods:

balance

transfer and send

In the current version of Solidity, transfer and send are considered legacy patterns. Prefer call{value: amount}("") and always check the returned success value.

call

(bool success, bytes memory result) = target.call{value: amount}(data);

delegatecall

Delegatecall uses the code of the target address, taking all other aspects (storage, balance, …) from the calling contract. The purpose of delegatecall is to use library or proxy code which is stored in another contract. The user has to ensure that the layout of storage in both contracts is suitable for delegatecall to be used.

contract A {
    uint256 value;
    address public sender;
    address target;

    function makeDelegateCall(uint256 _value) public {
        (bool success, ) = target.delegatecall(
            abi.encodeWithSignature("setValue(uint256)", _value)
        );
        require(success, "delegatecall failed");
    }
}

contract B {
    uint256 value;
    address public sender;

    function setValue(uint256 _value) public {
        value = _value;
        sender = msg.sender;
    }
}

Use call options such as {gas: gasAmount} and {value: amount} instead of the older .gas() and .value() syntax.

callcode

callcode is obsolete and should not be used in modern Solidity.

Array

Arrays can be dynamic or have a fixed size.

uint256[] dynamicSizeArray;

uint256[7] fixedSizeArray;

Fixed byte arrays

bytes1, bytes2, bytes3, …, bytes32.

Operators:

Comparisons: <=, <, ==, !=, >=, > (evaluate to bool)

Bit operators: &, |, ^ (bitwise exclusive or), ~ (bitwise negation), << (left shift), >> (right shift)

Index access: If x is of type bytesI, then x[k] for 0 <= k < I returns the k th byte (read-only).

Members

Dynamic byte arrays

bytes: Dynamically-sized byte array.

string: Dynamically-sized UTF-8 encoded string. It is similar to bytes, but does not allow length or index access directly.

Enum

Enum works just like in every other language.

enum ActionChoices {
    GoLeft,
    GoRight,
    GoStraight,
    SitStill
}

ActionChoices choice = ActionChoices.GoStraight;

Struct

New types can be declared using struct.

struct Funder {
    address addr;
    uint256 amount;
}

Funder funder;

Mapping

Declared as mapping(_KeyType => _ValueType)

Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value.

key can be any built-in value type, bytes, string, contract type, or enum. value can be almost any type, including mappings, arrays and structs.

Data locations

Reference types such as arrays, structs, bytes and string use data locations.

string public name;

function setName(string calldata _name) external {
    name = _name;
}

Mappings can only exist in storage.

Control Structures

Most of the control structures from JavaScript are available in Solidity except for switch and goto.

Functions

Structure

function <functionName>(<parameter types>) {internal|external|public|private} [pure|view|payable] [virtual|override] [returns (<return types>)]

Access modifiers

Parameters

Input parameters

Parameters are declared just like variables. Reference types should use an explicit data location such as memory or calldata.

function f(uint256 _a, uint256 _b) public pure {}

Output parameters

Output parameters are declared after the returns keyword

function f(uint256 _a, uint256 _b) public pure returns (uint256 _sum) {
    _sum = _a + _b;
}

Output can also be specified using return statement. In that case, we can omit parameter name returns (uint256).

Multiple return types are possible with return (v0, v1, ..., vn).

Constructor

Function that is executed during contract deployment. Defined using the constructor keyword.

contract C {
    address owner;
    uint256 status;

    constructor(uint256 _status) {
        owner = msg.sender;
        status = _status;
    }
}

Function Calls

Internal Function Calls

Functions of the current contract can be called directly (internally - via jumps) and also recursively

contract C {
    function funA() internal pure returns (uint256) {
        return 5;
    }

    function funB(uint256 _a) public pure returns (uint256 ret) {
        return funA() + _a;
    }
}

External Function Calls

this.g(8); and c.g(2); (where c is a contract instance) are also valid function calls, but the function will be called externally, via a message call.

Call options such as {gas: gasAmount, value: amount} can also be used with external function calls.

Named Calls

Function call arguments can also be given by name in any order as below.

function f(uint256 a, uint256 b) public pure {}

function g() public pure {
    f({b: 1, a: 2});
}

Unnamed function parameters

Parameters will be present on the stack, but are not accessible.

function f(uint256 a, uint256) public pure returns (uint256) {
    return a;
}

Function type

Pass function as a parameter to another function. Similar to callbacks and delegates

contract Oracle {
    struct Request {
        bytes data;
        function(bytes memory) external callback;
    }

    Request[] requests;
    event NewRequest(uint256);

    function query(bytes memory data, function(bytes memory) external callback) external {
        requests.push(Request(data, callback));
        emit NewRequest(requests.length - 1);
    }

    function reply(uint256 requestID, bytes memory response) external {
        requests[requestID].callback(response);
    }
}

contract OracleUser {
    Oracle public oracle;

    constructor(address _oracle) {
        oracle = Oracle(_oracle);
    }

    function buySomething() external {
        oracle.query(bytes("USD"), this.oracleResponse);
    }

    function oracleResponse(bytes memory) external {
        require(msg.sender == address(oracle), "only oracle");
    }
}

Function Modifier

Modifiers can automatically check a condition prior to executing the function.

address owner;
uint256 status;

modifier onlyOwner() {
    require(msg.sender == owner, "not owner");
    _;
}

function setStatus(uint256 _status) public onlyOwner {
    status = _status;
}

View or Constant Functions

Functions can be declared view in which case they promise not to modify the state, but can read from it. Older examples may use constant, but view is the current syntax.

uint256 b;

function f(uint256 a) public view returns (uint256) {
    return a * b;
}

Pure Functions

Functions can be declared pure in which case they promise not to read from or modify the state.

function f(uint256 a) public pure returns (uint256) {
    return a * 42;
}

Payable Functions

Functions that receive Ether are marked as payable function.

Fallback Function

A contract can have a receive() function and a fallback() function. These functions cannot return anything. receive() is executed on plain Ether transfers with empty calldata. fallback() is executed if none of the other functions match the given function identifier.

Receive vs Fallback

receive() external payable {
    // Receive Ether
}

fallback() external payable {
    // Do something
}

Contracts

Creating contracts using new

Contracts can be created from another contract using new keyword. The source of the contract has to be known in advance.

contract A {
    function add(uint256 _a, uint256 _b) public pure returns (uint256) {
        return _a + _b;
    }
}

contract C {
    A a;

    function f() public {
        a = new A();
    }
}

Contract Inheritance

Solidity supports multiple inheritance and polymorphism.

contract Owned {
    address owner;

    constructor() {
        owner = msg.sender;
    }
}

contract Mortal is Owned {
    function close() public virtual {
        require(msg.sender == owner, "not owner");
    }
}

contract Final is Mortal {
    function close() public override {
        super.close();
    }
}

Multiple inheritance

contract A {}
contract B {}
contract C is A, B {}

Constructor of base class

contract A {
    uint256 a;

    constructor(uint256 _a) {
        a = _a;
    }
}

contract B is A {
    constructor(uint256 _b) A(_b) {}
}

Abstract Contracts

Contracts that contain implemented and non-implemented functions. Such contracts cannot be deployed, but they can be used as base contracts.

abstract contract A {
    function c() public view virtual returns (bytes32);
}

contract B is A {
    function c() public pure override returns (bytes32) {
        return "c";
    }
}

Interface

Interfaces are similar to abstract contracts, but they have restrictions:

interface Token {
    function transfer(address recipient, uint256 amount) external returns (bool);
}

Events

Events allow the convenient usage of the EVM logging facilities, which in turn can be used to “call” JavaScript callbacks in the user interface of a dapp, which listen for these events.

Up to three parameters can receive the attribute indexed, which will cause the respective arguments to be searchable.

All non-indexed arguments will be stored in the data part of the log.

contract ClientReceipt {
    event Deposit(
        address indexed _from,
        bytes32 indexed _id,
        uint256 _value
    );

    function deposit(bytes32 _id) external payable {
        emit Deposit(msg.sender, _id, msg.value);
    }
}

Library

Libraries are similar to contracts, but they are deployed only once at a specific address, and their code can be reused by other contracts. Public library functions use delegatecall; internal ones are included in the caller.

library Arithmetic {
    function add(uint256 _a, uint256 _b) internal pure returns (uint256) {
        return _a + _b;
    }
}

contract C {
    uint256 sum;

    function f() public {
        sum = Arithmetic.add(2, 3);
    }
}

Using - For

using A for B; can be used to attach library functions to any type.

library Arithmetic {
    function add(uint256 _a, uint256 _b) internal pure returns (uint256) {
        return _a + _b;
    }
}

contract C {
    using Arithmetic for uint256;

    uint256 sum;

    function f(uint256 _a) public {
        sum = _a.add(3);
    }
}

Units

Ether units:

Time units:

uint256 public oneEther = 1 ether;
uint256 public oneMinute = 1 minutes;

Time units are simple suffixes for readability. Older examples may include years, but that is not supported in the current version of Solidity.

Error Handling

Custom Errors

Custom errors are a gas-efficient way to return structured failure information.

address owner;

error Unauthorized(address caller);

function adminOnly() public view {
    if (msg.sender != owner) revert Unauthorized(msg.sender);
}

error SendFailed();

function sendHalf(address payable addr) public payable returns (uint256 balance) {
    require(msg.value % 2 == 0, "Only allow even numbers");
    uint256 balanceBeforeTransfer = address(this).balance;

    (bool success, ) = addr.call{value: msg.value / 2}("");
    if (!success) revert SendFailed();

    assert(address(this).balance == balanceBeforeTransfer - msg.value / 2);
    return address(this).balance;
}

Catching exceptions is possible using try/catch for external calls and contract creation.

Try / Catch

try/catch can be used with external function calls and contract creation.

try otherContract.doWork() returns (uint256 value) {
    return value;
} catch {
    revert("external call failed");
}

Global variables

Block variables

Transaction variables

Mathematical and Cryptographic Functions

Older examples may use sha3(...). In recent versions of Solidity use keccak256(...).

Older examples may use now and suicide. In recent versions of Solidity use block.timestamp and selfdestruct.