Solidity Fast Learning…

Version pragma

Import files



  • Logical : ! (logical negation), && (AND), || (OR)
  • Comparisons : == (equality), != (inequality)


  • Comparisons: <=, <, ==, !=, >= and >
  • Bit operators: &, |, ^ (bitwise exclusive or) and ~ (bitwise negation)
  • Arithmetic operators: +, -, unary -, unary +, *, /, %, ** (exponentiation), << (left shift) and >> (right shift)


  • Comparisons: <=, <, ==, !=, >= and >


  • <address>.balance (uint256): balance of the Address in Wei

transfer and send

  • <address>.transfer(uint256 amount): send given amount of Wei to Address, throws on failure
  • <address>.send(uint256 amount) returns (bool): send given amount of Wei to Address, returns false on failure


  • <address>.call(...) returns (bool): issue low-level CALL, returns false on failure


  • <address>.delegatecall(...) returns (bool): issue low-level DELEGATECALL, returns false on failure
contract A {
uint value;
address public sender;
address a = "0x"; // address of contract B
function makeDelegateCall(uint _value) {
a.delegatecall(bytes4(keccak256("setValue(uint)")), _value); // Value of A is modified

contract B {
uint value;
address public sender;
setValue(uint _value) {
value = _value;
sender = msg.sender; // msg.sender is preserved in delegatecall. It was not available in callcode.


  • <address>.callcode(...) returns (bool): issue low-level CALLCODE, returns false on failure


uint[] dynamicSizeArray;uint[7] fixedSizeArray;

Fixed byte arrays

  • .length : read-only

Dynamic byte arrays


enum ActionChoices { 
ActionChoices choice = ActionChoices.GoStraight;


struct Funder {
address addr;
uint amount;
Funder funders;


Control Structures

  • if else
  • while
  • do
  • for
  • break
  • continue
  • return
  • ? :



Access modifiers

  • public - Accessible from this contract, inherited contracts and externally
  • private - Accessible only from this contract
  • internal - Accessible only from this contract and contracts inheriting from it
  • external - Cannot be accessed internally, only externally. Recommended to reduce gas. Access internally with this.f.


Input parameters

function f(uint _a, uint _b) {}

Output parameters

function f(uint _a, uint _b) returns (uint _sum) {
_sum = _a + _b;


contract C {
address owner;
uint status;
constructor(uint _status) {
owner = msg.sender;
status = _status;

Function Calls

Internal Function Calls

contract C {
function funA() returns (uint) {
return 5;

function FunB(uint _a) returns (uint ret) {
return funA() + _a;

External Function Calls

Named Calls

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

Unnamed function parameters

function f(uint a, uint) returns (uint) {
return a;

Function type

pragma solidity ^0.4.18;contract Oracle {
struct Request {
bytes data;
function(bytes memory) external callback;
Request[] requests;
event NewRequest(uint);
function query(bytes data, function(bytes memory) external callback) {
requests.push(Request(data, callback));
NewRequest(requests.length - 1);
function reply(uint requestID, bytes response) {
// Here goes the check that the reply comes from a trusted source
contract OracleUser {
Oracle constant oracle = Oracle(0x1234567); // known contract
function buySomething() {
oracle.query("USD", this.oracleResponse);
function oracleResponse(bytes response) {
require(msg.sender == address(oracle));

Function Modifier

modifier onlyOwner {
require(msg.sender == owner);
function close() onlyOwner {

View or Constant Functions

function f(uint a) view returns (uint) {
return a * b; // where b is a storage variable

Pure Functions

function f(uint a) pure returns (uint) {
return a * 42;

Payable Functions

Fallback Function

function() {
// Do something


Creating contracts using new

contract A {
function add(uint _a, uint _b) returns (uint) {
return _a + _b;
contract C {
address a;
function f(uint _a) {
a = new A();

Contract Inheritance

contract owned {
function owned() { owner = msg.sender; }
address owner;
contract mortal is owned {
function kill() {
if (msg.sender == owner) selfdestruct(owner);
contract final is mortal {
function kill() {
super.kill(); // Calls kill() of mortal.

Multiple inheritance

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

Constructor of base class

contract A {
uint a;
constructor(uint _a) { a = _a; }
contract B is A(1) {
constructor(uint _b) A(_b) {

Abstract Contracts

pragma solidity ^0.4.0;contract A {
function C() returns (bytes32);
contract B is A {
function C() returns (bytes32) { return "c"; }


  • Cannot have any functions implemented.
  • Cannot inherit other contracts or interfaces.
  • Cannot define constructor.
  • Cannot define variables.
  • Cannot define structs.
  • Cannot define enums.
pragma solidity ^0.4.11;interface Token {
function transfer(address recipient, uint amount);


pragma solidity ^0.4.0;contract ClientReceipt {
event Deposit(
address indexed _from,
bytes32 indexed _id,
uint _value
function deposit(bytes32 _id) payable {
emit Deposit(msg.sender, _id, msg.value);


library arithmatic {
function add(uint _a, uint _b) returns (uint) {
return _a + _b;
contract C {
uint sum;
function f() {
sum = arithmatic.add(2, 3);

Using — For

library arithmatic {
function add(uint _a, uint _b) returns (uint) {
return _a + _b;
contract C {
using arithmatic for uint;

uint sum;
function f(uint _a) {
sum = _a.add(3);

Error Handling

  • assert(bool condition): throws if the condition is not met - to be used for internal errors.
  • require(bool condition): throws if the condition is not met - to be used for errors in inputs or external components.
  • revert(): abort execution and revert state changes
function sendHalf(address addr) payable returns (uint balance) {
require(msg.value % 2 == 0); // Only allow even numbers
uint balanceBeforeTransfer = this.balance;
addr.transfer(msg.value / 2);
assert(this.balance == balanceBeforeTransfer - msg.value / 2);
return this.balance;

Global variables

Block variables

  • block.blockhash(uint blockNumber) returns (bytes32): hash of the given block - only works for the 256 most recent blocks excluding current
  • block.coinbase (address): current block miner’s address
  • block.difficulty (uint): current block difficulty
  • block.gaslimit (uint): current block gaslimit
  • block.number (uint): current block number
  • block.timestamp (uint): current block timestamp as seconds since unix epoch
  • now (uint): current block timestamp (alias for block.timestamp)

Transaction variables

  • (bytes): complete calldata
  • msg.gas (uint): remaining gas
  • msg.sender (address): sender of the message (current call)
  • msg.sig (bytes4): first four bytes of the calldata (i.e. function identifier)
  • msg.value (uint): number of wei sent with the message
  • tx.gasprice (uint): gas price of the transaction
  • tx.origin (address): sender of the transaction (full call chain)

Mathematical and Cryptographic Functions

  • addmod(uint x, uint y, uint k) returns (uint): compute (x + y) % k where the addition is performed with arbitrary precision and does not wrap around at 2**256.
  • mulmod(uint x, uint y, uint k) returns (uint): compute (x * y) % k where the multiplication is performed with arbitrary precision and does not wrap around at 2**256.
  • keccak256(...) returns (bytes32): compute the Ethereum-SHA-3 (Keccak-256) hash of the (tightly packed) arguments
  • sha256(...) returns (bytes32): compute the SHA-256 hash of the (tightly packed) arguments
  • sha3(...) returns (bytes32): alias to keccak256
  • ripemd160(...) returns (bytes20): compute RIPEMD-160 hash of the (tightly packed) arguments
  • ecrecover(bytes32 hash, uint8 v, bytes32 r, bytes32 s) returns (address): recover the address associated with the public key from elliptic curve signature or return zero on error (example usage)

Contract Related

  • this (current contract’s type): the current contract, explicitly convertible to Address
  • selfdestruct(address recipient): destroy the current contract, sending its funds to the given Address
  • suicide(address recipient): alias to selfdestruct. Soon to be deprecated.




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