Solidity Fast Learning…

This document is 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.

Version pragma

pragma solidity ^0.4.21; will compile with a compiler version >= 0.4.21 and < 0.5.0.

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:

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

Integer

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

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

Operators:

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

Address

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

Operators:

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

Methods:

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

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 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.

gas() option is available for call, callcode and delegatecall. value() option is not supported for delegatecall.

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

Prior to homestead, only a limited variant called callcode was available that did not provide access to the original msg.sender and msg.value values.

Array

Arrays can be dynamic or have a fixed size.

Fixed byte arrays

bytes1(byte), 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

  • .length : read-only

Dynamic byte arrays

bytes: Dynamically-sized byte array. It is similar to byte[], but it is packed tightly in calldata. Not a value-type!

string: Dynamically-sized UTF-8-encoded string. It is equal to bytes but does not allow length or index access. Not a value-type!

Enum

Enum works just like in every other language.

Struct

New types can be declared using struct.

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 almost any type except for a mapping, a dynamically sized array, a contract, an enum, or a struct. value can actually be any type, including mappings.

Control Structures

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

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

Functions

Structure

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

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.

Parameters

Parameters are declared just like variables and are memory variables.

Output parameters are declared after the returns keyword

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

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

Constructor

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

Function Calls

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

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.

.gas() and .value() can also be used with external function calls.

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

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

Function type

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

Function Modifier

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

View or Constant Functions

Functions can be declared view or constant in which case they promise not to modify the state, but can read from them.

The compiler does not enforce yet that a view method is not modifying state.

Pure Functions

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

Payable Functions

Functions that receive Ether are marked as payable function.

Fallback Function

A contract can have exactly one unnamed function. This function cannot have arguments and cannot return anything. It is executed on a call to the contract if none of the other functions match the given function identifier (or if no data was supplied at all).

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 Inheritance

Solidity supports multiple inheritance and polymorphism.

Abstract Contracts

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

Interface

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

  • Cannot have any functions implemented.
  • Cannot inherit other contracts or interfaces.
  • Cannot define constructor.
  • Cannot define variables.
  • Cannot define structs.
  • Cannot define enums.

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 searched for.

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

Library

Libraries are similar to contracts, but they are deployed only once at a specific address, and their code is used with delegatecall (callcode).

Using — For

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

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

Catching exceptions is not yet possible.

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

  • msg.data (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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