Solidity simple tutorialen - Ethereum smart contract

ethereum
smart contract
林修平


online compiler

•

online compiler :
/>

MANUAL COMPILE(using geth)

•

connect to console
•

set up a chain
•

connect to main chain：go-ethereum/build/bin/geth console

•

connect to testnet：go-ethereum/build/bin/geth --testnet console

•

build a private net：go-ethereum/build/bin/geth --datadir “your_directory"

--rpc --rpcport port --rpccorsdomain "*" --port "30303" --nodiscover -ipcapi "admin,db,eth,debug,miner,net,shh,txpool,personal,web3" --rpcapi
"db,eth,net,web3" --autodag --networkid number --nat "any" console


MANUAL COMPILE(using geth)

•

in console
•

var contractABI = web3.eth.contract([{contractABI}]);

•

var contract123 = contractABI.new(
parameters,
{from: address,
data: bytecode,
gas: gas
}, callback_function)


Quick Intro


Quick Intro
•

price unit：ether

•

address：you can withdraw from or save ether to it.
•

it can represent an user account
•

•

•

no code

or represent a contract
•

tied with a code

•

NOTE: same code on different address is different contract

transaction：
•

send ether

•

or to execute function in a contract

•

or both


Quick Intro
•

contract: comprised of state and function
•

state: used by user to keep information regarding the contract

•

function: change state of a contract

•

NOTE: Ethereum discourage users from using state to preserve information, so it cost a lot of gas to either
create or change the state.
•

•

WHY? Since every node(miner) needs to keep a full copy of the blockchain, they have to be compensated
for storage cost of every contract.

gas
•

every function comprised of many operations and there’s a price to every type of operation

•

there’s a fixed amount of gas cost to function you design and you have to pay for it when you want to execute a
function

•

you can decide how much ether you want to pay per gas and that becomes transaction fee needed for this
execution


Quick Intro

•

If you are on a public chain, every storage cost matters.

•

But what if you are on a private chain?
•

you can have ether as many as you want so you don’t need to
worry about transaction fee

•

but does it mean that you can use as many storage as you want?
•

YES! but still, the costs of storing these states are to be
taken by all nodes in your private chain


How it work


How it work

• state
• function：operate

• design

on states

logic：use functions
to make your contract work


How it work

Transaction(Deploy)
contract Count123{

uint counts(0);
function incre(){
counts = count + 1;
}
}


How it work

contract Count123{
uint counts(0);
function incre(){
counts = count + 1;
}
}


How it work

Transaction(Invoke)
Count123.incre()
contract Count123{
uint counts(0);
function incre(){
counts = count + 1;
}
}


How it work

let’s take slock.it for example and write a simple bike renting contract!

slock.it


state declaration


state declaration

contract bikeRenting {
address public owner;
address public currentRenter;
uint public expireTime;

…
}


state declaration
•

type , visibility , variable_name
contract bikeRenting {

address public owner;
address public currentRenter;

uint public expireTime;
…

}


state declaration
•

type , visibility , variable_name
•

type : bool, int, uint, address, mapping, bytes, string, struct

•

visibility : public or private(default)
•

•

public: accessible externally
(declare in your contract) uint public totalCount = 3;
(access in a console) mycontract.totalCount(); //3
(access from other contract) thatcontract.totalCount() //3

variable_name


state declaration
•

Array in solidity :
•

address[] owners;

•

push item into array:

•

delete item:
•

address[3] threeOwners;
owners.push(address)

delete owners[2]

pitfall: only changes the value of specified item to zero
instead of erase the item


state declaration
•

mapping :
•

mapping(typeA => typeB) variable_name;

•

example
•

declare a mapping:
•

•

map address 0x123456789abcdef to integer 10:
•

•

mapping(address => uint) deposits;

deposits[0x123456789abcdef] = 10;

uninitialized or undeclared values are zero instead of NULL
•

NOTE: there’s no NULL in Solidity


state declaration

•

Units and Globally available variables
•

Ether unit：ether、finney、wei

•

time unit：seconds、weeks、years、now
•

now: present time
•

more specifically, the time when this block is mined, i.e,
the time_stamp parameter in the block header


state declaration

•

Special variables and functions
•

•

msg: information regarding this transaction
•

msg.sender: address who send the transaction

•

msg.value: value sent with the transaction

address related functions
•

address.balance

•

address.send(amount)：send ether to address


state declaration

•

Special variables and functions
•

throw
•

reverts all changes made so far by the underlying transaction

•

confiscate all gas provided by underlying transaction

initialization


initialization

contract bikeRenting {
address public owner;
address public currentRenter;
uint public expireTime;
uint public unitPrice;
function bikeRenting(uint _unitPrice){
owner = msg.sender;
currentRenter = 0x0;
expireTime = now;
unitPrice = _unitPrice;
}
}