Solidity Documentation, Release 0.4.20
A contract in the sense of Solidity is a collection of code (its functions) and data (its state) that resides at a specific
address on the Ethereum blockchain. The line uint storedData; declares a state variable called storedData
of type uint (unsigned integer of 256 bits). You can think of it as a single slot in a database that can be queried and
altered by calling functions of the code that manages the database. In the case of Ethereum, this is always the owning
contract. And in this case, the functions set and get can be used to modify or retrieve the value of the variable.
To access a state variable, you do not need the prefix this. as is common in other languages.
This contract does not do much yet (due to the infrastructure built by Ethereum) apart from allowing anyone to store
a single number that is accessible by anyone in the world without a (feasible) way to prevent you from publishing this
number. Of course, anyone could just call set again with a different value and overwrite your number, but the number
will still be stored in the history of the blockchain. Later, we will see how you can impose access restrictions so that
only you can alter the number.
Note: All identifiers (contract names, function names and variable names) are restricted to the ASCII character set.
It is possible to store UTF-8 encoded data in string variables.
Warning: Be careful with using Unicode text as similarly looking (or even identical) characters can have different
code points and as such will be encoded as a different byte array.
Subcurrency Example
The following contract will implement the simplest form of a cryptocurrency. It is possible to generate coins out
of thin air, but only the person that created the contract will be able to do that (it is trivial to implement a different
issuance scheme). Furthermore, anyone can send coins to each other without any need for registering with username
and password - all you need is an Ethereum keypair.
pragma solidity ^0.4.0;
contract Coin {
// The keyword "public" makes those variables
// readable from outside.
address public minter;
mapping (address => uint) public balances;
// Events allow light clients to react on
// changes efficiently.
event Sent(address from, address to, uint amount);
// This is the constructor whose code is
// run only when the contract is created.
function Coin() public {
minter = msg.sender;
}
function mint(address receiver, uint amount) public {
if (msg.sender != minter) return;
balances[receiver] += amount;
}
function send(address receiver, uint amount) public {
if (balances[msg.sender] < amount) return;
balances[msg.sender] -= amount;
16 Chapter 7. Contents
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