Solidity示例程序
投票
以下合约非常复杂，但展示了很多Solidity的功能。它执行投票合约。当然，电子投票的主要问题是如何为正确的人员分配投票权，以及如何防止操纵。我们不会在这里解决所有问题，但至少我们会展示如何进行委派投票，以便计票自动且完全透明。
这个想法是为每个选票创建一个合约，为每个选项提供一个简称。然后，担任主席的合同创建者将分别给予每个地址的投票权。
然后，地址背后的人可以选择自己投票，或者将他们的投票委托给他们信任的人。
在投票时间结束时，winningProposal（）将返回投票数最多的提案。
pragma solidity ^0.4.11;
/// @title Voting with delegation.
contract Ballot {
// This declares a new complex type which will
// be used for variables later.
// It will represent a single voter.
struct Voter {
uint weight; // weight is accumulated by delegation
bool voted; // if true, that person already voted
address delegate; // person delegated to
uint vote; // index of the voted proposal
}
// This is a type for a single proposal.
struct Proposal {
bytes32 name; // short name (up to 32 bytes)
uint voteCount; // number of accumulated votes
}
address public chairperson;
// This declares a state variable that
// stores a `Voter` struct for each possible address.
mapping(address => Voter) public voters;
// A dynamically-sized array of `Proposal` structs.
Proposal[] public proposals;
/// Create a new ballot to choose one of `proposalNames`.
function Ballot(bytes32[] proposalNames) {
chairperson = msg.sender;
voters[chairperson].weight = 1;
// For each of the provided proposal names,
// create a new proposal object and add it
// to the end of the array.
for (uint i = 0; i < proposalNames.length; i++) {
// `Proposal({...})` creates a temporary
// Proposal object and `proposals.push(...)`
// appends it to the end of `proposals`.
proposals.push(Proposal({
name: proposalNames[i],
voteCount: 0
}));
}
}
// Give `voter` the right to vote on this ballot.
// May only be called by `chairperson`.
function giveRightToVote(address voter) {
// If the argument of `require` evaluates to `false`,
// it terminates and reverts all changes to
// the state and to Ether balances. It is often
// a good idea to use this if functions are
// called incorrectly. But watch out, this
// will currently also consume all provided gas
// (this is planned to change in the future).
require((msg.sender == chairperson) && !voters[voter].voted && (voters[voter].weight == 0));
voters[voter].weight = 1;
}
/// Delegate your vote to the voter `to`.
function delegate(address to) {
// assigns reference
Voter storage sender = voters[msg.sender];
require(!sender.voted);
// Self-delegation is not allowed.
require(to != msg.sender);
// Forward the delegation as long as
// `to` also delegated.
// In general, such loops are very dangerous,
// because if they run too long, they might
// need more gas than is available in a block.
// In this case, the delegation will not be executed,
// but in other situations, such loops might
// cause a contract to get "stuck" completely.
while (voters[to].delegate != address(0)) {
to = voters[to].delegate;
// We found a loop in the delegation, not allowed.
require(to != msg.sender);
}
// Since `sender` is a reference, this
// modifies `voters[msg.sender].voted`
sender.voted = true;
sender.delegate = to;
Voter storage delegate = voters[to];
if (delegate.voted) {
// If the delegate already voted,
// directly add to the number of votes
proposals[delegate.vote].voteCount += sender.weight;
} else {
// If the delegate did not vote yet,
// add to her weight.
delegate.weight += sender.weight;
}
}
/// Give your vote (including votes delegated to you)
/// to proposal `proposals[proposal].name`.
function vote(uint proposal) {
Voter storage sender = voters[msg.sender];
require(!sender.voted);
sender.voted = true;
sender.vote = proposal;
// If `proposal` is out of the range of the array,
// this will throw automatically and revert all
// changes.
proposals[proposal].voteCount += sender.weight;
}
/// @dev Computes the winning proposal taking all
/// previous votes into account.
function winningProposal() constant
returns (uint winningProposal)
{
uint winningVoteCount = 0;
for (uint p = 0; p < proposals.length; p++) {
if (proposals[p].voteCount > winningVoteCount) {
winningVoteCount = proposals[p].voteCount;
winningProposal = p;
}
}
}
// Calls winningProposal() function to get the index
// of the winner contained in the proposals array and then
// returns the name of the winner
function winnerName() constant
returns (bytes32 winnerName)
{
winnerName = proposals[winningProposal()].name;
}
}
可能的改进
目前，需要许多交易来将投票权分配给所有参与者。你能想出更好的方法吗？
盲拍
在本节中，我们将展示在以太坊创建一个完全失明的拍卖合同是多么容易。我们将从公开拍卖开始，每个人都可以看到所拍的投标，然后将此合同扩展到一个盲目拍卖，在竞标期结束之前无法看到实际出价。
简单的公开拍卖
以下简单的拍卖合同的总体思路是每个人都可以在投标期内发送他们的出价。出价已包括发送货币/以太币以使投标人与他们的出价相结合。如果提高最高出价，以前出价最高的出价者就可以拿回她的钱了。在投标期结束后，合同必须手动为受益人接收他的钱 - 合同不能激活自己。
pragma solidity ^0.4.11;
contract SimpleAuction {
// Parameters of the auction. Times are either
// absolute unix timestamps (seconds since 1970-01-01)
// or time periods in seconds.
address public beneficiary;
uint public auctionStart;
uint public biddingTime;
// Current state of the auction.
address public highestBidder;
uint public highestBid;
// Allowed withdrawals of previous bids
mapping(address => uint) pendingReturns;
// Set to true at the end, disallows any change
bool ended;
// Events that will be fired on changes.
event HighestBidIncreased(address bidder, uint amount);
event AuctionEnded(address winner, uint amount);
// The following is a so-called natspec comment,
// recognizable by the three slashes.
// It will be shown when the user is asked to
// confirm a transaction.
/// Create a simple auction with `_biddingTime`
/// seconds bidding time on behalf of the
/// beneficiary address `_beneficiary`.
function SimpleAuction(
uint _biddingTime,
address _beneficiary
) {
beneficiary = _beneficiary;
auctionStart = now;
biddingTime = _biddingTime;
}
/// Bid on the auction with the value sent
/// together with this transaction.
/// The value will only be refunded if the
/// auction is not won.
function bid() payable {
// No arguments are necessary, all
// information is already part of
// the transaction. The keyword payable
// is required for the function to
// be able to receive Ether.
// Revert the call if the bidding
// period is over.
require(now <= (auctionStart + biddingTime));
// If the bid is not higher, send the
// money back.
require(msg.value > highestBid);
if (highestBidder != 0) {
// Sending back the money by simply using
// highestBidder.send(highestBid) is a security risk
// because it could execute an untrusted contract.
// It is always safer to let the recipients
// withdraw their money themselves.
pendingReturns[highestBidder] += highestBid;
}
highestBidder = msg.sender;
highestBid = msg.value;
HighestBidIncreased(msg.sender, msg.value);
}
/// Withdraw a bid that was overbid.
function withdraw() returns (bool) {
uint amount = pendingReturns[msg.sender];
if (amount > 0) {
// It is important to set this to zero because the recipient
// can call this function again as part of the receiving call
// before `send` returns.
pendingReturns[msg.sender] = 0;
if (!msg.sender.send(amount)) {
// No need to call throw here, just reset the amount owing
pendingReturns[msg.sender] = amount;
return false;
}
}
return true;
}
/// End the auction and send the highest bid
/// to the beneficiary.
function auctionEnd() {
// It is a good guideline to structure functions that interact
// with other contracts (i.e. they call functions or send Ether)
// into three phases:
// 1. checking conditions
// 2. performing actions (potentially changing conditions)
// 3. interacting with other contracts
// If these phases are mixed up, the other contract could call
// back into the current contract and modify the state or cause
// effects (ether payout) to be performed multiple times.
// If functions called internally include interaction with external
// contracts, they also have to be considered interaction with
// external contracts.
// 1. Conditions
require(now >= (auctionStart + biddingTime)); // auction did not yet end
require(!ended); // this function has already been called
// 2. Effects
ended = true;
AuctionEnded(highestBidder, highestBid);
// 3. Interaction
beneficiary.transfer(highestBid);
}
}
盲拍
以前的公开拍卖会延伸到以下的盲目拍卖。盲目拍卖的优势在于投标期结束时没有时间压力。在一个透明的计算平台上创建一个盲目拍卖可能听起来像是一个矛盾，但是密码学可以解救。
在投标期间，投标人实际上并没有发出她的投标，而只是一个散列版本。由于目前认为实际上不可能找到两个（足够长）的哈希值相等的值，因此投标人承诺通过该投标。投标结束后，投标人必须公开他们的投标：他们将他们的价值未加密并且合同检查散列值与投标期间提供的散列值相同。
另一个挑战是如何在同一时间使拍卖具有约束力和盲目性：在赢得拍卖后，防止投标人不发送货币的唯一方法是让她在拍卖中一并发送。既然价值转移不能在以太坊蒙蔽，任何人都可以看到价值。
以下合同通过接受任何大于最高出价的值来解决此问题。因为这当然只能在披露阶段进行检查，所以有些出价可能是无效的，这是有意的（它甚至提供了一个明确的标志，以无效的投标和高价值转让）：投标人可以通过放置几个高或低无效出价。
pragma solidity ^0.4.11;
contract BlindAuction {
struct Bid {
bytes32 blindedBid;
uint deposit;
}
address public beneficiary;
uint public auctionStart;
uint public biddingEnd;
uint public revealEnd;
bool public ended;
mapping(address => Bid[]) public bids;
address public highestBidder;
uint public highestBid;
// Allowed withdrawals of previous bids
mapping(address => uint) pendingReturns;
event AuctionEnded(address winner, uint highestBid);
/// Modifiers are a convenient way to validate inputs to
/// functions. `onlyBefore` is applied to `bid` below:
/// The new function body is the modifier's body where
/// `_` is replaced by the old function body.
modifier onlyBefore(uint _time) { require(now < _time); _; }
modifier onlyAfter(uint _time) { require(now > _time); _; }
function BlindAuction(
uint _biddingTime,
uint _revealTime,
address _beneficiary
) {
beneficiary = _beneficiary;
auctionStart = now;
biddingEnd = now + _biddingTime;
revealEnd = biddingEnd + _revealTime;
}
/// Place a blinded bid with `_blindedBid` = keccak256(value,
/// fake, secret).
/// The sent ether is only refunded if the bid is correctly
/// revealed in the revealing phase. The bid is valid if the
/// ether sent together with the bid is at least "value" and
/// "fake" is not true. Setting "fake" to true and sending
/// not the exact amount are ways to hide the real bid but
/// still make the required deposit. The same address can
/// place multiple bids.
function bid(bytes32 _blindedBid)
payable
onlyBefore(biddingEnd)
{
bids[msg.sender].push(Bid({
blindedBid: _blindedBid,
deposit: msg.value
}));
}
/// Reveal your blinded bids. You will get a refund for all
/// correctly blinded invalid bids and for all bids except for
/// the totally highest.
function reveal(
uint[] _values,
bool[] _fake,
bytes32[] _secret
)
onlyAfter(biddingEnd)
onlyBefore(revealEnd)
{
uint length = bids[msg.sender].length;
require(_values.length == length);
require(_fake.length == length);
require(_secret.length == length);
uint refund;
for (uint i = 0; i < length; i++) {
var bid = bids[msg.sender][i];
var (value, fake, secret) =
(_values[i], _fake[i], _secret[i]);
if (bid.blindedBid != keccak256(value, fake, secret)) {
// Bid was not actually revealed.
// Do not refund deposit.
continue;
}
refund += bid.deposit;
if (!fake && bid.deposit >= value) {
if (placeBid(msg.sender, value))
refund -= value;
}
// Make it impossible for the sender to re-claim
// the same deposit.
bid.blindedBid = bytes32(0);
}
msg.sender.transfer(refund);
}
// This is an "internal" function which means that it
// can only be called from the contract itself (or from
// derived contracts).
function placeBid(address bidder, uint value) internal
returns (bool success)
{
if (value <= highestBid) {
return false;
}
if (highestBidder != 0) {
// Refund the previously highest bidder.
pendingReturns[highestBidder] += highestBid;
}
highestBid = value;
highestBidder = bidder;
return true;
}
/// Withdraw a bid that was overbid.
function withdraw() {
uint amount = pendingReturns[msg.sender];
if (amount > 0) {
// It is important to set this to zero because the recipient
// can call this function again as part of the receiving call
// before `send` returns (see the remark above about
// conditions -> effects -> interaction).
pendingReturns[msg.sender] = 0;
msg.sender.transfer(amount);
}
}
/// End the auction and send the highest bid
/// to the beneficiary.
function auctionEnd()
onlyAfter(revealEnd)
{
require(!ended);
AuctionEnded(highestBidder, highestBid);
ended = true;
// We send all the money we have, because some
// of the refunds might have failed.
beneficiary.transfer(this.balance);
}
}
安全远程购买
pragma solidity ^0.4.11;
contract Purchase {
uint public value;
address public seller;
address public buyer;
enum State { Created, Locked, Inactive }
State public state;
// Ensure that `msg.value` is an even number.
// Division will truncate if it is an odd number.
// Check via multiplication that it wasn't an odd number.
function Purchase() payable {
seller = msg.sender;
value = msg.value / 2;
require((2 * value) == msg.value);
}
modifier condition(bool _condition) {
require(_condition);
_;
}
modifier onlyBuyer() {
require(msg.sender == buyer);
_;
}
modifier onlySeller() {
require(msg.sender == seller);
_;
}
modifier inState(State _state) {
require(state == _state);
_;
}
event Aborted();
event PurchaseConfirmed();
event ItemReceived();
/// Abort the purchase and reclaim the ether.
/// Can only be called by the seller before
/// the contract is locked.
function abort()
onlySeller
inState(State.Created)
{
Aborted();
state = State.Inactive;
seller.transfer(this.balance);
}
/// Confirm the purchase as buyer.
/// Transaction has to include `2 * value` ether.
/// The ether will be locked until confirmReceived
/// is called.
function confirmPurchase()
inState(State.Created)
condition(msg.value == (2 * value))
payable
{
PurchaseConfirmed();
buyer = msg.sender;
state = State.Locked;
}
/// Confirm that you (the buyer) received the item.
/// This will release the locked ether.
function confirmReceived()
onlyBuyer
inState(State.Locked)
{
ItemReceived();
// It is important to change the state first because
// otherwise, the contracts called using `send` below
// can call in again here.
state = State.Inactive;
// NOTE: This actually allows both the buyer and the seller to
// block the refund - the withdraw pattern should be used.
buyer.transfer(value);
seller.transfer(this.balance);
}
}