Ethereum – Sample Smart Contracts
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Sometimes a good idea takes a lot of funds and collective effort. You could ask for donations, but donors prefer to give to projects they are more certain will get traction and proper funding. This is an example where a crowdfunding would be ideal: If you miss your goal, the donations are returned, therefore reducing the risk for donors. Since the code is open and auditable, there is no need for a centralized, trusted platform and therefore the only fees everyone will pay are just ethereum crowdsale contract example gas fees.
In this example we will make a better crowdfunding by solving two important problems: Rewards in crowdfundings ethereum crowdsale contract example usually handled by a central unchangeable database that keeps track of all donors: Instead we are going to do this the decentralized way and just create a token to keep track of rewards, anyone who contributes gets a token that they can trade, sell or keep for later.
When ethereum crowdsale contract example time comes to give the physical reward the producer only needs to exchange the tokens for real products. Donors get to keep their tokens, even if the project doesn't achieve its goals, as ethereum crowdsale contract example souvenir.
Also, generally those who are funding can't have any say on how the money is spent after the funds are raised and mismanagement often causes projects never to deliver anything at all. In this project we ethereum crowdsale contract example use a Democratic Organization ethereum crowdsale contract example will have to approve any ethereum crowdsale contract example coming out of the system. This is often called a crowdsale or crowd equity and is so fundamental that in some cases the token can be the reward itself, especially in projects where a group of people gather together to build a common public good.
First ethereum crowdsale contract example all, create a fixed supply token. Deploy it and save the address. Now create a shareholder association. In this example we are going to use the address of the token we just created as the Shares Addressa minimum quorum of 10and minutes 25 hours as the voting time.
Deploy this contract and save the address. Notice that in the Crowdsale function the one that is called upon contract creationhow the variables deadline and fundingGoal are set:. Inside the system all ether amounts are kept track in weithe smallest divisible unit of ether. The code above converts the funding goal into wei by multiplying it by 1,,, which is what the special keyword ether converts into.
The next line creates a timestamp that is exactly X minutes away from today by also using a combination of the special keywords now and minutes. For more global keywords, check the solidity documentation on Globally available variables. Notice that the contract understands what a token is because ethereum crowdsale contract example defined it earlier by starting the code with:.
This doesn't fully describe how the contract works or all the functions it has, but describes only the ones this contract needs: Put the address of the organization you just created in the field if successful, send to. If you are just doing it for a test or demonstration, put the crowdsale duration as minutes, but if you are really raising funds you can put a larger amount, like 45, 31 days.
The ether cost of each token should be calculated based on how many tokens you are putting up for sale a maximum of how many you added as "initial supply" of your token on the previous step. In this example, put 5 ethers. The address of the token you created should be added to the token reward address. Put a gas price, click deploy and wait for your crowdsale to be created. Once the crowdsale page is created, you now need to deposit enough rewards so it can pay the rewards back.
Click the address of the crowdsale, then deposit ethereum crowdsale contract example send 50 gadgets to the crowdsale. This is a very important point. The crowdsale we are building will be completely controlled by the token holders.
You can try to create special code on the association contract to prevent these hostile takeovers, or you can instead have all the funds sent to a simple address. To simplify we are simply selling off half of all the gadgets: Once the crowdsale has all the necessary tokens, contributing to it is easy and you can do ethereum crowdsale contract example from any ethereum wallet: You can see the relevant code bit here:.
The unnamed function is the default function executed whenever a contract receives ether. This function will automatically check if the crowdsale is active, calculate how many tokens the caller bought and send the equivalent. If the crowdsale has ended or if the contract is out of tokens the contract will throw meaning the execution will be stopped and the ethereum crowdsale contract example sent will be returned but all the gas will be spent.
This has the advantage that the contract prevents falling into a situation that someone will be left without their ether or tokens. In a previous version of this ethereum crowdsale contract example we would also self destruct the contract after the crowdsale ended: By creating a fallback function that throws when the sale is over, we prevent anyone losing money. The contract has a safeWithdrawl function, without any parameters, that can be executed by the beneficiary to access the amount raised or by the funders to get back their funds in the case of a failed fundraise.
In our code, only two things can happen: Since the token amount is limited, it means that once the goal has been reached no one else can contribute. But the history of crowdfunding is full of projects that overshoot their goals in much less time than predicted or that raised many times over the required amount.
So we are going to modify our project slightly so that instead of sending a limited set of tokens, the ethereum crowdsale contract example actually creates a new token out of thin air whenever someone sends them ether.
First of all, we need to create a Mintable token. Then modify the crowdsale to ethereum crowdsale contract example all mentions of transfer ethereum crowdsale contract example mintToken:.
Once you published the crowdsale contract, get its address and go into your Token Contract to execute a Change Ownership function. This will allow your crowdsale to call the Mint Token function as much as it wants.
This opens you to the danger of hostile takeover. At any point during the crowdsale anyone who donates more than the amount already raised will be able to control the whole pie and steal it. There are ethereum crowdsale contract example strategies to prevent that, but implementing will be left as an exercise to the reader:. You have to hire managers, find a trustworthy CFO to handle the accounts, run board meetings and do a bunch of paperwork. Or you can simply leave all that to an Ethereum contract.
It will collect proposals from your backers and submit them through a completely transparent voting process. Crowdsale Raising funds from friends without a third party. Crowdfund your idea Sometimes a good idea takes a lot of funds and collective effort. Tokens and DAOs In this example we will make a better crowdfunding by solving two important problems: If you are just testing, ethereum crowdsale contract example the wallet to the testnet and start mining.
The code Now copy this code and let's create the crowdsale: The following line will instantiate a contract at a given address: How to use Go to contracts and then deploy contract: Put ethers as the funding goal Ethereum crowdsale contract example you are just doing it for a test or demonstration, put the crowdsale duration as minutes, but if you are really raising funds you can put a larger amount, like 45, 31 days.
The address of the token you created should be added to the token reward address Put a gas price, click deploy and wait for your crowdsale to be created. I have gadgets.
Why not sell them all? Raise funds Once the crowdsale has all the necessary tokens, contributing to it is easy and you can do it from any ethereum wallet: You can see the relevant code bit here: Extending the crowdsale What if the crowdsale overshoots its target?
Unlimited crowdsale So we are going to modify our project slightly so that instead of sending a limited set of tokens, the project actually creates a new token out of thin air whenever someone sends them ether. Then modify the crowdsale to rename all mentions of transfer to mintToken: There are many strategies to prevent that, but implementing will be left as an exercise to the reader: A virtual organization where members vote on issues A transparent association based on shareholder voting Your own country with an unchangeable constitution A better delegative democracy.
