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Blockchain use cases
We are going to create a digital token. Tokens in the Ethereum ecosystem can represent any fungible tradable good: Since all tokens implement some basic features in a standard way, this also means that your token will be instantly compatible with the Ethereum wallet and any other client or contract that uses the same standards.
The standard token contract can be quite complex. But in essence a very basic token boils down to this:. So let's start with the basics. On the Solidity Contract Source code text field, type the code below:. A mapping means an associative array, where you associate addresses with balances.
The addresses are in the basic hexadecimal Ethereum format, while the balances are integers, ranging from 0 to quattuorvigintillion. If you don't know how much a quattuorvigintillion is, it's many vigintillions more than anything you are planning to use your tokens for. The public keyword, means that this variable will be accessible by anyone on the blockchain, meaning all balances are public as they need to be, in order for clients to display them. If you published your contract right away, it would work but wouldn't be very useful: So we are going to create a few tokens on startup.
Add this code before the last closing bracket, just under the mapping.. Notice that the function MyToken has the same name as the contract MyToken. This is very important and if you rename one, you have to rename the other too: This function will set the balance of msg.
The choice of 21 million was rather arbitrary, and you can change it to anything you want in the code, but there's a better way: Take a look at the right column beside the contract and you'll see a drop-down list, written pick a contract. Select the "MyToken" contract and you'll see that now it shows a section called Constructor parameters. These are changeable parameters for your token, so you can reuse the same code and only change these variables in the future.
Right now you have a functional contract that created balances of tokens but since there isn't any function to move it, all it does is stay on the same account. So we are going to implement that now. Write the following code before the last bracket. This is a very straightforward function: Right away there's an obvious problem: Since we don't want to handle debt in this particular contract, we are simply going to make a quick check and if the sender doesn't have enough funds the contract execution will simply stop.
It's also to check for overflows, to avoid having a number so big that it becomes zero again. To stop a contract execution mid-execution you can either return or throw The former will cost less gas but it can be more headache as any changes you did to the contract so far will be kept. In the other hand, 'throw' will cancel all contract execution, revert any changes that transaction could have made and the sender will lose all Ether he sent for gas.
But since the Wallet can detect that a contract will throw, it always shows an alert, therefore preventing any Ether to be spent at all. Now all that is missing is having some basic information about the contract. In the near future this can be handled by a token registry, but for now we'll add them directly to the contract:.
And now we update the constructor function to allow all those variables to be set up at the start:. Finally, we now need something called Events. These are special, empty functions that you call to help clients like the Ethereum Wallet keep track of activities happening in the contract. Events should start with a capital letter.
Add this line at the beginning of the contract to declare the event:. What are those notice and param comments, you might ask? That's Natspec an emerging standard for a natural language specification, which allows wallets to show the user a natural language description of what the contract is about to do.
While not currently supported by many wallets, this will change in the future, so it's nice to be prepared. If you aren't there already, open the Ethereum Wallet, go to the contracts tab and then click "deploy new contract".
Now get the token source from above and paste it into the "Solidity source field". If the code compiles without any error, you should see a "pick a contract" drop-down list on the right.
Get it and select the "MyToken" contract. On the right column, you'll see all the parameters you need to personalize your own token. You can tweak them as you please, but for the purpose of this tutorial we recommend you to pick these parameters: Your app should be looking like this:. Scroll to the end of the page and you'll see an estimate of the computation cost of that contract and you can select a fee on how much Ether you are willing to pay for it.
Any excess Ether you don't spend will be returned to you so you can leave the default settings if you wish.
Press "deploy", type your account password and wait a few seconds for your transaction to be picked up. You'll be redirected to the front page where you can see your transaction waiting for confirmations. To send some to a few friends: If you send it to a friend, they will not see anything in their wallet yet. This is because the wallet only tracks tokens it knows about, and you have to add these manually. Now go to the "Contracts" tab and you should see a link to your newly created contract.
Click on it to go to its page. Since this is a very simple contract page there isn't much to do here, just click "copy address" and paste the contract address into a text editor, you'll need it shortly. To add a token to watch, go to the contracts page and then click "Watch Token". A pop-up will appear and you only need to paste the contract address. The token name, symbol and decimal number should be automatically filled but if it's not you can put anything you want it will only affect how it displays on your wallet.
Once you do this, you'll automatically be shown any balance you have of that token and you'll be able to send it to anyone else. And now you have your own crypto token! Tokens by themselves can be useful as value exchange on local communities , ways to keep track of worked hours or other loyalty programs. But can we make a currency have an intrinsic value by making it useful? You can deploy your whole crypto token without ever touching a line of code, but the real magic happens when you start customizing it.
The following sections will be suggestions on functions you can add to your token to make it fit your needs more. You'll notice that there some more functions in your basic token contract, like approve, sendFrom and others. These functions are there for your token to interact with other contracts: So for contracts, you should first approve an amount of tokens they can move from your account and then ping them to let them know they should do their thing - or do the two actions in one, with approveAndCall.
Because many of these functions are having to reimplement the transferring of tokens, it makes sense to change them to an internal function, which can only be called by the contract itself:. Now all your functions that result in the transfer of coins, can do their own checks and then call transfer with the correct parameters.
Notice that this function will move coins from any account to any other, without requiring anyone's permission to do so: All dapps are fully decentralized by default, but that doesn't mean they can't have some sort of central manager, if you want them to.
Maybe you want the ability to mint more coins, maybe you want to ban some people from using your currency. You can add any of those features, but the catch is that you can only add them at the beginning, so all the token holders will always know exactly the rules of the game before they decide to own one. For that to happen, you need a central controller of currency. This could be a simple account, but could also be a contract and therefore the decision on creating more tokens will depend on the contract: In order to do that we'll learn a very useful property of contracts: Inheritance allows a contract to acquire properties of a parent contract, without having to redefine all of them.
This makes the code cleaner and easier to reuse. This creates a very basic contract that doesn't do anything except define some generic functions about a contract that can be "owned". Now the next step is just to add the text is owned to your contract:.
This means that all the functions inside MyToken now can access the variable owner and the modifier onlyOwner.
The contract also gets a function to transfer ownership. Since it might be interesting to set the owner of the contract at startup, you can also add this to the constructor function:. Suppose you want the amount of coins in circulation to change. This is the case when your tokens actually represent an off blockchain asset like gold certificates or government currencies and you want the virtual inventory to reflect the real one.
This might also be the case when the currency holders expect some control of the price of the token, and want to issue or remove tokens from circulation. First, we need to add a variable to store the totalSupply and assign it to our constructor function. Notice the modifier onlyOwner on the end of the function name. This means that this function will be rewritten at compilation to inherit the code from the modifier onlyOwner we had defined before.
This function's code will be inserted where there's an underline on the modifier function, meaning that this particular function can only be called by the account that is set as the owner. Just add this to a contract with an owner modifier and you'll be able to create more coins. Depending on your use case, you might need to have some regulatory hurdles on who can and cannot use your tokens. For that to happen, you can add a parameter that enables the contract owner to freeze or unfreeze assets.
Add this variable and function anywhere inside the contract. You can put them anywhere but for good practice we recommend you put the mappings with the other mappings and events with the other events. With this code, all accounts are unfrozen by default but the owner can set any of them into a freeze state by calling Freeze Account.