= Creating ERC20 Supply

In this guide, you will learn how to create an ERC20 token with a custom supply mechanism. We will showcase two idiomatic ways to use OpenZeppelin Contracts for this purpose that you will be able to apply to your smart contract development practice.

The standard interface implemented by tokens built on Ethereum is called ERC20, and Contracts includes a widely used implementation of it: the aptly named xref:api:token/ERC20.adoc[`ERC20`] contract. This contract, like the standard itself, is quite simple and bare-bones. In fact, if you try to deploy an instance of `ERC20` as-is it will be quite literally useless... it will have no supply! What use is a token with no supply?

The way that supply is created is not defined in the ERC20 document. Every token is free to experiment with its own mechanisms, ranging from the most decentralized to the most centralized, from the most naive to the most researched, and more.

[[fixed-supply]]
== Fixed Supply

Let's say we want a token with a fixed supply of 1000, initially allocated to the account that deploys the contract. If you've used Contracts v1, you may have written code like the following:

[source,solidity]
----
contract ERC20FixedSupply is ERC20 {
    constructor() {
        totalSupply += 1000;
        balances[msg.sender] += 1000;
    }
}
----

Starting with Contracts v2, this pattern is not only discouraged, but disallowed. The variables `totalSupply` and `balances` are now private implementation details of `ERC20`, and you can't directly write to them. Instead, there is an internal xref:api:token/ERC20.adoc#ERC20-_mint-address-uint256-[`_mint`] function that will do exactly this:

[source,solidity]
----
contract ERC20FixedSupply is ERC20 {
    constructor() ERC20("Fixed", "FIX") {
        _mint(msg.sender, 1000);
    }
}
----

Encapsulating state like this makes it safer to extend contracts. For instance, in the first example we had to manually keep the `totalSupply` in sync with the modified balances, which is easy to forget. In fact, we omitted something else that is also easily forgotten: the `Transfer` event that is required by the standard, and which is relied on by some clients. The second example does not have this bug, because the internal `_mint` function takes care of it.

[[rewarding-miners]]
== Rewarding Miners

The internal xref:api:token/ERC20.adoc#ERC20-_mint-address-uint256-[`_mint`] function is the key building block that allows us to write ERC20 extensions that implement a supply mechanism.

The mechanism we will implement is a token reward for the miners that produce Ethereum blocks. In Solidity, we can access the address of the current block's miner in the global variable `block.coinbase`. We will mint a token reward to this address whenever someone calls the function `mintMinerReward()` on our token. The mechanism may sound silly, but you never know what kind of dynamic this might result in, and it's worth analyzing and experimenting with!

[source,solidity]
----
contract ERC20WithMinerReward is ERC20 {
    constructor() ERC20("Reward", "RWD") {}

    function mintMinerReward() public {
        _mint(block.coinbase, 1000);
    }
}
----

As we can see, `_mint` makes it super easy to do this correctly.

[[automating-the-reward]]
== Automating the Reward

So far our supply mechanism was triggered manually, but `ERC20` also allows us to extend the core functionality of the token through the xref:api:token/ERC20.adoc#ERC20-_update-address-address-uint256-[`_update`] function.

Adding to the supply mechanism from the previous section, we can use this function to mint a miner reward for every token transfer that is included in the blockchain.

```solidity
include::api:example$ERC20WithAutoMinerReward.sol[]
```

[[wrapping-up]]
== Wrapping Up

We've seen how to implement a ERC20 supply mechanism: internally through `_mint`. Hopefully this has helped you understand how to use OpenZeppelin Contracts and some of the design principles behind it, and you can apply them to your own smart contracts.
