= ERC20

An ERC20 token contract keeps track of xref:tokens.adoc#different-kinds-of-tokens[_fungible_ tokens]: any one token is exactly equal to any other token; no tokens have special rights or behavior associated with them. This makes ERC20 tokens useful for things like a *medium of exchange currency*, *voting rights*, *staking*, and more.

OpenZeppelin Contracts provides many ERC20-related contracts. On the xref:api:token/ERC20.adoc[`API reference`] you'll find detailed information on their properties and usage.

[[constructing-an-erc20-token-contract]]
== Constructing an ERC20 Token Contract

Using Contracts, we can easily create our own ERC20 token contract, which will be used to track _Gold_ (GLD), an internal currency in a hypothetical game.

Here's what our GLD token might look like.

[source,solidity]
----
// contracts/GLDToken.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

contract GLDToken is ERC20 {
    constructor(uint256 initialSupply) public ERC20("Gold", "GLD") {
        _mint(msg.sender, initialSupply);
    }
}
----

Our contracts are often used via https://solidity.readthedocs.io/en/latest/contracts.html#inheritance[inheritance], and here we're reusing xref:api:token/ERC20.adoc#erc20[`ERC20`] for both the basic standard implementation and the xref:api:token/ERC20.adoc#ERC20-name--[`name`], xref:api:token/ERC20.adoc#ERC20-symbol--[`symbol`], and xref:api:token/ERC20.adoc#ERC20-decimals--[`decimals`] optional extensions. Additionally, we're creating an `initialSupply` of tokens, which will be assigned to the address that deploys the contract.

TIP: For a more complete discussion of ERC20 supply mechanisms, see xref:erc20-supply.adoc[Creating ERC20 Supply].

That's it! Once deployed, we will be able to query the deployer's balance:

[source,javascript]
----
> GLDToken.balanceOf(deployerAddress)
1000000000000000000000
----

We can also xref:api:token/ERC20.adoc#IERC20-transfer-address-uint256-[transfer] these tokens to other accounts:

[source,javascript]
----
> GLDToken.transfer(otherAddress, 300000000000000000000)
> GLDToken.balanceOf(otherAddress)
300000000000000000000
> GLDToken.balanceOf(deployerAddress)
700000000000000000000
----

[[a-note-on-decimals]]
== A Note on `decimals`

Often, you'll want to be able to divide your tokens into arbitrary amounts: say, if you own `5 GLD`, you may want to send `1.5 GLD` to a friend, and keep `3.5 GLD` to yourself. Unfortunately, Solidity and the EVM do not support this behavior: only integer (whole) numbers can be used, which poses an issue. You may send `1` or `2` tokens, but not `1.5`.

To work around this, xref:api:token/ERC20.adoc#ERC20[`ERC20`] provides a xref:api:token/ERC20.adoc#ERC20-decimals--[`decimals`] field, which is used to specify how many decimal places a token has. To be able to transfer `1.5 GLD`, `decimals` must be at least `1`, since that number has a single decimal place.

How can this be achieved? It's actually very simple: a token contract can use larger integer values, so that a balance of `50` will represent `5 GLD`, a transfer of `15` will correspond to `1.5 GLD` being sent, and so on.

It is important to understand that `decimals` is _only used for display purposes_. All arithmetic inside the contract is still performed on integers, and it is the different user interfaces (wallets, exchanges, etc.) that must adjust the displayed values according to `decimals`. The total token supply and balance of each account are not specified in `GLD`: you need to divide by `10^decimals` to get the actual `GLD` amount.

You'll probably want to use a `decimals` value of `18`, just like Ether and most ERC20 token contracts in use, unless you have a very special reason not to. When minting tokens or transferring them around, you will be actually sending the number `num GLD * 10^decimals`.

NOTE: By default, `ERC20` uses a value of `18` for `decimals`. To use a different value, you will need to call xref:api:token/ERC20.adoc#ERC20-_setupDecimals-uint8-[_setupDecimals] in your constructor.

So if you want to send `5` tokens using a token contract with 18 decimals, the the method to call will actually be:

```solidity
transfer(recipient, 5 * 10^18);
```

[[Presets]]
== Preset ERC20 contract
A preset ERC20 is available, xref:api:presets/ERC20PresetMinterPauser[ERC20PresetMinterPauser]. It is preset to allow for token minting (create), stop all token transfers (pause) and allow holders to burn (destroy) their tokens. The contract uses xref:access-control.adoc[Access Control] to control access to the minting and pausing functionality.  The account that deploys the contract will be granted the minter and pauser roles, as well as the default admin role.

This contract is ready to deploy without having to write any Solidity code.  It can be used as-is for quick prototyping and testing, but is also suitable for production environments.
