= Extending Contracts

Most of the OpenZeppelin Contracts are expected to be used via https://solidity.readthedocs.io/en/latest/contracts.html#inheritance[inheritance]: you will _inherit_ from them when writing your own contracts.

This is the commonly found `is` syntax, like in `contract MyToken is ERC20`.

[NOTE]
====
Unlike ``contract``s, Solidity ``library``s are not inherited from and instead rely on the https://solidity.readthedocs.io/en/latest/contracts.html#using-for[`using for`] syntax.

OpenZeppelin Contracts has some ``library``s: most are in the xref:api:utils.adoc[Utils] directory.
====

== Overriding

Inheritance is often used to add the parent contract's functionality to your own contract, but that's not all it can do. You can also _change_ how some parts of the parent behave using _overrides_.

For example, imagine you want to change xref:api:access.adoc#AccessControl[`AccessControl`] so that xref:api:access.adoc#AccessControl-revokeRole-bytes32-address-[`revokeRole`] can no longer be called. This can be achieved using overrides:

```solidity
// contracts/ModifiedAccessControl.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import "@openzeppelin/contracts/access/AccessControl.sol";

contract ModifiedAccessControl is AccessControl {
    // Override the revokeRole function
    function revokeRole(bytes32, address) public override {
        revert("ModifiedAccessControl: cannot revoke roles");
    }
}
```

The old `revokeRole` is then replaced by our override, and any calls to it will immediately revert. We cannot _remove_ the function from the contract, but reverting on all calls is good enough.

=== Calling `super`

Sometimes you want to _extend_ a parent's behavior, instead of outright changing it to something else. This is where `super` comes in.

The `super` keyword will let you call functions defined in a parent contract, even if they are overridden. This mechanism can be used to add additional checks to a function, emit events, or otherwise add functionality as you see fit.

TIP: For more information on how overrides work, head over to the https://solidity.readthedocs.io/en/latest/contracts.html#index-17[official Solidity documentation].

Here is a modified version of xref:api:access.adoc#AccessControl[`AccessControl`] where xref:api:access.adoc#AccessControl-revokeRole-bytes32-address-[`revokeRole`] cannot be used to revoke the `DEFAULT_ADMIN_ROLE`:


```solidity
// contracts/ModifiedAccessControl.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import "@openzeppelin/contracts/access/AccessControl.sol";

contract ModifiedAccessControl is AccessControl {
    function revokeRole(bytes32 role, address account) public override {
        require(
            role != DEFAULT_ADMIN_ROLE,
            "ModifiedAccessControl: cannot revoke default admin role"
        );

        super.revokeRole(role, account);
    }
}
```

The `super.revokeRole` statement at the end will invoke ``AccessControl``'s original version of `revokeRole`, the same code that would've run if there were no overrides in place.

NOTE: As of v3.0.0, `view` functions are not `virtual` in OpenZeppelin, and therefore cannot be overridden. We're considering https://github.com/OpenZeppelin/openzeppelin-contracts/issues/2154[lifting this restriction] in an upcoming release. Let us know if this is something you care about!

[[using-hooks]]
== Using Hooks

Sometimes, in order to extend a parent contract you will need to override multiple related functions, which leads to code duplication and increased likelihood of bugs.

For example, consider implementing safe xref:api:token/ERC20.adoc#ERC20[`ERC20`] transfers in the style of xref:api:token/ERC721.adoc#IERC721Receiver[`IERC721Receiver`]. You may think overriding xref:api:token/ERC20.adoc#ERC20-transfer-address-uint256-[`transfer`] and xref:api:token/ERC20.adoc#ERC20-transferFrom-address-address-uint256-[`transferFrom`] would be enough, but what about  xref:api:token/ERC20.adoc#ERC20-_transfer-address-address-uint256-[`_transfer`] and xref:api:token/ERC20.adoc#ERC20-_mint-address-uint256-[`_mint`]? To prevent you from having to deal with these details, we introduced **hooks**.

Hooks are simply functions that are called before or after some action takes place. They provide a centralized point to _hook into_ and extend the original behavior.

Here's how you would implement the `IERC721Receiver` pattern in `ERC20`, using the xref:api:token/ERC20.adoc#ERC20-_beforeTokenTransfer-address-address-uint256-[`_beforeTokenTransfer`] hook:

```solidity
pragma solidity ^0.6.0;

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

contract ERC20WithSafeTransfer is ERC20 {
    function _beforeTokenTransfer(address from, address to, uint256 amount)
        internal virtual override
    {
        super._beforeTokenTransfer(from, to, amount);

        require(_validRecipient(to), "ERC20WithSafeTransfer: invalid recipient");
    }

    function _validRecipient(address to) private view returns (bool) {
        ...
    }
    
    ...
}
```

Using hooks this way leads to cleaner and safer code, without having to rely on a deep understanding of the parent's internals.

[NOTE]
====
Hooks are a new feature of OpenZeppelin Contracts v3.0.0, and we're eager to learn how you plan to use them!

So far, the only available hook is `_beforeTransferHook`, in all of xref:api:token/ERC20.adoc#ERC20-_beforeTokenTransfer-address-address-uint256-[`ERC20`], xref:api:token/ERC721.adoc#ERC721-_beforeTokenTransfer-address-address-uint256-[`ERC721`], xref:api:token/ERC777.adoc#ERC777-_beforeTokenTransfer-address-address-address-uint256-[`ERC777`] and xref:api:token/ERC1155.adoc#ERC1155-_beforeTokenTransfer-address-address-address-uint256---uint256---bytes-[`ERC1155`]. If you have ideas for new hooks, let us know!
====

=== Rules of Hooks

There's a few guidelines you should follow when writing code that uses hooks in order to prevent issues. They are very simple, but do make sure you follow them:

1. Whenever you override a parent's hook, re-apply the `virtual` attribute to the hook. That will allow child contracts to add more functionality to the hook.
2. **Always** call the parent's hook in your override using `super`. This will make sure all hooks in the inheritance tree are called: contracts like xref:api:token/ERC20.adoc#ERC20Pausable[`ERC20Pausable`] rely on this behavior.

```solidity
contract MyToken is ERC20 {
    function _beforeTokenTransfer(address from, address to, uint256 amount)
        internal virtual override // Add virtual here!
    {
        super._beforeTokenTransfer(from, to, amount); // Call parent hook
        ...
    }
}
```
That's it! Enjoy simpler code using hooks!

