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merge api docs changes Squashed commit of the following: commit 06243c3e8e86074ff8e9e3f22b7829a2c315d707 Merge: 991882ec 99373558 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 18:15:37 2019 -0300 Merge branch 'api-docs' into api-docs-merge commit 991882eca5bb8a3391995154bfb9d53d8a69cb4f Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 18:08:02 2019 -0300 manually apply docs changes and renamings commit fa1f6e97dd67a76c3cd828d0a5e19b4ac6c37acb Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 17:36:03 2019 -0300 move functions to new order commit 99373558e3af4905d29bc6f3f542ba93d28a24ab Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 16:23:40 2019 -0300 add missing docs links commit d180d6c36a6f5460e85473ee5a18992d1449a6ff Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 16:14:12 2019 -0300 update solidity-docgen dependency fixes uri encoded links commit faab0e50da91cd2f0a409e3ad32e2db127ad319a 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5ec47d62785e1d6e5f8e91edca58f2dc7f87d7a3 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 15:14:49 2019 -0300 fix escrow docs ordering commit cdcdc909b16f219a9a3272036b6a8f21e34b48ef Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 13:35:07 2019 -0300 add wip notice commit 987e2951ae93211c8c70c8288e30573555c57830 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 13:09:35 2019 -0300 update openzeppelin-docsite dependency fixes links to old versions commit b00d22c0affac2e2108df8b773dfa1706afcb44e Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 13:09:28 2019 -0300 fix guide links commit f112a9400c5e5ad495c8e0fdb972e26987b34244 Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 20:42:37 2019 -0300 update docsite commit 68aacdd56a29e35a84f6732f9293612bbcaf7520 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Wed May 22 20:00:39 2019 -0300 ERC20Capped commit 4edce78bab2c6d140f3ea3f33db71e92ca4d8aaf 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22 16:58:31 2019 -0300 update docgen commit 2a8c7a378e8962a5baeb334b2492815f05075f98 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Wed May 22 14:36:35 2019 -0300 Util docs. commit 327ae8ff45a1a523c7591bf4996c4a9b52d7ec7a Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 13:08:50 2019 -0300 add missing drafts commit 5e7f71335ac8423c0e363ae8c7ad9b2977f202f8 Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 12:47:41 2019 -0300 tweak ierc20 docs commit cd0e86a0b712f74ffd406e072d4b1fbf4dd2c176 Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 12:46:45 2019 -0300 add some erc721 docs commit e081184159417f71da14bc0fc110b7b11e29d75d Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 12:41:46 2019 -0300 update docsite commit 0beb75784022419d47123c2a9fe7a5f1eb87f9b2 Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 12:22:27 2019 -0300 correct drafts structure commit 2e94b287c7cead7a6c0603205670566461c31abb 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<nicolas.venturo@gmail.com> Date: Thu May 16 20:01:46 2019 -0300 First draft of ERC777 docs. commit 985c5d305329fd9d400120d86dce5c386e19cd50 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 16 19:14:32 2019 -0300 Final draft for IERC777. commit bf53f133d987b67f938a329e6d659ba3483aab0b Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 16 19:13:37 2019 -0300 more work on ERC20 api docs commit b7c250b7cb4669448cfab5535c4ff70b94a15635 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 16 17:08:47 2019 -0300 Fix typo. commit 197bbfbfc67a09607ead492b834879c62b3d905a Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 16 17:05:14 2019 -0300 Initial draft of IERC777. commit 7dc3b55161c860437a8f13a2ce5808b1c3dd70a2 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 16 11:58:32 2019 -0300 add payment docs structure commit da16fc4480243181e58c3440e977e76a91a1839a Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 16 16:05:33 2019 -0300 Initial ERC777 docstrings. commit 9f6a7e35bd2f045e6063ca2f93c67b792c0ef47c Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 15 22:13:17 2019 -0300 partial pass through ERC20 docs
6 years ago
---
id: utilities
title: Utilities
---
OpenZeppelin provides a ton of useful utilities that you can use in your project. Here are some of the more popular ones:
## Cryptography
- [`ECDSA`](api/cryptography#ecdsa) — provides functions for recovering and managing Ethereum account ECDSA signatures:
- to use it, declare: `using ECDSA for bytes32;`
- signatures are tightly packed, 65 byte `bytes` that look like `{v (1)} {r (32)} {s (32)}`
- this is the default from `web3.eth.sign` so you probably don't need to worry about this format
- recover the signer using [`myDataHash.recover(signature)`](api/cryptography#ECDSA.recover(bytes32,bytes))
- if you are using `eth_personalSign`, the signer will hash your data and then add the prefix `\x19Ethereum Signed Message:\n`, so if you're attempting to recover the signer of an Ethereum signed message hash, you'll want to use [`toEthSignedMessageHash`](api/cryptography#ECDSA.toEthSignedMessageHash(bytes32))
Use these functions in combination to verify that a user has signed some information on-chain:
```solidity
keccack256(
abi.encodePacked(
someData,
moreData
)
)
.toEthSignedMessageHash()
.recover(signature)
```
- [`MerkleProof`](api/cryptography#merkleproof) — provides [`verify`](api/cryptography#MerkleProof.verify(bytes32[],bytes32,bytes32)) for verifying merkle proofs.
## Introspection
In Solidity, it's frequently helpful to know whether or not a contract supports an interface you'd like to use. ERC165 is a standard that helps do runtime interface detection. OpenZeppelin provides some helpers, both for implementing ERC165 in your contracts and querying other contracts:
- [`IERC165`](api/introspection#ierc165) — this is the ERC165 interface that defines [`supportsInterface`](api/introspection#IERC165.supportsInterface(bytes4)). When implementing ERC165, you'll conform to this interface.
- [`ERC165`](api/introspection#erc165) — inherit this contract if you'd like to support interface detection using a lookup table in contract storage. You can register interfaces using [`_registerInterface(bytes4)`](api/introspection#ERC165._registerInterface(bytes4)): check out example usage as part of the ERC721 implementation.
- [`ERC165Checker`](api/introspection#erc165checker) — ERC165Checker simplifies the process of checking whether or not a contract supports an interface you care about.
- include with `using ERC165Checker for address;`
- [`myAddress._supportsInterface(bytes4)`](api/introspection#ERC165Checker._supportsInterface(address,bytes4))
- [`myAddress._supportsAllInterfaces(bytes4[])`](api/introspection#ERC165Checker._supportsAllInterfaces(address,bytes4[]))
```solidity
contract MyContract {
using ERC165Checker for address;
bytes4 private InterfaceId_ERC721 = 0x80ac58cd;
/**
* @dev transfer an ERC721 token from this contract to someone else
*/
function transferERC721(
address token,
address to,
uint256 tokenId
)
public
{
require(token.supportsInterface(InterfaceId_ERC721), "IS_NOT_721_TOKEN");
IERC721(token).transferFrom(address(this), to, tokenId);
}
}
```
## Math
The most popular math related library OpenZeppelin provides is [`SafeMath`](api/math#safemath), which provides mathematical functions that protect your contract from overflows and underflows.
Include the contract with `using SafeMath for uint256;` and then call the functions:
- `myNumber.add(otherNumber)`
- `myNumber.sub(otherNumber)`
- `myNumber.div(otherNumber)`
- `myNumber.mul(otherNumber)`
- `myNumber.mod(otherNumber)`
Easy!
## Payment
Want to split some payments between multiple people? Maybe you have an app that sends 30% of art purchases to the original creator and 70% of the profits to the current owner; you can build that with [`PaymentSplitter`](api/payment#paymentsplitter)!
In solidity, there are some security concerns with blindly sending money to accounts, since it allows them to execute arbitrary code. You can read up on these security concerns in the [Ethereum Smart Contract Best Practices](https://consensys.github.io/smart-contract-best-practices/) website. One of the ways to fix reentrancy and stalling problems is, instead of immediately sending Ether to accounts that need it, you can use [`PullPayment`](api/payment#pullpayment), which offers an [`_asyncTransfer`](api/payment#PullPayment._asyncTransfer(address,uint256)) function for sending money to something and requesting that they [`withdrawPayments()`](api/payment#PullPayment.withdrawPayments(address%20payable)) it later.
If you want to Escrow some funds, check out [`Escrow`](api/payment#escrow) and [`ConditionalEscrow`](api/payment#conditionalescrow) for governing the release of some escrowed Ether.
### Misc
Want to check if an address is a contract? Use [`Address`](api/utils#address) and [`Address.isContract()`](api/utils#Address.isContract(address)).
merge api docs changes Squashed commit of the following: commit 06243c3e8e86074ff8e9e3f22b7829a2c315d707 Merge: 991882ec 99373558 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 18:15:37 2019 -0300 Merge branch 'api-docs' into api-docs-merge commit 991882eca5bb8a3391995154bfb9d53d8a69cb4f Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 18:08:02 2019 -0300 manually apply docs changes and renamings commit fa1f6e97dd67a76c3cd828d0a5e19b4ac6c37acb Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 17:36:03 2019 -0300 move functions to new order commit 99373558e3af4905d29bc6f3f542ba93d28a24ab Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 16:23:40 2019 -0300 add missing docs links commit d180d6c36a6f5460e85473ee5a18992d1449a6ff Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 16:14:12 2019 -0300 update solidity-docgen dependency fixes uri encoded links commit faab0e50da91cd2f0a409e3ad32e2db127ad319a Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 16:05:03 2019 -0300 update openzeppelin-docsite and solidity-docgen dependencies add visibility specifiers commit ef305268bb2735e488e35d16819a4b432b3a35e3 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 23 15:36:45 2019 -0300 Fix guide links. commit 339b20dbfa2d5f6ea02e63c2f3fdcba0fe879c3c Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 23 13:37:51 2019 -0300 Fix typos. commit 6c7b97460578b9eea90b53c280454e361f8f0052 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 15:26:29 2019 -0300 fix utilities guide links commit 0e7692a8fd8516a11becc4121d77d792439600b1 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 15:23:19 2019 -0300 update solidity-docgen dependency commit ebb8a8651516ece21736c6c3b2577eb1b3487651 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 15:15:01 2019 -0300 fix utilities guide links commit 5ec47d62785e1d6e5f8e91edca58f2dc7f87d7a3 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 15:14:49 2019 -0300 fix escrow docs ordering commit cdcdc909b16f219a9a3272036b6a8f21e34b48ef Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 13:35:07 2019 -0300 add wip notice commit 987e2951ae93211c8c70c8288e30573555c57830 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 13:09:35 2019 -0300 update openzeppelin-docsite dependency fixes links to old versions commit b00d22c0affac2e2108df8b773dfa1706afcb44e Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 23 13:09:28 2019 -0300 fix guide links commit f112a9400c5e5ad495c8e0fdb972e26987b34244 Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 20:42:37 2019 -0300 update docsite commit 68aacdd56a29e35a84f6732f9293612bbcaf7520 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Wed May 22 20:00:39 2019 -0300 ERC20Capped commit 4edce78bab2c6d140f3ea3f33db71e92ca4d8aaf 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Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 11:56:25 2019 -0300 fix docsite-start script commit 0fa4160484309d0851584fe57c0d81a3600977db Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 11:47:44 2019 -0300 improve docsite start script (automatically watch docgen) commit 9d571897cc03bee92035964cf7a2cfeda1e2f690 Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 22 11:30:37 2019 -0300 update solidity-docgen commit 82980f5aefbdfb8a9815a3b7b0e88e970b65dd5d Author: Francisco Giordano <frangio.1@gmail.com> Date: Tue May 21 19:15:13 2019 -0300 edit docs for Secondary commit 00d7a005b0530bee730b77a1b69a95f1b4ffe315 Author: Francisco Giordano <frangio.1@gmail.com> Date: Tue May 21 19:15:13 2019 -0300 edit docs for ownable commit b0c4c2bdf83eca5d4a71792daac603236733d46e Author: Francisco Giordano <frangio.1@gmail.com> Date: Tue May 21 18:27:13 2019 -0300 change title of Math section commit deb788583f191780e55b7f673520eaf13a5c7e23 Author: Francisco Giordano <frangio.1@gmail.com> Date: Tue May 21 18:26:59 2019 -0300 capitalization commit f2bcf85d343ea4a0739fe22a77b1e22c2296ddea Author: Francisco Giordano <frangio.1@gmail.com> Date: Tue May 21 18:26:06 2019 -0300 edit docs for Pausable commit 73ba0cf43dbb44c39c1bf2ee63ef9fe558faa919 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Sat May 18 19:08:06 2019 -0300 Crypto docs. commit 9d6fc6223f51cf2321b2e3217c512579654c3917 Merge: 7e21f8f7 9f1cec12 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Fri May 17 17:23:15 2019 -0300 Merge branch 'api-docs-777' into api-docs commit 9f1cec12e3351fb1b5fc0b59f5ded39928064a56 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Fri May 17 17:22:54 2019 -0300 ERC777 done. commit 7e21f8f7b6982d2f92df302cdf6ec62522d8ffff Author: Francisco Giordano <frangio.1@gmail.com> Date: Fri May 17 16:39:47 2019 -0300 add math docs commit f18d1f17023b6e5b42ae04fc38aa1170e6863864 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 16 20:01:46 2019 -0300 First draft of ERC777 docs. commit 985c5d305329fd9d400120d86dce5c386e19cd50 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 16 19:14:32 2019 -0300 Final draft for IERC777. commit bf53f133d987b67f938a329e6d659ba3483aab0b Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 16 19:13:37 2019 -0300 more work on ERC20 api docs commit b7c250b7cb4669448cfab5535c4ff70b94a15635 Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 16 17:08:47 2019 -0300 Fix typo. commit 197bbfbfc67a09607ead492b834879c62b3d905a Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 16 17:05:14 2019 -0300 Initial draft of IERC777. commit 7dc3b55161c860437a8f13a2ce5808b1c3dd70a2 Author: Francisco Giordano <frangio.1@gmail.com> Date: Thu May 16 11:58:32 2019 -0300 add payment docs structure commit da16fc4480243181e58c3440e977e76a91a1839a Author: Nicolás Venturo <nicolas.venturo@gmail.com> Date: Thu May 16 16:05:33 2019 -0300 Initial ERC777 docstrings. commit 9f6a7e35bd2f045e6063ca2f93c67b792c0ef47c Author: Francisco Giordano <frangio.1@gmail.com> Date: Wed May 15 22:13:17 2019 -0300 partial pass through ERC20 docs
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Want to keep track of some numbers that increment by 1 every time you want another one? Check out [`Counter`](api/drafts#counter). This is especially useful for creating incremental ERC721 `tokenId`s like we did in the last section.