Drafts
Contracts in this category should be considered unstable. They are as thoroughly reviewed as everything else in OpenZeppelin Contracts, but we have doubts about their API so we don’t commit to backwards compatibility. This means these contracts can receive breaking changes in a minor version, so you should pay special attention to the changelog when upgrading. For anything that is outside of this category you can read more about API Stability.
| This page is incomplete. We’re working to improve it for the next release. Stay tuned! |
ERC 20
ERC20Migrator
This contract can be used to migrate an ERC20 token from one
contract to another, where each token holder has to opt-in to the migration.
To opt-in, users must approve for this contract the number of tokens they
want to migrate. Once the allowance is set up, anyone can trigger the
migration to the new token contract. In this way, token holders "turn in"
their old balance and will be minted an equal amount in the new token.
The new token contract must be mintable. For the precise interface refer to
OpenZeppelin’s ERC20Mintable, but the only functions that are needed are
MinterRole.isMinter and ERC20Mintable.mint. The migrator will check
that it is a minter for the token.
The balance from the legacy token will be transferred to the migrator, as it
is migrated, and remain there forever.
Although this contract can be used in many different scenarios, the main
motivation was to provide a way to migrate ERC20 tokens into an upgradeable
version of it using ZeppelinOS. To read more about how this can be done
using this implementation, please follow the official documentation site of
ZeppelinOS: https://docs.zeppelinos.org/docs/erc20_onboarding.html
Example of usage:
const migrator = await ERC20Migrator.new(legacyToken.address);
await newToken.addMinter(migrator.address);
await migrator.beginMigration(newToken.address);beginMigration(contract ERC20Mintable newToken_) public
Begins the migration by setting which is the new token that will be minted. This contract must be a minter for the new token.
ERC20Snapshot
Inspired by Jordi Baylina’s MiniMeToken to record historical balances.
When a snapshot is made, the balances and total supply at the time of the snapshot are recorded for later access.
To make a snapshot, call the Snapshot function, which will emit the Snapshot event and return a snapshot id.
To get the total supply from a snapshot, call the function totalSupplyAt with the snapshot id.
To get the balance of an account from a snapshot, call the balanceOfAt function with the snapshot id and the
account address.
TokenVesting
A token holder contract that can release its token balance gradually like a typical vesting scheme, with a cliff and vesting period. Optionally revocable by the owner.
Miscellaneous
Counters
Provides counters that can only be incremented or decremented by one. This can be used e.g. to track the number of elements in a mapping, issuing ERC721 ids, or counting request ids.
Include with using Counters for Counters.Counter;
Since it is not possible to overflow a 256 bit integer with increments of one, increment can skip the SafeMath
overflow check, thereby saving gas. This does assume however correct usage, in that the underlying _value is never
directly accessed.