author: Witek Radomski <witek@enjin.io>, Andrew Cooke <ac0dem0nk3y@gmail.com>, Philippe Castonguay <pc@horizongames.net>, James Therien <james@turing-complete.com>, Eric Binet <eric@enjin.io>, Ronan Sandford <wighawag@gmail.com>
A standard interface for contracts that manage multiple token types. A single deployed contract may include any combination of fungible tokens, non-fungible tokens or other configurations (e.g. semi-fungible tokens).
This standard outlines a smart contract interface that can represent any number of fungible and non-fungible token types. Existing standards such as ERC-20 require deployment of separate contracts per token type. The ERC-721 standard's token ID is a single non-fungible index and the group of these non-fungibles is deployed as a single contract with settings for the entire collection. In contrast, the ERC-1155 Multi Token Standard allows for each token ID to represent a new configurable token type, which may have its own metadata, supply and other attributes.
Tokens standards like ERC-20 and ERC-721 require a separate contract to be deployed for each token type or collection. This places a lot of redundant bytecode on the Ethereum blockchain and limits certain functionality by the nature of separating each token contract into its own permissioned address. With the rise of blockchain games and platforms like Enjin Coin, game developers may be creating thousands of token types, and a new type of token standard is needed to support them. However, ERC-1155 is not specific to games and many other applications can benefit from this flexibility.
New functionality is possible with this design such as transferring multiple token types at once, saving on transaction costs. Trading (escrow / atomic swaps) of multiple tokens can be built on top of this standard and it removes the need to "approve" individual token contracts separately. It is also easy to describe and mix multiple fungible or non-fungible token types in a single contract.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
**Smart contracts implementing the ERC-1155 standard MUST implement the ERC-165 `supportsInterface` function and MUST return the constant value `true` if `0xd9b67a26` is passed through the `interfaceID` argument.**
@dev Either `TransferSingle` or `TransferBatch` MUST emit when tokens are transferred, including zero value transfers as well as minting or burning (see "Safe Transfer Rules" section of the standard).
@dev Either `TransferSingle` or `TransferBatch` MUST emit when tokens are transferred, including zero value transfers as well as minting or burning (see "Safe Transfer Rules" section of the standard).
The `_from` argument MUST be the address of the holder whose balance is decreased.
The `_to` argument MUST be the address of the recipient whose balance is increased.
The `_ids` argument MUST be the list of tokens being transferred.
The `_values` argument MUST be the list of number of tokens (matching the list and order of tokens specified in _ids) the holder balance is decreased by and match what the recipient balance is increased by.
When minting/creating tokens, the `_from` argument MUST be set to `0x0` (i.e. zero address).
When burning/destroying tokens, the `_to` argument MUST be set to `0x0` (i.e. zero address).
@dev MUST emit when approval for a second party/operator address to manage all tokens for an owner address is enabled or disabled (absence of an event assumes disabled).
MUST emit the `TransferSingle` event to reflect the balance change (see "Safe Transfer Rules" section of the standard).
After the above conditions are met, this function MUST check if `_to` is a smart contract (e.g. code size > 0). If so, it MUST call `onERC1155Received` on `_to` and act appropriately (see "Safe Transfer Rules" section of the standard).
MUST revert if any of the balance(s) of the holder(s) for token(s) in `_ids` is lower than the respective amount(s) in `_values` sent to the recipient.
MUST emit `TransferSingle` or `TransferBatch` event(s) such that all the balance changes are reflected (see "Safe Transfer Rules" section of the standard).
Balance changes and events MUST follow the ordering of the arrays (_ids[0]/_values[0] before _ids[1]/_values[1], etc).
After the above conditions for the transfer(s) in the batch are met, this function MUST check if `_to` is a smart contract (e.g. code size > 0). If so, it MUST call the relevant `ERC1155TokenReceiver` hook(s) on `_to` and act appropriately (see "Safe Transfer Rules" section of the standard).
**Smart contracts MUST implement all of the functions in the `ERC1155TokenReceiver` interface to accept transfers. See "Safe Transfer Rules" for further detail.**
**Smart contracts MUST implement the ERC-165 `supportsInterface` function and signify support for the `ERC1155TokenReceiver` interface to accept transfers. See "ERC1155TokenReceiver ERC-165 rules" for further detail.**
@dev An ERC1155-compliant smart contract MUST call this function on the token recipient contract, at the end of a `safeTransferFrom` after the balance has been updated.
This function MUST return `bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"))` (i.e. 0xf23a6e61) if it accepts the transfer.
@dev An ERC1155-compliant smart contract MUST call this function on the token recipient contract, at the end of a `safeBatchTransferFrom` after the balances have been updated.
This function MUST return `bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))` (i.e. 0xbc197c81) if it accepts the transfer(s).
To be more explicit about how the standard `safeTransferFrom` and `safeBatchTransferFrom` functions MUST operate with respect to the `ERC1155TokenReceiver` hook functions, a list of scenarios and rules follows.
- If the token(s) being sent are part of a hybrid implementation of another standard, that particular standard's rules on sending to a contract MAY now be followed instead. See "Compatibility with other standards" section.
**_Scenario#6 :_** The receiver implements the `ERC1155TokenReceiver` interface and is the recipient of one and only one balance change (e.g. `safeTransferFrom` called).
* The balances for the transfer MUST have been updated before the `ERC1155TokenReceiver` hook is called on a recipient contract.
* The transfer event MUST have been emitted to reflect the balance changes before the `ERC1155TokenReceiver` hook is called on the recipient contract.
* One of `onERC1155Received` or `onERC1155BatchReceived` MUST be called on the recipient contract.
**_Scenario#7 :_** The receiver implements the `ERC1155TokenReceiver` interface and is the recipient of more than one balance change (e.g. `safeBatchTransferFrom` called).
* All balance transfers that are referenced in a call to an `ERC1155TokenReceiver` hook MUST be updated before the `ERC1155TokenReceiver` hook is called on the recipient contract.
* All transfer events MUST have been emitted to reflect current balance changes before an `ERC1155TokenReceiver` hook is called on the recipient contract.
*`onERC1155Received` or `onERC1155BatchReceived` MUST be called on the recipient as many times as necessary such that every balance change for the recipient in the scenario is accounted for.
**_Scenario#8 :_** You are the creator of a contract that implements the `ERC1155TokenReceiver` interface and you forward the token(s) onto another address in one or both of `onERC1155Received` and `onERC1155BatchReceived`.
* Forwarding should be considered acceptance and then initiating a new `safeTransferFrom` or `safeBatchTransferFrom` in a new context.
- The prescribed keccak256 acceptance value magic for the receiver hook being called MUST be returned after forwarding is successful.
* The `_data` argument MAY be re-purposed for the new context.
**_Scenario#9 :_** You are transferring tokens via a non-standard API call i.e. an implementation specific API and NOT `safeTransferFrom` or `safeBatchTransferFrom`.
* In this scenario all balance updates and events output rules are the same as if a standard transfer function had been called.
- i.e. an external viewer MUST still be able to query the balance via a standard function and it MUST be identical to the balance as determined by `TransferSingle` and `TransferBatch` events alone.
* If the receiver is a contract the `ERC1155TokenReceiver` hooks still need to be called on it and the return values respected the same as if a standard transfer function had been called.
- However while the `safeTransferFrom` or `safeBatchTransferFrom` functions MUST revert if a receiving contract does not implement the `ERC1155TokenReceiver` interface, a non-standard function MAY proceed with the transfer.
* MUST emit the `TransferSingle` event to reflect the balance change (see "TransferSingle and TransferBatch event rules" section).
* After the above conditions are met, this function MUST check if `_to` is a smart contract (e.g. code size > 0). If so, it MUST call `onERC1155Received` on `_to` and act appropriately (see "onERC1155Received rules" section).
- The `_data` argument provided by the sender for the transfer MUST be passed with its contents unaltered to the `onERC1155Received` hook function via its `_data` argument.
* MUST revert if length of `_ids` is not the same as length of `_values`.
* MUST revert if any of the balance(s) of the holder(s) for token(s) in `_ids` is lower than the respective amount(s) in `_values` sent to the recipient.
* MUST emit `TransferSingle` or `TransferBatch` event(s) such that all the balance changes are reflected (see "TransferSingle and TransferBatch event rules" section).
* The balance changes and events MUST occur in the array order they were submitted (_ids[0]/_values[0] before _ids[1]/_values[1], etc).
* After the above conditions are met, this function MUST check if `_to` is a smart contract (e.g. code size > 0). If so, it MUST call `onERC1155Received` or `onERC1155BatchReceived` on `_to` and act appropriately (see "onERC1155Received and onERC1155BatchReceived rules" section).
- The `_data` argument provided by the sender for the transfer MUST be passed with its contents unaltered to the `ERC1155TokenReceiver` hook function(s) via their `_data` argument.
*`TransferSingle` SHOULD be used to indicate a single balance transfer has occurred between a `_from` and `_to` pair.
- It MAY be emitted multiple times to indicate multiple balance changes in the transaction, but note that `TransferBatch` is designed for this to reduce gas consumption.
*`TransferBatch` SHOULD be used to indicate multiple balance transfers have occurred between a `_from` and `_to` pair.
- It MAY be emitted with a single element in the list to indicate a singular balance change in the transaction, but note that `TransferSingle` is designed for this to reduce gas consumption.
- When minting/creating tokens, the `_from` argument MUST be set to `0x0` (i.e. zero address). See "Minting/creating and burning/destroying rules".
- When burning/destroying tokens, the `_to` argument MUST be set to `0x0` (i.e. zero address). See "Minting/creating and burning/destroying rules".
* The total value transferred from address `0x0` minus the total value transferred to `0x0` observed via the `TransferSingle` and `TransferBatch` events MAY be used by clients and exchanges to determine the "circulating supply" for a given token ID.
* To broadcast the existence of a token ID with no initial balance, the contract SHOULD emit the `TransferSingle` event from `0x0` to `0x0`, with the token creator as `_operator`, and a `_value` of 0.
* All `TransferSingle` and `TransferBatch` events MUST be emitted to reflect all the balance changes that have occurred before any call(s) to `onERC1155Received` or `onERC1155BatchReceived`.
- To make sure event order is correct in the case of valid re-entry (e.g. if a receiver contract forwards tokens on receipt) state balance and events balance MUST match before calling an external contract.
* The recipient contract MAY accept an increase of its balance by returning the acceptance magic value `bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"))`
- If the return value is `bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"))` the transfer MUST be completed or MUST revert if any other conditions are not met for success.
* If the return value is anything other than `bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"))` the transaction MUST be reverted.
*`onERC1155Received` (and/or `onERC1155BatchReceived`) MAY be called multiple times in a single transaction and the following requirements must be met:
- The set of all calls to `onERC1155Received` and `onERC1155BatchReceived` describes all balance changes that occurred during the transaction in the order submitted.
* The `_values` argument MUST be the list of number of tokens (matching the list and order of tokens specified in `_ids`) the holder balance is decreased by and match what the recipient balance is increased by.
* The recipient contract MAY accept an increase of its balance by returning the acceptance magic value `bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))`
- If the return value is `bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))` the transfer MUST be completed or MUST revert if any other conditions are not met for success.
* If the return value is anything other than `bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))` the transaction MUST be reverted.
*`onERC1155BatchReceived` (and/or `onERC1155Received`) MAY be called multiple times in a single transaction and the following requirements must be met:
- The set of all calls to `onERC1155Received` and `onERC1155BatchReceived` describes all balance changes that occurred during the transaction in the order submitted.
- It MUST return the constant value `true` if `0x01ffc9a7` is passed through the `interfaceID` argument. This signifies ERC-165 support.
- It MUST return the constant value `true` if `0x4e2312e0` is passed through the `interfaceID` argument. This signifies ERC-1155 `ERC1155TokenReceiver` support.
- It MUST NOT consume more than 10,000 gas.
- This keeps it below the ERC-165 requirement of 30,000 gas, reduces the gas reserve needs and minimises possible side-effects of gas exhaustion during the call.
* If an implementation specific API function is used to transfer ERC-1155 token(s) to a contract, the `safeTransferFrom` or `safeBatchTransferFrom` (as appropriate) rules MUST still be followed if the receiver implements the `ERC1155TokenReceiver` interface. If it does not the non-standard implementation SHOULD revert but MAY proceed.
1. An approved user calls a function such as `function myTransferFrom(address _from, address _to, uint256[] calldata _ids, uint256[] calldata _values);`.
5.`myTransferFrom` calls `onERC1155BatchReceived` on `_to` and it reverts or returns an unknown value (if it had returned `bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))` the transfer can be considered successful).
6. At this point `myTransferFrom` SHOULD revert the transaction immediately as receipt of the token(s) was not explicitly accepted by the `onERC1155BatchReceived` function.
7. If however `myTransferFrom` wishes to continue it MUST call `supportsInterface(0x4e2312e0)` on `_to` and if it returns the constant value `true` the transaction MUST be reverted, as it is now known to be a valid receiver and the previous acceptance step failed.
- NOTE: You could have called `supportsInterface(0x4e2312e0)` at a previous step if you wanted to gather and act upon that information earlier, such as in a hybrid standards scenario.
8. If the above call to `supportsInterface(0x4e2312e0)` on `_to` reverts or returns a value other than the constant value `true` the `myTransferFrom` function MAY consider this transfer successful.
* The above example is not exhaustive but illustrates the major points (and shows that most are shared with `safeTransferFrom` and `safeBatchTransferFrom`):
- Balances that are updated MUST have equivalent transfer events emitted.
- A receiver address has to be checked if it is a contract and if so relevant `ERC1155TokenReceiver` hook function(s) have to be called on it.
- Balances (and events associated) that are referenced in a call to an `ERC1155TokenReceiver` hook MUST be updated (and emitted) before the `ERC1155TokenReceiver` hook is called.
- Only non-standard transfer functions MAY allow tokens to be sent to a recipient contract that does NOT implement the necessary `ERC1155TokenReceiver` hook functions. `safeTransferFrom` and `safeBatchTransferFrom` MUST revert in that case (unless it is a hybrid standards implementation see "Compatibility with other standards").
**_Minting/creating and burning/destroying rules:_**
* A mint/create operation is essentially a specialized transfer and MUST follow these rules:
- To broadcast the existence of a token ID with no initial balance, the contract SHOULD emit the `TransferSingle` event from `0x0` to `0x0`, with the token creator as `_operator`, and a `_value` of 0.
- The "TransferSingle and TransferBatch event rules" MUST be followed as appropriate for the mint(s) (i.e. singles or batches) however the `_from` argument MUST be set to `0x0` (i.e. zero address) to flag the transfer as a mint to contract observers.
- __NOTE:__ This includes tokens that are given an initial balance in the contract. The balance of the contract MUST also be able to be determined by events alone meaning initial contract balances (for eg. in construction) MUST emit events to reflect those balances too.
* A burn/destroy operation is essentially a specialized transfer and MUST follow these rules:
- The "TransferSingle and TransferBatch event rules" MUST be followed as appropriate for the burn(s) (i.e. singles or batches) however the `_to` argument MUST be set to `0x0` (i.e. zero address) to flag the transfer as a burn to contract observers.
- When burning/destroying you do not have to actually transfer to `0x0` (that is impl specific), only the `_to` argument in the event MUST be set to `0x0` as above.
* The total value transferred from address `0x0` minus the total value transferred to `0x0` observed via the `TransferSingle` and `TransferBatch` events MAY be used by clients and exchanges to determine the "circulating supply" for a given token ID.
* As mentioned above mint/create and burn/destroy operations are specialized transfers and so will likely be accomplished with custom transfer functions rather than `safeTransferFrom` or `safeBatchTransferFrom`. If so the "Implementation specific transfer API rules" section would be appropriate.
* A contract MAY skip calling the `ERC1155TokenReceiver` hook function(s) if the mint operation is transferring the token(s) to itself. In all other cases the `ERC1155TokenReceiver` rules MUST be followed as appropriate for the implementation (i.e. safe, custom and/or hybrid).
bytes4 constant public ERC1155_ERC165 = 0xd9b67a26; // ERC-165 identifier for the main token standard.
bytes4 constant public ERC1155_ERC165_TOKENRECEIVER = 0x4e2312e0; // ERC-165 identifier for the `ERC1155TokenReceiver` support (i.e. `bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)")) ^ bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))`).
bytes4 constant public ERC1155_ACCEPTED = 0xf23a6e61; // Return value from `onERC1155Received` call if a contract accepts receipt (i.e `bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"))`).
bytes4 constant public ERC1155_BATCH_ACCEPTED = 0xbc197c81; // Return value from `onERC1155BatchReceived` call if a contract accepts receipt (i.e `bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))`).
There have been requirements during the design discussions to have this standard be compatible with existing standards when sending to contract addresses, specifically ERC-721 at time of writing.
To cater for this scenario, there is some leeway with the revert logic should a contract not implement the `ERC1155TokenReceiver` as per "Safe Transfer Rules" section above, specifically "Scenario#3 : The receiver does not implement the necessary `ERC1155TokenReceiver` interface function(s)".
Hence in a hybrid ERC-1155 contract implementation an extra call MUST be made on the recipient contract and checked before any hook calls to `onERC1155Received` or `onERC1155BatchReceived` are made.
1. The implementation MUST call the function `supportsInterface(0x4e2312e0)` on the recipient contract, providing at least 10,000 gas.
2. If the function call succeeds and the return value is the constant value `true` the implementation proceeds as a regular ERC-1155 implementation, with the call(s) to the `onERC1155Received` or `onERC1155BatchReceived` hooks and rules associated.
3. If the function call fails or the return value is NOT the constant value `true` the implementation can assume the recipient contract is not an `ERC1155TokenReceiver` and follow its other standard's rules for transfers.
*__Note that a pure implementation of a single standard is recommended__* rather than a hybrid solution, but an example of a hybrid ERC-1155/ERC-721 contract is linked in the references section under implementations.
An important consideration is that even if the tokens are sent with another standard's rules the *__ERC-1155 transfer events MUST still be emitted.__* This is so the balances can still be determined via events alone as per ERC-1155 standard rules.
The URI value allows for ID substitution by clients. If the string `{id}` exists in any URI, clients MUST replace this with the actual token ID in hexadecimal form. This allows for a large number of tokens to use the same on-chain string by defining a URI once, for that large number of tokens.
Example of such a URI: `https://token-cdn-domain/{id}.json` would be replaced with `https://token-cdn-domain/000000000000000000000000000000000000000000000000000000000004cce0.json` if the client is referring to token ID 314592/0x4CCE0.
The optional `ERC1155Metadata_URI` extension can be identified with the (ERC-165 Standard Interface Detection)[https://eips.ethereum.org/EIPS/eip-165].
If the optional `ERC1155Metadata_URI` extension is included:
* The ERC-165 `supportsInterface` function MUST return the constant value `true` if `0x0e89341c` is passed through the `interfaceID` argument.
* _Changes_ to the URI MUST emit the `URI` event if the change can be expressed with an event (i.e. it isn't dynamic/programmatic).
- An implementation MAY emit the `URI` event during a mint operation but it is NOT mandatory. An observer MAY fetch the metadata uri at mint time from the `uri` function if it was not emitted.
* The `uri` function SHOULD be used to retrieve values if no event was emitted.
* The `uri` function MUST return the same value as the latest event for an `_id` if it was emitted.
* The `uri` function MUST NOT be used to check for the existence of a token as it is possible for an implementation to return a valid string even if the token does not exist.
This JSON schema is loosely based on the "ERC721 Metadata JSON Schema", but includes optional formatting to allow for ID substitution by clients. If the string `{id}` exists in any JSON value, it MUST be replaced with the actual token ID, by all client software that follows this standard.
"description": "The number of decimal places that the token amount should display - e.g. 18, means to divide the token amount by 1000000000000000000 to get its user representation.",
"description": "Describes the asset to which this token represents",
},
"image": {
"type": "string",
"description": "A URI pointing to a resource with mime type image/* representing the asset to which this token represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive.",
},
"properties": {
"type": "object",
"description": "Arbitrary properties. Values may be strings, numbers, object or arrays.",
},
}
}
```
An example of an ERC-1155 Metadata JSON file follows. The properties array proposes some SUGGESTED formatting for token-specific display properties and metadata.
Metadata localization should be standardized to increase presentation uniformity across all languages. As such, a simple overlay method is proposed to enable localization. If the metadata JSON file contains a `localization` attribute, its content MAY be used to provide localized values for fields that need it. The `localization` attribute should be a sub-object with three attributes: `uri`, `default` and `locales`. If the string `{locale}` exists in any URI, it MUST be replaced with the chosen locale by all client software.
"description": "Identifies the asset to which this token represents",
},
"decimals": {
"type": "integer",
"description": "The number of decimal places that the token amount should display - e.g. 18, means to divide the token amount by 1000000000000000000 to get its user representation.",
},
"description": {
"type": "string",
"description": "Describes the asset to which this token represents",
},
"image": {
"type": "string",
"description": "A URI pointing to a resource with mime type image/* representing the asset to which this token represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive.",
},
"properties": {
"type": "object",
"description": "Arbitrary properties. Values may be strings, numbers, object or arrays.",
},
"localization": {
"type": "object",
"required": ["uri", "default", "locales"],
"properties": {
"uri": {
"type": "string",
"description": "The URI pattern to fetch localized data from. This URI should contain the substring `{locale}` which will be replaced with the appropriate locale value before sending the request."
},
"default": {
"type": "string",
"description": "The locale of the default data within the base JSON"
},
"locales": {
"type": "array",
"description": "The list of locales for which data is available. These locales should conform to those defined in the Unicode Common Locale Data Repository (http://cldr.unicode.org/)."
The function `setApprovalForAll` allows an operator to manage one's entire set of tokens on behalf of the approver. To permit approval of a subset of token IDs, an interface such as [ERC-1761 Scoped Approval Interface](https://eips.ethereum.org/EIPS/eip-1761) is suggested.
An owner SHOULD be assumed to always be able to operate on their own tokens regardless of approval status, so should SHOULD NOT have to call `setApprovalForAll` to approve themselves as an operator before they can operate on them.
The `symbol` function (found in the ERC-20 and ERC-721 standards) was not included as we do not believe this is a globally useful piece of data to identify a generic virtual item / asset and are also prone to collisions. Short-hand symbols are used in tickers and currency trading, but they aren't as useful outside of that space.
The `name` function (for human-readable asset names, on-chain) was removed from the standard to allow the Metadata JSON to be the definitive asset name and reduce duplication of data. This also allows localization for names, which would otherwise be prohibitively expensive if each language string was stored on-chain, not to mention bloating the standard interface. While this decision may add a small burden on implementers to host a JSON file containing metadata, we believe any serious implementation of ERC-1155 will already utilize JSON Metadata.
The requirement to emit `TransferSingle` or `TransferBatch` on balance change implies that a valid implementation of ERC-1155 redeploying to a new contract address MUST emit events from the new contract address to replicate the deprecated contract final state. It is valid to only emit a minimal number of events to reflect only the final balance and omit all the transactions that led to that state. The event emit requirement is to ensure that the current state of the contract can always be traced only through events. To alleviate the need to emit events when changing contract address, consider using the proxy pattern, such as described in ERC-1538. This will also have the added benefit of providing a stable contract address for users.
The standard supports `safeTransferFrom` and `onERC1155Received` functions because they are significantly cheaper for single token-type transfers, which is arguably a common use case.
The standard only supports safe-style transfers, making it possible for receiver contracts to depend on `onERC1155Received` or `onERC1155BatchReceived` function to be always called at the end of a transfer.
As the Ethereum ecosystem continues to grow, many dapps are relying on traditional databases and explorer API services to retrieve and categorize data. The ERC-1155 standard guarantees that event logs emitted by the smart contract will provide enough data to create an accurate record of all current token balances. A database or explorer may listen to events and be able to provide indexed and categorized searches of every ERC-1155 token in the contract.
The function `setApprovalForAll` allows an operator to manage one's entire set of tokens on behalf of the approver. It enables frictionless interaction with exchange and trade contracts.
Restricting approval to a certain set of token IDs, quantities or other rules MAY be done with an additional interface or an external contract. The rationale is to keep the ERC-1155 standard as generic as possible for all use-cases without imposing a specific approval scheme on implementations that may not need it. Standard token approval interfaces can be used, such as the suggested [ERC-1761 Scoped Approval Interface](https://github.com/ethereum/EIPs/issues/1761) which is compatible with ERC-1155.
This standard can be used to represent multiple token types for an entire domain. Both fungible and non-fungible tokens can be stored in the same smart-contract.
The `safeBatchTransferFrom` function allows for batch transfers of multiple token IDs and values. The design of ERC-1155 makes batch transfers possible without the need for a wrapper contract, as with existing token standards. This reduces gas costs when more than one token type is included in a batch transfer, as compared to single transfers with multiple transactions.
Another advantage of standardized batch transfers is the ability for a smart contract to respond to the batch transfer in a single operation using `onERC1155BatchReceived`.
It is RECOMMENDED that clients and wallets sort the token IDs and associated values (in ascending order) when posting a batch transfer, as some ERC-1155 implementations offer significant gas cost savings when IDs are sorted. See [Horizon Games - Multi-Token Standard](https://github.com/horizon-games/multi-token-standard) "packed balance" implementation for an example of this.
In order to keep storage requirements light for contracts implementing ERC-1155, enumeration (discovering the IDs and values of tokens) must be done using event logs. It is RECOMMENDED that clients such as exchanges and blockchain explorers maintain a local database containing the token ID, Supply, and URI at the minimum. This can be built from each TransferSingle, TransferBatch, and URI event, starting from the block the smart contract was deployed until the latest block.
ERC-1155 contracts must therefore carefully emit `TransferSingle` or `TransferBatch` events in any instance where tokens are created, minted, transferred or destroyed.
The following strategies are examples of how you MAY mix fungible and non-fungible tokens together in the same contract. The standard does NOT mandate how an implementation must do this.
The top 128 bits of the uint256 `_id` parameter in any ERC-1155 function MAY represent the base token ID, while the bottom 128 bits MAY represent the index of the non-fungible to make it unique.
Non-fungible tokens can be interacted with using an index based accessor into the contract/token data set. Therefore to access a particular token set within a mixed data contract and a particular non-fungible within that set, `_id` could be passed as `<uint128: base token id><uint128: index of non-fungible>`.
To identify a non-fungible set/category as a whole (or a fungible) you COULD just pass in the base id via the `_id` argument as `<uint128: base token id><uint128: zero>`. If your implementation uses this technique this naturally means the index of a non-fungible SHOULD be 1-based.
Inside the contract code the two pieces of data needed to access the individual non-fungible can be extracted with uint128(~0) and the same mask shifted by 128.
balanceOf(baseTokenNFT, msg.sender); // Get balance of the base token for non-fungible set 12345 (this MAY be used to get balance of the user for all of this token set if the implementation wishes as a convenience).
balanceOf(baseTokenNFT + indexNFT, msg.sender); // Get balance of the token at index 50 for non-fungible set 12345 (should be 1 if user owns the individual non-fungible token or 0 if they do not).
balanceOf(baseTokenFT, msg.sender); // Get balance of the fungible base token 54321.
Note that 128 is an arbitrary number, an implementation MAY choose how they would like this split to occur as suitable for their use case. An observer of the contract would simply see events showing balance transfers and mints happening and MAY track the balances using that information alone.
For an observer to be able to determine type (non-fungible or fungible) from an ID alone they would have to know the split ID bits format on a implementation by implementation basis.
The [ERC-1155 Reference Implementation](https://github.com/enjin/erc-1155) is an example of the split ID bits strategy.
Another simple way to represent non-fungibles is to allow a maximum value of 1 for each non-fungible token. This would naturally mirror the real world, where unique items have a quantity of 1 and fungible items have a quantity greater than 1.
- [Here Be Dragons - Going Beyond ERC-20 and ERC-721 To Reduce Gas Cost by ~80%](https://medium.com/horizongames/going-beyond-erc20-and-erc721-9acebd4ff6ef)
