EIPs/EIPS/eip-165.md

## Preamble

```
EIP: 165
Title: ERC-165 Standard Interface Detection
Author: Christian Reitwießner <chris@ethereum.org>, Nick Johnson <nick@ethereum.org>, Fabian Vogelsteller <fabian@frozeman.de>, Jordi Baylina <jordi@baylina.cat>, Konrad Feldmeier <konrad.feldmeier@brainbot.com>, William Entriken <github.com@phor.net>
Type: Standard Track
Category: ERC
Status: Draft
Created: 2018-01-23
```

## Simple Summary

Creates a standard method to publish and detect what interfaces a smart contract implements.

## Abstract

Herein, we standardize the following:

1. How interfaces are identified
2. How a contract will publish the interfaces it implements
3. How to detect if a contract implements ERC-165
4. How to detect if a contract implements any given interface

## Motivation

For some "standard interfaces" like [the ERC-20 token interface](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md), it is sometimes useful to query whether a contract supports the interface and if yes, which version of the interface, in order to adapt the way in which the contract is to be interacted with. Specifically for ERC-20, a version identifier has already been proposed. This proposal stadardizes the concept of interfaces and standardizes the identification (naming) of interfaces.

## Specification

### How Interfaces are Identified

For this standard, an *interface* is a set of [function selectors as defined by the Ethereum ABI](http://solidity.readthedocs.io/en/develop/abi-spec.html#function-selector). This a subset of [Solidity's concept of interfaces](http://solidity.readthedocs.io/en/develop/abi-spec.html) and the  `interface` keyword definition which also defines return types, mutability and events.

We define the interface identifier as the XOR of all function selectors in the interface. This code example shows how to calculate an interface identifier:

```solidity
pragma solidity ^0.4.20;

interface Solidity101 {
    function hello() external pure;
    function world(int) external pure;
}

contract Selector {
    function calculateSelector() public pure returns (bytes4) {
        Solidity101 i;
        return i.hello.selector ^ i.world.selector;
    }
}
```

Note: interfaces do not permit optional functions, therefore, the interface identity will not include them.

### How a Contract will Publish the Interfaces it Implements

A contract that is compliant with ERC-165 shall implement the following interface (referred as `ERC165.sol`):

```solidity
pragma solidity ^0.4.20;

interface ERC165 {
    /// @notice Query if a contract implements an interface
    /// @param interfaceID The interface identifier, as specified in ERC-165
    /// @dev Interface identification is specified in ERC-165. This function
    ///  uses less than 30,000 gas.
    /// @return `true` if the contract implements `interfaceID` and
    ///  `interfaceID` is not 0xffffffff, `false` otherwise
    function supportsInterface(bytes4 interfaceID) external view returns (bool);
}
```

The interface identifier for this interface is `0x01ffc9a7`. You can calculate this by running `        bytes4(keccak256('supportsInterface(bytes4)'));` or using the `Selector` contract above.

Therefore the implementing contract will have a `supportsInterface` function that returns:

- `true` when `interfaceID` is `0x01ffc9a7` (EIP165 interface)
- `false` when `interfaceID` is `0xffffffff`
- `true` for any other `interfaceID` this contract implements
- `false` for any other `interfaceID`

This function must return a bool and use at most 30,000 gas.

Implementation note, there are several logical ways to implement this function. Please see the example implementations and the discussion on gas usage.

### How to Detect if a Contract Implements ERC-165

1. The source contact makes a `STATICCALL` to the destination address with input data: `0x01ffc9a701ffc9a700000000000000000000000000000000000000000000000000000000` and gas 30,000. This corresponds to `contract.supportsInterface(0x01ffc9a7)`.
2. If the call fails or return false, the destination contract does not implement ERC-165.
3. If the call returns true, a second call is made with input data `0x01ffc9a7ffffffff00000000000000000000000000000000000000000000000000000000`.
4. If the second call fails or returns true, the destination contract does not implement ERC-165.
5. Otherwise it implements ERC-165.

### How to Detect if a Contract Implements any Given Interface

1. If you are not sure if the contract implements ERC-165, use the above procedure to confirm.
2. If it does not implement ERC-165, then you will have to see what methods it uses the old-fashioned way.
3. If it implements ERC-165 then just call `supportsInterface(interfaceID)` to determine if it implements an interface you can use.

## Rationale

We tried to keep this specification as simple as possible. This implementation is also compatible with the current Solidity version.

## Backwards Compatibility

The mechanism described above (with `0xffffffff`) should work with most of the contracts previous to this standard to determine that they do not implement ERC-165.

Also [the ENS](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-137.md) already implements this EIP.

## Test Cases

Following is a contract that detects which interfaces other contracts implement. From @fulldecent and @jbaylina.

```solidity
pragma solidity ^0.4.20;

contract ERC165Query {
    bytes4 constant InvalidID = 0xffffffff;
    bytes4 constant ERC165ID = 0x01ffc9a7;

    function doesContractImplementInterface(address _contract, bytes4 _interfaceId) external view returns (bool) {
        uint256 success;
        uint256 result;
        
        (success, result) = noThrowCall(_contract, ERC165ID);
        if ((success==0)||(result==0)) {
            return false;
        }

        (success, result) = noThrowCall(_contract, InvalidID);
        if ((success==0)||(result!=0)) {
            return false;
        }

        (success, result) = noThrowCall(_contract, _interfaceId);
        if ((success==1)&&(result==1)) {
            return true;
        }
        return false;
    }

    function noThrowCall(address _contract, bytes4 _interfaceId) constant internal returns (uint256 success, uint256 result) {
        bytes4 erc165ID = ERC165ID;

        assembly {
                let x := mload(0x40)               // Find empty storage location using "free memory pointer"
                mstore(x, erc165ID)                // Place signature at begining of empty storage
                mstore(add(x, 0x04), _interfaceId) // Place first argument directly next to signature

                success := staticcall(
                                    30000,         // 30k gas
                                    _contract,     // To addr
                                    x,             // Inputs are stored at location x
                                    0x20,          // Inputs are 32 bytes long
                                    x,             // Store output over input (saves space)
                                    0x20)          // Outputs are 32 bytes long

                result := mload(x)                 // Load the result
        }
    }
}
```

## Implementation

This approach uses a `view` function implementation of `supportsInterface`. The execution cost is 586 gas for any input. But contract initialization requires storing each interface (`SSTORE` is 20,000 gas). The `ERC165MappingImplementation` contract is generic and reusable.

```solidity
pragma solidity ^0.4.20;

import "./ERC165.sol";

contract ERC165MappingImplementation is ERC165 {
    /// @dev You must not set element 0xffffffff to true
    mapping(bytes4 => bool) internal supportedInterfaces;

    function ERC165MappingImplementation() internal {
        supportedInterfaces[this.supportsInterface.selector] = true;
    }

    function supportsInterface(bytes4 interfaceID) external view returns (bool) {
        return supportedInterfaces[interfaceID];
    }
}

interface Simpson {
    function is2D() external returns (bool);
    function skinColor() external returns (string);
}

contract Lisa is ERC165MappingImplementation, Simpson {
    function Lisa() public {
        supportedInterfaces[this.is2D.selector ^ this.skinColor.selector] = true;
    }
    
    function is2D() external returns (bool){}
    function skinColor() external returns (string){}
}
```

Following is a `pure` function implementation of `supportsInterface`. The worst-case execution cost is 236 gas, but increases linearly with a higher number of supported interfaces.

```solidity
pragma solidity ^0.4.20;

import "./ERC165.sol";

interface Simpson {
    function is2D() external returns (bool);
    function skinColor() external returns (string);
}

contract Homer is ERC165, Simpson {
    function supportsInterface(bytes4 interfaceID) external view returns (bool) {
        return 
          interfaceID == this.supportsInterface.selector || // ERC165
          interfaceID == this.is2D.selector 
                         ^ this.skinColor.selector; // Simpson
    }
    
    function is2D() external returns (bool){}
    function skinColor() external returns (string){}
}
```

With three or more supported interfaces (including ERC165 itself as a required supported interface), the mapping approach (in every case) costs less gas than the pure approach (at worst case).

## Copyright

Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).