Solidity – How to Make Use of Arbitrary Bytes Array in Solidity

solidity

I'm implementing an ERC-721 contract that supports the function:

safeTransferFrom(address from, address to, uint256 tokenId, bytes data)

The last parameter of this function is an arbitrary byte array. I imagine most implementations of this function ignore this parameter, but I am interested in using it. Specifically, I want to pass two additional uint256 fields in this parameter and decode them within the contract. How can I achieve this? For example:

function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) public {
    (uint256 startDate, uint256 endDate) = decode(data);
    // etc...
}

In the above example, I'm asking how to implement the decode function.

Best Answer

You can use abi.decode() function of solidity to extract your uint256 values.

Reference: abi.decoder in solidity

First make the bytes data using in the script, or using web3:

var data = await abi.encode(11111111,2222222);

Then extract those uint256 in solidity:

function Name(bytes memory data) public pure returns(uint8 number1, uint256 number2) {
    
        (number1, number2) = abi.decode(data, (uint8, uint256));

    }

And this should do the trick!

Related Solutions

Solidity Log Events – How to Split Calldata Bytes to Multiple Log Events?

Arbitrary Split approach

Edit: Added this section to address arbitrary split destinations

By the time you are splitting, you should know the length of the target data. Below is an example implementation of copying the bytes to their destination inside solidity, which should be trivially extendable to N buckets.

pragma solidity ^0.4.15;

contract HelpLogs {

  event LogFirstHalf(bytes _data);
  event LogSecondHalf(bytes _data);

  function logit(bytes data) external {
      uint midpoint = data.length / 2;
      bytes memory data1 = new bytes(midpoint);
      for (uint i = 0; i < midpoint; i++) {
          data1[i] = data[i];
      }
      bytes memory data2 = new bytes(data.length - midpoint);
      for (i = 0; i < data.length - midpoint; i++) {
          data2[i] = data[i + midpoint];
      }
      LogFirstHalf(data1);
      LogSecondHalf(data2);
  }
}

Note that the gas usage is higher than it needs to be, because it works byte-by-byte. It would be faster to use 32-byte words, with bitmasking. A good reference is memcpy from Arachnid's solidity-string utils library.

[Edit: old] Fixed Bucket Approach

You can split the data externally without the 21 kgas overhead. Send the pre-split data as two parameters to a single function call:

pragma solidity ^0.4.15;

contract HelpLogs {

  event LogFirstHalf(bytes _data);
  event LogSecondHalf(bytes _data);

  function logit(bytes dataPart1, bytes dataPart2) external {
    LogFirstHalf(dataPart1);
    LogSecondHalf(dataPart2);
  }
}

This will cost less gas than splitting inside the EVM.

Solidity – Handling Calldata with Too Many Parameters

Yes, the extra calldata bytes will be ignored, and yes, calldata read past the end of the length is 0s.

Best Answer

Related Solutions

Solidity Log Events – How to Split Calldata Bytes to Multiple Log Events?

Solidity – Handling Calldata with Too Many Parameters

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