WERC721

WERC721 (wrapped ERC721) tokens are redeemable extensions of their ERC721 counterparts with the following benefits:

• Significantly-reduced transfer gas costs;
• Native call-batching; and
• Meta transactions.

Contracts: Overview

The project is comprised of two (2) key contracts:

• WERC721Factory: Deploys WERC721 contracts against existing ERC721 contracts.
• WERC721: Handles wrapping, unwrapping, and transfers for a single ERC721 contract.

Contracts: WERC721Factory

The WERC721Factory contract enables anyone to easily and cheaply deploy a WERC721 contract that is associated with a ERC721 contract (if one does not already exist - each ERC721 contract may only have one WERC721 contract) by employing the Clones With Immutable Args (CWIA) pattern.

The CWIA implementation contract is deployed by the WERC721Factory contract, and is admin-less, contains no selfdestruct calls, and does not delegate calls to any contracts other than itself (see Multicallable#L46).

Reference material:

• Clones with immutable args by wighawag, zefram.eth, Saw-mon & Natalie.
• Minimal proxy library contract by vectorized.eth.

Contracts: WERC721

The WERC721 contract is a partially-compliant implementation of the ERC721 interface with significantly-reduced token transfer gas costs and additional (broadly-useful) utility built in: call-batching and meta transactions.

The decision to be partially-compliant was for the sake of reducing friction with regards to developer adoption and ease of integration; any application which handles ERC721 tokens and does not make use of the missing ERC721 interface can support WERC721 tokens with minimal changes.

The following items from the ERC721 interface are removed in WERC721:

event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);

function balanceOf(address _owner) external view returns (uint256);

function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data) external payable;

function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;

function approve(address _approved, uint256 _tokenId) external payable;

function getApproved(uint256 _tokenId) external view returns (address);

Significantly-Reduced Transfer Gas Costs

The following operations below are removed from the transferFrom function, reducing gas costs. Below each operation are details about their gas costs, enabling the reader to better understand the gas savings from using WERC721 (resources are linked below for verifying the figures).

NOTE: Solmate's ERC721 implementation is used for comparison since the library is popular and the contracts are written in plain Solidity (generally easier to read). There may be ERC721 implementations which the list below does not apply to (e.g. an ERC721 implementation which uses a loop to determine an account's token balance vs. maintaining a storage variable).

For the sake of simplicity, EIP2930 is not considered.

• SLOAD for checking whether msg.sender is approved to transfer tokenId.
  • Incurs a 2,100 gas cost (cold access).
• SSTORE for decrementing the balance of from.
  • Incurs a 2,100 gas cost (cold access).
  • Incurs a 2,900 gas cost (slot started non-zero, pending change).
  • If the new balance of from is zero, results in a 4,800 gas refund.
  • Net gas cost = 200 or 5,000.
• SSTORE for incrementing the balance of to (we are assuming that from and to are not the same account).
  • Incurs a 2,100 gas cost (cold access).
  • If the original balance of to was zero, incurs a 20,000 gas cost.
  • Else incurs a 2,900 gas cost.
  • Net gas cost = 5,000 or 22,100.
• SSTORE for deleting the token approval.
  • If msg.sender did not have an approval for this token, incurs a 100 gas cost (no op).
  • Else incurs a 2,900 gas cost (slot started non-zero, pending change) and results in a 4,800 gas refund.
  • Net gas cost = -1,900 or 100.

Based on the above, the gas savings from switching to WERC721.transferFrom ranges from 3,300 and 27,200 gas.

It's important to note that wrapping ERC721 tokens costs gas and should be considered when deciding whether or not to use WERC721. For tokens that are transferred frequently, the gas savings will likely recoup the wrapping gas costs quickly.

Reference material:

• Dynamic gas cost appendix by wolflo.
• EVM Codes.

Contract Deployments: WERC721Factory

Chain ID	Chain	Contract Address	Deployment Tx
1	Ethereum Mainnet	0x9246B771Ee83877c8D6Cba3780747F603754728D	Etherscan

Installation

The steps below assume that the code repo has already been cloned and the reader has navigated to the root of the project directory.

1. Install Foundry: https://book.getfoundry.sh/.
2. Run forge i to install project dependencies.
3. Run forge test to compile contracts and run tests.

Audits

• Krum Pashov (@pashov) independent smart contract security researcher.