EIP4521 - 721/20-compatible transfer

# Abstract

ERC-721, the popular standard for non-fungible tokens (NFTs), includes send functions, such as transferFrom() and safeTransferFrom(), but does not include a backwards-compatible transfer() found in fungible ERC-20 tokens. This standard provides references to add such a transfer().

# Motivation

This standard proposes a simple extension to allow NFTs to work with contracts designed to manage ERC-20s and many consumer wallets which expect to be able to execute a token transfer(). For example, if an NFT is inadvertently sent to a contract that typically handles ERC-20, that NFT will be locked. It should also simplify the task for contract programmers if they can rely on transfer() to both handle ERC-20 and NFTs.

# Specification

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.

The interface for ERC-4521 transfer() MUST conform to ERC-20 and resulting transfers MUST fire the Transfer event as described in ERC-721.

function transfer(address to, uint256 tokenId) external returns (bool success);

# Rationale

Replicating ERC-20 transfer() with just a minor change to accurately reflect that a unique tokenId rather than fungible sum is being sent is desirable for code simplicity and to make integration easier.

# Backwards Compatibility

This EIP does not introduce any known backward compatibility issues.

# Reference Implementation

A reference implementation of an ERC-4521 transfer():

function transfer(address to, uint256 tokenId) public virtual returns (bool success) {
        require(msg.sender == ownerOf[tokenId], "NOT_OWNER");

        unchecked {
            balanceOf[msg.sender]--; 
        
            balanceOf[to]++;
        }
        
        delete getApproved[tokenId];
        
        ownerOf[tokenId] = to;
        
        emit Transfer(msg.sender, to, tokenId); 
        
        success = true;
}

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# Security Considerations

Implementers must be sure to include the relevant return bool value for an ERC-4521 in order to conform with existing contracts that use ERC-20 interfaces, otherwise, NFTs may be locked unless a safeTransfer is used in such contracts.