ERC-4521: 721/20-compatible transfer


  Ethereum Improvement Proposals
        
        
          
            
              
            
          
        

        AllCoreNetworkingInterfaceERCMetaInformational

      



      
        
  
    

  
    
      🚧 Stagnant
    
    
    
    
    
      Standards Track: ERC
    
  
  
    
      ERC-4521: 721/20-compatible transfer
    
    
      
        
      
    
    
      
        
      
    
  

  Recommends a simple extension to make NFTs compatible with apps and contracts that handle fungibles.

  	Authors	Ross Campbell (@z0r0z)
	Created	2021-12-13
	Discussion Link	https://ethereum-magicians.org/t/eip-4521-721-20-compatible-transfer/7903
	Requires	

  EIP-721




  


  

  
    Table of Contents

    	Abstract
	Motivation
	Specification
	Rationale
	Backwards Compatibility
	Reference Implementation
	Security Considerations
	Copyright



  


  
      
      
         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;
}


  
    
      
      
         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.

  
    
      
      
         Copyright
      
      
    

Copyright and related rights waived via CC0.


  Citation

  Please cite this document as:

  
  Ross Campbell (@z0r0z), "ERC-4521: 721/20-compatible transfer [DRAFT]," Ethereum Improvement Proposals, no. 4521, December 2021. [Online serial]. Available: https://eips.ethereum.org/EIPS/eip-4521.






      
    
  

  

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        Ethereum Improvement Proposals (EIPs) describe standards for the Ethereum platform, including core protocol specifications, client APIs, and contract standards.