What does SoulBound mean?

According to Vitalik’s writings here:

A soulbound item, once picked up, cannot be transferred or sold to another player.

In this article, he also mentions bringing this property to NFTs where once you acquire (mint or airdropped) this token, you can’t transfer it. This means you won’t be able to sell it in any marketplace.

One of the clear use cases of this property is in POAP. As Vitalik writes:

Perhaps the best example of a project trying to do this is POAP, the “proof of attendance protocol”. POAP is a standard by which projects can send NFTs that represent the idea that the recipient personally participated in some event.

This is a pretty interesting concept to me so I wanted to implement it myself for one of my projects (more on that in future posts). Therefore I am also sharing a quick tutorial to write a very simple ERC-1155 contract with this property.

Prerequisites

• Basic knowledge of smart contracts

• Basic knowledge about Solidity and ERC-1155

• Foundry installed — You can also use hardhat if you are more familiar with it, but this tutorial assumes you are using Foundry

Steps

1. Create a new project using

forge init soulbound

This will create a new project with a sample smart contract Counter and your project structure should look like this

2. You can either create a new contract under the src directory or work with the existing Counter.sol

3. We will be using the OpenZepplin contracts library for implementing our token so we need to install that. Foundry uses git submodules for importing libraries. The below command will help you with this installation

forge install OpenZeppelin/openzeppelin-contracts

4. Once installed you are ready to write your smart contract. The next step is to import your dependencies into your contract. You can use the below command to import the ERC1155 library you just installed

import "openzeppelin-contracts/contracts/token/ERC1155/ERC1155.sol";

5. Since we are focused on soulbound tokens in this tutorial, we will just see how to make your contract behave like one. The rest of the code remains the same as what a normal ERC-1155 contract would look like. ERC-1155 has a method called _beforeTokenTransfer which is a Hook that will be called before any transfer of token is done. We will override this method so that it behaves like a soulbound token

function _beforeTokenTransfer(
   address operator,
   address from,
   address to,
   uint256[] memory ids,
   uint256[] memory amounts,
   bytes memory data
) pure override internal {
  require(from == address(0) || to == address(0), "This a Soulbound     token. It cannot be transferred. It can only be burned by the token   owner.");
}

Let me explain the most important piece of code from == address(0) || to == address(0). The first condition says that if the token is minted (minted token originates from 0 address) and the second condition says if the token is burned (tokens get burned to 0 address). So only if the token is minted or burned, this method will execute the transfer otherwise it will fail in this require condition. One line of code and you can make it a soulbound token. Pretty impressive.

6. You can now deploy your contract using forge create command and after you mint a token if you try to transfer that it won’t allow you to transfer. The only thing you can do with this token is to burn it.

If you want to look at the full example you can take a look here. I hope this was useful. Please subscribe to my newsletter if you want my writings directly in your inbox.

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