Index

Chapter 2: ERC721 Standard, Multiple Inheritance

Let's take a look at the ERC721 standard:

contract ERC721 {
  event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);
  event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);

  function balanceOf(address _owner) external view returns (uint256);
  function ownerOf(uint256 _tokenId) external view returns (address);
  function transferFrom(address _from, address _to, uint256 _tokenId) external payable;
  function approve(address _approved, uint256 _tokenId) external payable;
}

This is the list of methods we'll need to implement, which we'll be doing over the coming chapters in pieces.
It looks like a lot, but don't get overwhelmed! We're here to walk you through it.

Implementing a token contract

When implementing a token contract, the first thing we do is copy the interface to its own Solidity file and import it, import "./erc721.sol";. Then we have our contract inherit from it, and we override each method with a function definition.
But wait — ZombieOwnership is already inheriting from ZombieAttack — how can it also inherit from ERC721?
Luckily in Solidity, your contract can inherit from multiple contracts as follows:

contract SatoshiNakamoto is NickSzabo, HalFinney {
// Omg, the secrets of the universe revealed!
}

As you can see, when using multiple inheritance, you just separate the multiple contracts you're inheriting from with a comma, ,. In this case, our contract is inheriting from NickSzabo and HalFinney.
Let's give it a try.

Putting it to the Test

We've already created erc721.sol with the interface above for you.
1. Import erc721.sol into zombieownership.sol

2. Declare that ZombieOwnership inherits from ZombieAttack AND ERC721

zombieownership.sol

pragma solidity ^0.4.25;

import "./zombieattack.sol";
// Import file here
import "./erc721.sol";

// Declare ERC721 inheritance here
contract ZombieOwnership is ZombieAttack, ERC721 {

}