Index

Lesson 3: Advanced Solidity Concepts

Chapter 1: Immutability of Contracts

Up until now, Solidity has looked quite similar to other languages like JavaScript. But there are a number of ways that Ethereum DApps are actually quite different from normal applications.
To start with, after you deploy a contract to Ethereum, it’s immutable, which means that it can never be modified or updated again.
The initial code you deploy to a contract is there to stay, permanently, on the blockchain. This is one reason security is such a huge concern in Solidity. If there's a flaw in your contract code, there's no way for you to patch it later. You would have to tell your users to start using a different smart contract address that has the fix.
But this is also a feature of smart contracts. The code is law. If you read the code of a smart contract and verify it, you can be sure that every time you call a function it's going to do exactly what the code says it will do. No one can later change that function and give you unexpected results.

External dependencies

In Lesson 2, we hard-coded the CryptoKitties contract address into our DApp. But what would happen if the CryptoKitties contract had a bug and someone destroyed all the kitties?
It's unlikely, but if this did happen it would render our DApp completely useless — our DApp would point to a hardcoded address that no longer returned any kitties. Our zombies would be unable to feed on kitties, and we'd be unable to modify our contract to fix it.
For this reason, it often makes sense to have functions that will allow you to update key portions of the DApp.
For example, instead of hard coding the CryptoKitties contract address into our DApp, we should probably have a setKittyContractAddress function that lets us change this address in the future in case something happens to the CryptoKitties contract.

Put it to the test

Let's update our code from Lesson 2 to be able to change the CryptoKitties contract address.
1. Delete the line of code where we hard-coded ckAddress.

2. Change the line where we created kittyContract to just declare the variable — i.e. don't set it equal to anything.

3. Create a function called setKittyContractAddress. It will take one argument, _address (an address), and it should be an external function.

4. Inside the function, add one line of code that sets kittyContract equal to KittyInterface(_address).

Note: If you notice a security hole with this function, don't worry — we'll fix it in the next chapter ;)

Old code:

// 1. Remove this:
  address ckAddress = 0x06012c8cf97BEaD5deAe237070F9587f8E7A266d;
  // 2. Change this to just a declaration:
  KittyInterface kittyContract = KittyInterface(ckAddress);

  // 3. Add setKittyContractAddress method here

New code:

KittyInterface kittyContract;
function setKittyContractAddress(address _address) external {
kittyContract = KittyInterface(_address);
}

Total code:

pragma solidity ^0.4.25;

import "./zombiefactory.sol";

contract KittyInterface {
  function getKitty(uint256 _id) external view returns (
    bool isGestating,
    bool isReady,
    uint256 cooldownIndex,
    uint256 nextActionAt,
    uint256 siringWithId,
    uint256 birthTime,
    uint256 matronId,
    uint256 sireId,
    uint256 generation,
    uint256 genes
  );
}

contract ZombieFeeding is ZombieFactory {

  KittyInterface kittyContract;
  function setKittyContractAddress(address _address) external {
    kittyContract = KittyInterface(_address);
  }

  function feedAndMultiply(uint _zombieId, uint _targetDna, string _species) public {
    require(msg.sender == zombieToOwner[_zombieId]);
    Zombie storage myZombie = zombies[_zombieId];
    _targetDna = _targetDna % dnaModulus;
    uint newDna = (myZombie.dna + _targetDna) / 2;
    if (keccak256(abi.encodePacked(_species)) == keccak256(abi.encodePacked("kitty"))) {
      newDna = newDna - newDna % 100 + 99;
    }
    _createZombie("NoName", newDna);
  }

  function feedOnKitty(uint _zombieId, uint _kittyId) public {
    uint kittyDna;
    (,,,,,,,,,kittyDna) = kittyContract.getKitty(_kittyId);
    feedAndMultiply(_zombieId, kittyDna, "kitty");
  }

}