Lesson 7: Questions about how the constructor works in this lesson

[pacelliv]

I'm trying to understand the following points:

1. I don't understand from where the constructor takes the parameter priceFeedAddress.
2. priceFeed is a variable or a contract ?

    AggregatorV3Interface public priceFeed;

    constructor(address priceFeedAddress) {
        i_owner = msg.sender;
        priceFeed = AggregatorV3Interface(priceFeedAddress);
    }

[othaime-en]

Hey @pacelliv if you remember in the initial lesson where we created FundMe.sol we hardcoded the priceFeedAddress inside the contract like this, AggregatorV3Interface priceFeed = AggregatorV3Interface(0x8A753747A1Fa494EC906cE90E9f37563A8AF630e); especially inside the priceConverter.sol. With this setup, priceFeed becomes sort of a contract instance (since it is tied to the interface AggregatorV3Interface) where you can call all the functions related to the contract at the given address (0x8A753747A1Fa494EC906cE90E9f37563A8AF630e). However, hardcoding the address like this is not really efficient because if the address changes after we have deployed our contract, we'd have to deploy a new one with the real address of the AggregatorV3Interface again. Hence we need to dynamically add the priceFeedAddress ourselves instead of hardcoding it in the contract. That's why we pass priceFeedAddress in the constructor then assing it to priceFeed inside the constructor

[pacelliv]

Hey @othaime-en now I totally understand what's being done with priceFeedAddress after the constructor gets the address.

But I'm still trying to understand how the constructor gets the address in the first place. This is how I think the constructor gets it:

In the helper-hardhat-config.js was established on which networks I could deploy the contract.

My deploy script have this lines:

let ethUsdPriceFeedAddress
    if (developmentChains.includes(network.name)) {
        const ethUsdAggregator = await deployments.get("MockV3Aggregator")
        ethUsdPriceFeedAddress = ethUsdAggregator.address
    } else {
        ethUsdPriceFeedAddress = networkConfig[chainId]["ethUsdPriceFeed"]
    }

    // what happens when we want to change chains?
    // when going to localhost or hardhat network we want to use a mock
    const args = [ethUsdPriceFeedAddress]
    const fundMe = await deploy("FundMe", {
        from: deployer,
        args: args,
        log: true,
        waitConfirmations: network.config.blockConfirmations || 1,
    })

Depending on which network I'm deploying my contract a ethUsdPriceFeedAddress will be determined and passed as an argument (args) to FundMe.sol and this how the constructor gets the address as a parameter and store it with the name priceFeedAddress.

It's like this or I'm missing something?

[othaime-en]

Exactly! You are absolutely right. That is how the priceFeedAddress gets passed on to the constructor

[krakxn]

1: I don't understand from where the constructor takes the parameter priceFeedAddress.

constructor(address priceFeedAddress) { -- this is what is called a parameterized constructor; it essentially is a constructor but just with parameters, which--in this case--is address priceFeedAddress. We had a similar constructor in Raffle as well with multiple parameters; now you would say, where are the parameters coming from? They are "given" in external scripts when deploying the contract; e.g. check the deploy scripts for Raffle, you will see a args argument which takes the constructor's parameters in the exact order as in the contract--which is used in initializing the contract.

2: priceFeed is a variable or a contract ?

AggregatorV3Interface public priceFeed; -- defined here is the variable priceFeed of data-type AggregatorV3Interface, which--essentially--is an instance of the contract AggregatorV3Interface--we call this an object in OOP.