Lesson 4: Interfaces & Price Feeds - how would we know that a parameter can be passed into the AggregatorV3Interface constructor?

[nsawit]

Hi,
In this example:
Function getPrice() public {
AggregatorV3Interface priceFeed = AggregatorV3INterface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);
}

In this line, AggregatorV3Interface priceFeed = AggregatorV3INterface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e)
I am confused why this can accept parameter?

[inzamamahmad3]

AggregatorV3Interface is an external library that takes that smart contract address. Suppose you want to know the price of Eth in terms of USD (ETH/USD) you take the address of the smart contract from the chainlink that referred to (ETH/USD) and you can use this address as a parameter inside AggregatorV3Interface and get the price.

[nsawit]

@inzamamahmad3 Okay, yes. The goal of the AggregatorV3Interface is clear, that it should take smart contract address.

However, when I look into the code of AggregatorV3Interface.sol (https://github.com/smartcontractkit/chainlink/blob/develop/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol)

I don't see nor understand where in the definition of the code that it can accept contract.

When we're calling an interface, is it automatic that the constructor can accept whatever parameter?

Or did we just 'override' the interface constructor?

When I look back on the previous lesson:

import "./SimpleStorage.sol";
contract StorageFactory{
SimpleStorage[] public simpleStorageArray;

function createSimpleStorageContract() public {
    SimpleStorage simpleStorage = new SimpleStorage(); // this would work
    SimpleStorage simpleStorage = new SimpleStorage(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);  // this would throw an error
    simpleStorageArray.push(simpleStorage);
}

when compiling the code above, this would throw an exception TypeError: Wrong argument count for function call: 1 arguments given but expected 0.

but when we pass parameter to the interface constructor, this works just fine:
AggregatorV3Interface priceFeed = AggregatorV3INterface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);

[othaime-en]

Hey @nsawit As we all know, interfaces allow us to talk to other contracts on the blockchain by just writing the function signatures without specifying the implementation of these function. When defining interfaces, we just write the functions we expect to call from the other contract and disregard their implementation altogether. In your case, we want to talk to a data feed that will give us the price of ethereum, so we "create" an interface for it. However, chainlink already created the interface for this contract and named it AggregatorV3Interface instead of copying and pasting this interface in our contract, we just import it from the github repo.

Okay now we have the interface, we need to have the address of the contract we want to interact with and in this case, we go to the chainlink documentation to learn more about the contract that gives us the data feed. If you go to ethereum data feeds in the chainlink documentation, you'll see a bunch of addresses where these contracts are hosted on different chains. In this case we are interested with ETH/USD conversion on goerli network so we are given the address 0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e. Remember the interface we created? it was just a skeleton, the real contract is in this address and for us to use the interface, we need to read the main contract first so we know what else is needed that isn't contained in interface. Reading the contract, we discover on line 139 that it has a constructor function that takes in an address parameter. So that's why we pass an address in the interface. To learn more about how we interact with this contract and pass the address argument, you simply go back to the chainlink documentation section titled Using Datafeeds In the solidity subsection, we see that we see exactly how we can pass the address and retrieve the price of ETH in USD.

TL;DR
In short, AggregatorV3Interface is just an interface to the main contract at 0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e which has the constructor argument that wants us to pass an address.
To learn more about the data feed, be sure to check out the Chainlink Data Feeds section in the chainlink documentation so that you understand more.

[nsawit]

it makes more sense now.