about interface

[akugone]

Hello
I try to understand the interface concept.
In the vid we have
AggregatorV3Interface priceFeed = AggregatorV3Interface(0x8A753747A1Fa494EC906cE90E9f37563A8AF630e);
return priceFede.version();

I understand that we pass the address as a parameter in the AggregatorV3Interface, but when you look the interface all the function are empty (ok that's normal for an interface :) but how it can process and send a result if there'is nothing in the functions to use the parameter ?

[technophile-04]

Hii @akugone ! I think its similar to #2129 you can take a look at this and if you have further doubt you can ask it 🙌

[akugone]

No not really, i will deep dive in interface to try to understand :)

[adityabhattad2021]

Interfaces as the name suggest are only to get the idea of what actual contract looks like( i.e. what functions it has what are its input parameters etc. ). In your case AggregatorV3Interface, when you decide to use aggregator v3 contract your contract will have no idea
about what functions it has and what are their input paramaters etc.
with this line

AggregatorV3Interface priceFeed = AggregatorV3Interface(0x8A753747A1Fa494EC906cE90E9f37563A8AF630e);

you specify that this 0x8A753747A1Fa494EC906cE90E9f37563A8AF630e address is aggregator v3 and that it has all the functions that are specified in its interface, allowing you to use those functions with dot operator like:

priceFeed.version();

without even having the actual code for aggregator v3 contract,
here when you call the function priceFeed.version() it is passed to this address 0x8A753747A1Fa494EC906cE90E9f37563A8AF630e
the same which you specified here AggregatorV3Interface priceFeed = AggregatorV3Interface(0x8A753747A1Fa494EC906cE90E9f37563A8AF630e);
and the address has the actual contract (aggregator v3) with all the code and here the function is processed and result is sent.
Tried my best to explain, hope that it would be helpful.

[krakxn]

"I understand that we pass the address as a parameter in the AggregatorV3Interface" Incorrect, that is type-casting. We are explicitly converting the address 0x8A753747A1Fa494EC906cE90E9f37563A8AF630e to type AggregatorV3Interface, not passing as a parameter.

A simple example would be:

string strNum = "1";
uint256 intNum = uint256(strNum);

"but how it can process and send a result if there'is nothing in the functions to use the parameter" That is a characteristic of interface (refer abstract contracts—primarily an OOP-concept).

A simple example to interact with other contracts via interface (from Solidity):

contract Counter {
    uint public count;

    function increment() external {
        count += 1;
    }
}

interface ICounter {
    function count() external view returns (uint);

    function increment() external;
}

contract MyContract {
    function incrementCounter(address _counter) external {
        ICounter(_counter).increment();
    }

    function getCount(address _counter) external view returns (uint) {
        return ICounter(_counter).count();
    }
}

[alymurtazamemon]

@akugone Hey! I think it is not clear to you, why are we using AggregatorV3Interface instead of just AggregatorV3 contract.

For using any contract in our contracts we required 2 things. 1. ABI and 2. Address of that contract.

We have an address of the aggregator contract, the only thing that we required is ABI.

Now 1 way is to just copy the whole implementation of the aggregator v3 contract into its separate file and use it in our contract by importing it, by doing so when you compile your contract it will generate the ABI of the aggregator contract and it will run fine.

Now some contract implementations are too big and sometimes in multiple contracts, so we have an easy solution for it. Instead of coping the whole implementation of the contract, we can just copy the interface of the main contract, it will generate the same ABI for it. And openzeppelin and other similar contracts have made it very easy for us, they have created packages for us, so now we do not even need to copy the interface of the main contract, just do this with the above line.