Type Error for Storage Factory's sfGet method.

[TilakMaddy]

Simple Storage Class
(Will compile successfully)

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0 <0.9.0;

contract SimpleStorage {

    uint256 public number; // Note that I made this variable public 

    function store(uint256 num) public virtual {
        number = num;
    }

    function retrieve() public view returns(uint256) {
        return number;
    }

}

Storage Factory class
(Doesn't compile)
I get the following error

TypeError: Return argument type function () view external returns (uint256) is not implicitly convertible to expected type (type of first return variable) uint256.

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0 <0.9.0;

import "./SimpleStorage.sol";

contract StorageFactory {

    SimpleStorage[] public simpleStorageArray;

    function createSimpleStorageContract() public {
        SimpleStorage simpleStorage = new SimpleStorage();
        simpleStorageArray.push(simpleStorage);
    }

    function sfStore(uint256 _simpleStorageIndex, uint256 _simpleStorageNumber) public {
        simpleStorageArray[_simpleStorageIndex].store(_simpleStorageNumber);
    }

    //function sfGet(uint256 _simpleStorageIndex) public view returns(uint256) {
    //    return simpleStorageArray[_simpleStorageIndex].retrieve(); 
    //}

    // My version of sfGet that directly tries to return the number w/o any intermediate method like retrieve
    function sfGet(uint256 _simpleStorageIndex) public view returns(uint256) {
        return simpleStorageArray[_simpleStorageIndex].number;
    }

}

What does the error mean? ..and why?

[conqryash007]

Just replace simpleStorageArray[_simpleStorageIndex].number; with simpleStorageArray[_simpleStorageIndex].number(); in sfGet function.
The code will compile correctly.

function sfGet(uint256 _simpleStorageIndex) public view returns(uint256) {
        return simpleStorageArray[_simpleStorageIndex].number();
    }

Solidity generates a getter function for every public variable which is used to access its content.

[PatrickAlphaC]

Boom!

[TilakMaddy]

Okay but I'm still confused.... if number is actually a getter function then retrieve() should be defined like this

 function retrieve() public view returns(uint256) {
        return number(); // Notice I added round brackets
 }

[conqryash007]

Actually, when you are inside a contract, you can directly access the variable defined in it, but as soon as you need to interact outside of a contract, you require an interface to interact with it, which is the ABI of the contract.

We get the getter function after the contract is compiled (via ABI), so we can't do number() right away in the current contract.

BTW, retrieve is a so-called "getter function".

---

In this case we are interacting with one of simpleStorage's contract via storageFactory contract and so you cannot access variable( of simpleStorage ) directly, we will require a getter function.

[TilakMaddy]

Oh...got it ! Thanks :)