Load another deployed contract's public values

Question

pragma solidity ^0.4.24;

I'm trying to call another contract's functions to return values.

I have the below which successfully returns a true/false

_childAddress.call(bytes4(keccak256("isValidChild(uint256, uint256)")), _childModule6, 6) == true)

But when I try to return an actual value like so:

uint256 tokenId = _childAddress.call(bytes4(keccak256("totalSupply()")));

I get the following error:

Type bool is not implicitly convertible to expected type uint256. tokenId

I can't find the syntax anywhere for returning a value.

Any help appreciated.

Answer 1

With solc v0.5 the signature of call changed it returns a tuple (bool, bytes). The first parameter indicates if the call successful and the second parameter is the result of the function.

    bool success;
    bytes memory result;
    uint totalSupply;

Make the call using abi.encodeWithSignature to encode parameters and function signature

    (success, result) = childContract.call(abi.encodeWithSignature("totalSupply()"));
    if (success) {

If the call was successful decode the result of the function

        (totalSupply) = abi.decode(result, (uint));
        return totalSupply;
    }

With v0.4 either you create an interface for your child contract or use assemply to obtain the value returned by the call.

Answer 2

The other answer is correct, and also widely used. however if you do a address.call the compiler will not allow to mark the function as view even if you're just reading the value. That's because address.call could be used to write the state as well.

if you want a view function with a low level call to totalSupply() do this

function readTotalSupply() external view returns(uint) {
    (, bytes memory data) = childContract.staticcall(abi.encodeWithSignature("totalSupply()"));
    return abi.decode(data, (uint));
}