How do I convert a uint256 variable to an int256 one?

Question

I was trying to print uint timeStamp by typing return timeStamp; right below return price; from this code:

pragma solidity ^0.6.7;

import "@chainlink/contracts/src/v0.6/interfaces/AggregatorV3Interface.sol";

contract PriceConsumerV3 {

AggregatorV3Interface internal priceFeed;

/**
 * Network: Kovan
 * Aggregator: BTC/USD
 * Address: 0x6135b13325bfC4B00278B4abC5e20bbce2D6580e
 */
constructor() public {
    priceFeed = AggregatorV3Interface(0x6135b13325bfC4B00278B4abC5e20bbce2D6580e);
}

/**
 * Returns the latest price
 */
function getThePrice() public view returns (int) {
    (
        uint80 roundID, 
        int price,
        uint startedAt,
        uint timeStamp,
        uint80 answeredInRound
    ) = priceFeed.latestRoundData();
    return price;
    return timeStamp;
}
}

When I compiled the code above on the Remix Compiler, it replied:

TypeError: Return argument type uint256 is not implicitly convertible to expected type (type of first return variable) int256. return timeStamp; ^-------^

I tend to think that I would just have to type int256 return timeStamp or something similar instead of return timeStamp; but I can't figure it out.

Feedback is appreciated.

Radiant

Answer 1

You can typecast the uint to int using this syntax:

return int(timeStamp);

Note: This will overflow (in Solidity 0.7.x and older) or throw an exception (in Solidity 0.8+) for values between 2^255 (int max value) and 2^256-1 (uint max value). But this is most likely just a theoretical case, because timestamp is not expected to have this large value.

---

Mind that this line is unreachable, because you're already returning price on the previous line.

(
    uint80 roundID, 
    int price,
    uint startedAt,
    uint timeStamp,
    uint80 answeredInRound
) = priceFeed.latestRoundData();
return price; // the `price` is returned, and the function doesn't execute after this line
return timeStamp; // this is ignored because of the early return on previous line

If you want to return multiple values, you can use this syntax:

// note the multiple datatypes in the `returns` block
function getThePriceAndTimestamp() public view returns (int, uint) {
    (
        uint80 roundID, 
        int price,
        uint startedAt,
        uint timeStamp,
        uint80 answeredInRound
    ) = priceFeed.latestRoundData();
    return (price, timeStamp); // here returning multiple values
}

Despinadespise

Oh, btw, does the above mean the return statement can only be used once per function declared? – Radiant

You can use multiple returns. It makes sense in a conditional return (example: if(user.isActive == false) { return 0; } else { return user.score; }) ... Just in your particular case the second would be always ignored. – Despinadespise

@PetrHejda I think you meant to say 2^255 instead of 2^255-1 in the note about the Solidity v0.8 exception? 2^255-1 is type(int256).max, and so it fits within int256. – Returnee

@PaulRazvanBerg Yes, that was my miscalculation. Thank you, I fixed in in the post. – Despinadespise