What does the tilde symbol mean in Solidity integer type casting in the example below?

Question

function _approveTokenIfNeeded(address token, address spender) private {
        if (IERC20(token).allowance(address(this), spender) == 0) {
            IERC20(token).safeApprove(spender, uint256(~0));
        }
    }

I found this function in a popular DeFi protocol.

uint256(~0) looks new to me.

Is this a different way of getting the largest available uint256 value? Like type(uint256).max?

The compiler version is v0.7.0+commit.9e61f92b.

Answer 1

The tilde (~) is the symbol for bitwise negation, i.e. it flips every bit of the value it operates on. In this case, yes, it's being used to essentially get type(uint256).max

You can find this operator by searching "~" on the types (specifically the Operators section) page of the Solidity Lang docs