What is the purpose of uint256(-1)

Question

I've seen a contract that stores -1 as an unsigned integer.

uint256 can = allowed ? uint256(-1) : 0;

Can anyone explain the benefit of this over something like:

uint256 can = allowed ? 1 : 0;

Answer 1

By 2s-complement, uint256(-1) is equal to the maximum value of uint256.

So it's essentially a shorter way to write 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF.

For bool can, your wondering would be correct (i.e., allowed ? true : false would suffice).

But since uint256 can is used, the answer to your question depends on what it is used for...

Answer 2

uint256(-1) is a trick to gives the maximum uint256 value.

Answer 3

Use type(uint256).max for better readability. Supports Solidity >= 0.6.8

Answer 4

Here is another REAL purpose

Tests are passing:

for (uint256 i = pendingDepositsLength - 1; i != uint256(-1); i--) {

Test are failing:

for (uint256 i = pendingDepositsLength - 1; i >= 0; i--) {

Explanation

i-- causes underflow and i becomes a really large number.

uint256(-1) solves that. Clever use case, not sure about readability.

I wanted to refactor but the failing tests raised the alarm.