what is the meaning of 0x0? say when variable gets assigned to it, example: keccak256(number) = 0x0;

Question

Is 0x0 just zero? thanks

keccak256(number) = 0x0; 

Or when not used as address I mean just to denote variable?

Answer 1

The 0x prefix means hexadecimal and it's a way to tell programs, contracts, APIs that the input should be interpreted as a hexadecimal number (we'll shorten to hex). 0x0 is actually 0 but in hex. Usually we use 0x0 to check whether the address is not set by us and it is set to default value by the solidity which is 0x0.

require(_addressIn != address(0))

E.g. 0x0 in Solidity is short for 0x0000000000000000000000000000000000000000, and if we use this as some address than it does have value greater than zero. check here.

Keccak256 computes the Ethereum-SHA-3 (Keccak-256) hash (doc) of the arguments passed into the function. So the above line of code is not correct because keccak256(number) is returning the hash value which you can store in some variable, instead you are trying to treat the output hash value as an variable and assigning the 0x0 to that.

I hope it helps.

Answer 2

The example given wouldn't actually work because you're trying to assign 0x0 to the keccak function.

It has the same meaning as bytes32(0). So, you can go:

require(bytes32(0) == 0x0);

That would be comparing equivalents. It was possible to compare address and 0x0 but the trend seems to be toward explicit type casting, so you would go address(0) with a recent compiler.

This type of expression is often used to validate inputs, in particular, catching important values that were not passed in. This is common:

function doSomething(bytes32 key) ... {
  require(key != 0x0);
  // carry on
}

Hope it helps.