1

In this example contract: https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/token/ERC721/ERC721Token.sol

The removeToken method is supposed to delete a token; the token _tokenId no longer exists. So why does it do tokenOwner[_tokenId] = 0; instead of delete tokenOwner[_tokenId];?

Surely setting the value to 0 does not actually delete it? Wouldn't that mean that the contract will always take more storage space, never going down, even when the tokens are burnt?

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4

As there is no null in Solidity all variables are considered to be empty if their value is 0 or similar (I guess "" for strings). So setting an integer to 0 basically deletes it.

delete also doesn't do anything else than set the value to 0 for integers. So doing tokenOwner[_tokenId] = 0; is equal to delete tokenOwner[_tokenId];

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  • Is it the same in terms of gas? Is setting a variable to 0 costs negative gas (returns gas to sender)? – takeshi Jun 22 '18 at 10:12
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    You never get gas actually returned to you but yes, setting variable to its default value reduces gas usage of the transaction – Lauri Peltonen Jun 22 '18 at 11:06

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