How to save and send tokens

Question

I have some basic questions about implementing Tokens. I want to use the ERC20 standard, which defines the following interfaces:

• balanceOf(address _owner) constant returns (uint256 balance)
• transfer(address _to, uint256 _value) returns (bool success)

Therefore i must somehow save, which address has got how many tokens. I looked around the internet and they implement it alwasy with an array definded by "mapping (address => uint256) public balanceOf". I know how to implement the transfer(..) function with this array, but i still i have two questions:

• The array balanceOf becomes bigger and bigger when more addresses own Tokens. Isn't there a risk, that the array becomes so big that i can't search users in it anymore because of the gas limit of a block?
• The array balanceOf is some kind of user registration, which i don't want. Is there a way how i can send tokens directly to a user wallet? If yes, how would it look like in solidity?

Thanks for your help, DerBär

Answer 1

• A mapping is similar to a hash table, so you should not run into issues regarding search/scaling as more users use your token. Unlike an array where you might need to search for an element, a mapping uses the key (address in this case) as a defined index for where the value (uint256 balance) will be located in memory.
• A common misconception is that tokens are "stored in the user's wallet". This is not true. A user "owning" a token is exactly equivalent of the user having a non zero amount in the balanceOf mapping we reference above. Even when you are trying to check what tokens a user has, you need to first look at the address of the token in question, and then make a query in the context of that contract. There is no place (within Ethereum) where at the account level, you will get a list of all the tokens that user owns.

Also, you should probably not be reinventing the wheel in terms of implementing the ERC20 standard. Instead, use something like this from a trusted source.