Solidity – Accessing Getter Functions for Public Variables

soliditystate-variable

Many token contracts have unrestricted getter functions for state variables, which seems redundant (eg, see OpenZepplin's ERC20 contract). For example, a contract could have it this way:

uint private _totalSupply;

function totalSupply() view public returns (uint) {
return _totalSupply
}

or this way:

uint public totalSupply;

Web3.js and solidity would use the exact same syntax–totalSupply–to retrieve the variable "totalSupply." As the former is more common, but also more verbose, I presume I am missing some way in which a getter function dominates querying a contract directly. Can someone enlighten me?

Best Answer

You're missing the fact that in the first case, only this contract can change the value of the totalSupply private variable, whereas in the second case, every inheriting contract can also change the value of the totalSupply public variable.

BTW, that is why OpenZepplin moved from public variables to private variables when they released v2.

Off-chain-wise (e.g., web3.js), both paradigms are identical of course, because the Solidity compiler auto-generates a public function for every public state-variable in the contract.

Related Solutions

Solidity Functions – Managing Functions with More Than 16 Arguments

One possible approach is to use a single dynamic-size byte array as you suggest, another is to group the parameters together by their type, so if you have three arguments of type address you instead pass a single array of addresses, of which you expect the user to send three. That avoids the need to construct then parse out a byte array. But often this will just move the problem from the place where you pass these arguments into your contract to the place where you try to use them.

However, needing all those parameters is often (not always) a sign that the design isn't right. For example, you may be trying to store treat the contract like a database with a lot of data fields that the contract doesn't actually need to do calculations on. This is needlessly expensive, and you're often better just storing the hash of some of that data, or JSON-encoding and storing it on IPFS, then just sending the contract a single parameter for the IPFS address. Even if you need all the data in contract storage for some reason, any fields that the contract doesn't need to do calculations on can be stored together in a single string, and parsed out on the client side when it needs them.

Solidity – Handling Unimplemented Functions in Contracts

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}

Best Answer

Related Solutions

Solidity Functions – Managing Functions with More Than 16 Arguments

Solidity – Handling Unimplemented Functions in Contracts

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