Is struct data returned from web3 guaranteed to be in the correct order?

Question

I have this solidity struct

struct Prices {
    uint sell_price;        // wei
    uint target_price;      // primary
    uint penalty_price;     // wei
}
Prices public prices

In truffle when I do contract.prices() I retrieve an array [sell_price, target_price, penalty_price] , the order which they are listed in the struct. I wanted to know if this order is guaranteed?

Answer 1

Yes

The getter function that Solidity generates uses the order listed in the struct. Solidity would generate a function of the following form:

function prices() constant returns (uint sell_price, uint target_price, uint penalty_price) {
  sell_price = prices.sell_price;
  target_price = prices.target_price;
  penalty_price = prices.penalty_price;
}

Here's the ABI and see that the outputs have a specific order:

[
  {
    "constant": true,
    "inputs": [],
    "name": "prices",
    "outputs": [
      {
        "name": "sell_price",
        "type": "uint256"
      },
      {
        "name": "target_price",
        "type": "uint256"
      },
      {
        "name": "penalty_price",
        "type": "uint256"
      }
    ],
    "payable": false,
    "type": "function"
  }
]

---

Additionally, a more complex example of a getter from http://solidity.readthedocs.io/en/develop/contracts.html#getter-functions

contract Complex {
    struct Data {
        uint a;
        bytes3 b;
        mapping (uint => uint) map;
    }
    mapping (uint => mapping(bool => Data[])) public data;
}

It will generate a function of the following form:

function data(uint arg1, bool arg2, uint arg3) returns (uint a, bytes3 b) {
    a = data[arg1][arg2][arg3].a;
    b = data[arg1][arg2][arg3].b;
}

Note that the mapping in the struct is omitted because there is no good way to provide the key for the mapping.

Answer 2

Yes.

The ABI is a collection of fixed-length arguments in a specific order. The elements will be reliably/consistently ordered.

Hope it helps.