Solidity元素访问

Question

在特弗斯的宠物店教程里。Adoption.sol是--

pragma solidity ^0.4.19;

contract Adoption {
    address[16] public adopters;

    // Adopting a pet
    function adopt(uint petId) public returns (uint) {
        require(petId >= 0 && petId <= 15);

        adopters[petId] = msg.sender;

        return petId;
    }   
}

TestAdoption.sol是--

pragma solidity ^0.4.19;

import "truffle/Assert.sol";
import "truffle/DeployedAddresses.sol";
import "../contracts/Adoption.sol";

contract TestAdoption {
    Adoption adoption = Adoption(DeployedAddresses.Adoption());

    // Testing retrieval of a single pet's owner
    function testGetAdopterAddressByPetId() public {
        // Expected owner is this contract
        address expected = this;

        address adopter = adoption.adopters(8);

        Assert.equal(adopter, expected, "Owner of pet ID 8.");
    }   
}

我的问题是address adopter = adoption.adopters(8); in TestAdoption.sol，我认为adoption.adopters是一个数组，它的元素应该通过[8]而不是(8)访问。

然而，马弗斯的代码是正确的。实际上，将(8)更改为[8]将导致可靠的编译错误--

TypeError: Indexed expression has to be a type, mapping or array (is function (uint256) view external returns (address))
        address adopter = adoption.adopters[8];
                          ^---------------^

背后的理论是什么？

Answer 1

无法从另一份合同直接访问这些字段。address[16] public adopters中的public关键字将生成一个名称相同的getter函数，因此是adoption.adopters(8)。