结构体

Solidity 提供了一种以结构的形式定义新类型的方法，如下例所示：

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.0 <0.9.0;

// Defines a new type with two fields.
// Declaring a struct outside of a contract allows
// it to be shared by multiple contracts.
// Here, this is not really needed.
struct Funder {
    address addr;
    uint amount;
}

contract CrowdFunding {
    // Structs can also be defined inside contracts, which makes them
    // visible only there and in derived contracts.
    struct Campaign {
        address payable beneficiary;
        uint fundingGoal;
        uint numFunders;
        uint amount;
        mapping (uint => Funder) funders;
    }

    uint numCampaigns;
    mapping (uint => Campaign) campaigns;

    function newCampaign(address payable beneficiary, uint goal) public returns (uint campaignID) {
        campaignID = numCampaigns++; // campaignID is return variable
        // We cannot use "campaigns[campaignID] = Campaign(beneficiary, goal, 0, 0)"
        // because the right hand side creates a memory-struct "Campaign" that contains a mapping.
        Campaign storage c = campaigns[campaignID];
        c.beneficiary = beneficiary;
        c.fundingGoal = goal;
    }

    function contribute(uint campaignID) public payable {
        Campaign storage c = campaigns[campaignID];
        // Creates a new temporary memory struct, initialised with the given values
        // and copies it over to storage.
        // Note that you can also use Funder(msg.sender, msg.value) to initialise.
        c.funders[c.numFunders++] = Funder({addr: msg.sender, amount: msg.value});
        c.amount += msg.value;
    }

    function checkGoalReached(uint campaignID) public returns (bool reached) {
        Campaign storage c = campaigns[campaignID];
        if (c.amount < c.fundingGoal)
            return false;
        uint amount = c.amount;
        c.amount = 0;
        c.beneficiary.transfer(amount);
        return true;
    }
}

该合约不提供众筹合约的全部功能，但它包含理解结构所需的基本概念。结构类型可以在映射和数组中使用，它们本身可以包含映射和数组。

结构不可能包含自己类型的成员，尽管结构本身可以是映射成员的值类型，也可以包含其类型的动态大小的数组。这个限制是必要的，因为结构的大小必须是有限的。

请注意，在所有函数中，结构类型如何分配给具有数据位置的局部变量storage。这不会复制结构，而只会存储一个引用，以便对局部变量成员的赋值实际上写入状态。

当然，您也可以直接访问该结构的成员，而无需将其分配给局部变量，如 .campaigns[campaignID].amount = 0

笔记

在 Solidity 0.7.0 之前，允许包含仅存储类型（例如映射）成员的内存结构，并且像 上面示例中的分配将起作用并且只是默默地跳过这些成员。campaigns[campaignID] = Campaign(beneficiary, goal, 0, 0)