Web3 & DApp Development Fundamentals · 课时

存储优化

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存储优化 是 CoddyKit 上的免费 Web3 & DApp Development Fundamentals 课时。这是第 2 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Web3 & DApp Development Fundamentals 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Web3 & DApp Development Fundamentals 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Storage Slots Basics

Contract storage is a giant array of 32-byte slots, indexed from slot 0. State variables are assigned to slots in declaration order.

Every slot you write costs gas, so the fewer slots your data occupies, the cheaper your contract is to use. Understanding the slot layout is the foundation of storage optimization.

contract Layout {
    uint256 a; // slot 0
    uint256 b; // slot 1
    uint256 c; // slot 2
}

Variable Packing

The Solidity compiler packs multiple small variables into a single 32-byte slot when they fit consecutively.

A uint128 uses 16 bytes, so two of them share one slot. A bool uses 1 byte and a uint8 uses 1 byte. Packing reduces both deployment and runtime storage costs.

contract Packed {
    uint128 a; // slot 0 (bytes 0-15)
    uint128 b; // slot 0 (bytes 16-31)
    uint256 c; // slot 1
}

Declaration Order Matters

Packing only works for variables that are adjacent in declaration. A poorly ordered layout wastes slots.

In the bad example below, the uint256 forces the two uint128s into separate slots. Reordering so the small types are together lets them pack.

// BAD: 3 slots
uint128 a; // slot 0
uint256 b; // slot 1
uint128 c; // slot 2

// GOOD: 2 slots
uint128 a; // slot 0
uint128 c; // slot 0
uint256 b; // slot 1

Choosing Integer Sizes

Smaller integers only save gas when they pack. A lone uint8 in its own slot is not cheaper than a uint256, and may even cost more because of masking operations.

Rule of thumb: use uint256 by default, and only downsize when you can fit several small values into one slot.

Packing Structs

Structs follow the same packing rules. Order struct members so small types group together. A well-packed struct can cut the number of SSTORE operations dramatically.

struct Order {
    uint128 amount;  // slot 0
    uint64 timestamp;// slot 0
    uint64 id;       // slot 0
    address maker;   // slot 1
}

Constants and Immutables

Values that never change should not live in storage at all.

constant — inlined into bytecode at compile time, zero storage
immutable — set once in the constructor, stored in bytecode not storage

Both avoid expensive SLOAD operations entirely.

uint256 public constant MAX_SUPPLY = 10000;
address public immutable owner;

constructor() {
    owner = msg.sender;
}

Mappings and Dynamic Arrays

Mappings and dynamic arrays do not pack. Each entry occupies its own slot computed by hashing the key with the base slot.

You cannot pack across mapping entries, but you can pack the struct stored as a mapping value. Designing the value struct to fit in fewer slots saves gas on every write.

mapping(address => Order) public orders;
// each Order packs into 2 slots regardless of mapping

Caching Storage Reads

Reading a storage variable repeatedly inside a function pays for each access. Copy it into a local memory variable once, then use the local.

This is especially valuable inside loops, where each iteration would otherwise re-read storage.

uint256 len = items.length; // cache length once
for (uint256 i = 0; i < len; i++) {
    // process items[i]
}

Batched Writes

When updating multiple fields of a packed struct, load the struct into memory, modify it, then write it back once. The compiler can combine adjacent fields into a single SSTORE.

Writing fields one at a time may trigger multiple read-modify-write cycles on the same slot.

Order memory o = orders[id];
o.amount = newAmount;
o.timestamp = uint64(block.timestamp);
orders[id] = o; // single packed write of slot 0

Zero to Non-Zero Penalty

Initializing a slot from zero to non-zero costs 20000 gas, while updating an already non-zero slot costs only 5000 gas.

This is why some contracts pre-warm slots or use a sentinel value (like 1 instead of 0) to keep slots non-zero, trading a one-time cost for cheaper repeated updates.

// Using 1 as 'false' and 2 as 'true' keeps the slot non-zero
uint256 private locked = 1; // never goes back to 0

Layout Audit Checklist

When auditing storage for gas:

Group small types so they pack into shared slots
Use constant / immutable for fixed values
Cache repeated storage reads in memory
Batch struct writes
Avoid unnecessary zero to non-zero transitions

Always verify with a gas report after changes.

Quick Check

You declare a uint128, then a uint256, then another uint128. How many storage slots do they use?

Recap

You learned to optimize storage layout:

Storage is 32-byte slots; adjacent small types pack together
Declaration order determines packing
constant and immutable avoid storage entirely
Cache reads and batch struct writes
Mind the zero to non-zero write penalty

Next we look at optimizing loops and calldata handling.

常见问题解答

「存储优化」课时是免费的吗？

是的 — 「存储优化」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Web3 & DApp Development Fundamentals 课程的其余内容，请升级到 CoddyKit PRO。 Web3 & DApp Development Fundamentals 课程共包含 4 节课。

「存储优化」这节课中我会学到什么？

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学习 Web3 & DApp Development Fundamentals 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Web3 & DApp Development Fundamentals 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 2 节课，共 4 节。

「存储优化」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Web3 & DApp Development Fundamentals 课中编写并运行代码吗？

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