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用于精确布局的 packed struct

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用于精确布局的 packed struct 是 CoddyKit 上的免费 Zig Academy 课时。这是第 1 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 Zig Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Zig Academy 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。

Layout Matters

Sometimes you need bytes laid out in an exact shape, like a hardware register or a network packet. That is when a packed struct shines.

Normal Structs Add Padding

A plain struct lets Zig reorder and pad fields for speed, so its in-memory size may be larger than the sum of its parts.

const Plain = struct {
    flag: bool,
    value: u32,
};

Enter packed struct

Write packed struct and Zig stores fields back-to-back with no padding, in the exact order you declared them.

const Tight = packed struct {
    flag: bool,
    value: u7,
};

It Is Bit-Packed

A packed struct is measured in bits, not bytes. The fields above use 1 + 7 bits, fitting neatly into a single byte.

A Known Backing Integer

Because the layout is exact, a packed struct has a guaranteed backing integer type that holds all its bits together.

const Flags = packed struct {
    a: bool,
    b: bool,
    rest: u6,
};

Convert to the Integer

Use @bitCast to turn a packed struct into its backing integer, so you can store or transmit it as a raw number.

const f = Flags{ .a = true, .b = false, .rest = 0 };
const bits: u8 = @bitCast(f);

Convert Back Again

The same @bitCast reads an integer back into a packed struct, decoding raw bits into named fields you can use.

const raw: u8 = 0b0000_0001;
const f: Flags = @bitCast(raw);

Check the Exact Size

Use @bitSizeOf to confirm a packed struct occupies precisely the bit count you expect, with zero hidden padding.

const n = @bitSizeOf(Flags); // 8

Declaration Order Is the Layout

In a packed struct, the first field occupies the lowest bits. Order is the layout, so declare fields exactly as the format demands.

Great for Protocols

Packed structs map cleanly onto binary formats, letting you describe a hardware register or wire packet as readable named fields.

Trade-Off to Know

Bit packing can make field access a touch slower than a normal struct. Reach for packed only when exact layout actually matters. 🧩

Quick Check

What does a packed struct guarantee about its fields?

Recap

You learned that packed struct gives an exact, padding-free bit layout, has a backing integer, and converts with @bitCast. Perfect for binary formats. 🎉

常见问题解答

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