Mojo Academy · レッスン

copyinitメソッド

型のコピー方法を定義します

「__copyinit__メソッド」はCoddyKit上の無料Mojo Academyレッスンです。これはレッスン2/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはMojo Academy学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Mojo Academyコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Defining How to Copy

When a value is copied, Mojo calls a special method named __copyinit__. It builds a fresh instance from an existing one. 📋

It Runs on Assignment

Every time a copy happens, like plain assignment, __copyinit__ fires. It is the hook that defines what copying your type means.

The Method Signature

It takes the new instance to fill and a read-only existing value to copy from. You set each field on the new one.

fn __copyinit__(out self, existing: Self):
    self.x = existing.x

Initialize Every Field

Inside __copyinit__ your job is to give the new instance valid values for all of its fields, usually mirrored from existing.

Deep vs Shallow Copies

For heap data you decide: copy the pointer, or allocate and clone the buffer. A true deep copy gives full independence.

Why Independence Matters

A correct __copyinit__ ensures the copy owns its own data. Then editing the copy never reaches back and changes the original.

Often Derived for You

You rarely write it by hand. Marking a type Copyable lets Mojo synthesize a field-by-field __copyinit__ automatically.

@fieldwise_init
struct Point(Copyable):
    var x: Int

When to Write It Yourself

Hand-write __copyinit__ only when the default is wrong, like when you manage a raw buffer that needs cloning.

It Defines Copy Semantics

This method is literally what makes your type copyable. Without it, Mojo cannot duplicate your value on assignment.

Keep It Correct and Cheap

A good __copyinit__ is correct first and lean second. Copy only what you must so duplication stays affordable.

A Building Block of Safety

Together with moves, __copyinit__ gives Mojo its predictable value semantics. You control exactly how data is duplicated.

Quick Check

Lets check the copy initializer.

Recap

__copyinit__ defines how your type copies itself, building an independent instance. Mojo often derives it, but you can customize it. 🎯

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