关于结构体/类/字典的区别

1. 结构体只包含数据 结构体可以包含子结构体(嵌套),结构体的数据变量可以不初始化
2. 类包含函数和数据和子类(嵌套)
3. 字典只包含数据 key和value不能为null

The issue describes differences between structs, classes, and dictionaries. Look for documentation files or language specification documents in the repository. Identify where these concepts are explained and update the documentation to clarify the distinctions, ensuring examples align with the language's implementation.

索引モデルが issue の本文から書いたものです。