Rust for C-Programmers

9.12 Visibility and Modules

By default, structs and their fields are private to the module they are defined in. Use pub to expose them.

// In module `geometry`

pub struct Shape { // Public struct
    pub name: String, // Public field
    sides: u32,      // Private field (default)
}

struct InternalData { // Private struct (default)
    pub value: i32,   // allowed, but pub has no effect
    config: u8,
}

impl Shape {
    pub fn new(name: String, sides: u32) -> Self { // Public constructor
        Shape { name, sides }
    }
    // ... methods ...
}

Key visibility rules:

• pub struct: Makes the struct type usable outside its defining module.
• pub field: Makes a field accessible outside the module if the struct itself is accessible.
• Private fields/methods: Cannot be accessed directly from outside the module, even if the struct type is public. Access is typically provided via public methods (like getters/setters).
• pub field in a private struct: A field marked pub inside a struct that is not pub has no effect.

This system enforces encapsulation, allowing modules to control their public API.