Visibility

When you start breaking down your code into multiple modules, you need to start thinking about visibility. Visibility determines which regions of your code (or other people's code) can access a given entity, be it a struct, a function, a field, etc.

Private by default

By default, everything in Rust is private.
A private entity can only be accessed:

1. within the same module where it's defined, or
2. by one of its submodules

We've used this extensively in the previous exercises:

• create_todo_ticket worked (once you added a use statement) because helpers is a submodule of the crate root, where Ticket is defined. Therefore, create_todo_ticket can access Ticket without any issues even though Ticket is private.
• All our unit tests are defined in a submodule of the code they're testing, so they can access everything without restrictions.

Visibility modifiers

You can modify the default visibility of an entity using a visibility modifier.
Some common visibility modifiers are:

• pub: makes the entity public, i.e. accessible from outside the module where it's defined, potentially from other crates.
• pub(crate): makes the entity public within the same crate, but not outside of it.
• pub(super): makes the entity public within the parent module.
• pub(in path::to::module): makes the entity public within the specified module.

You can use these modifiers on modules, structs, functions, fields, etc. For example:

pub struct Configuration {
    pub(crate) version: u32,
    active: bool,
}

Configuration is public, but you can only access the version field from within the same crate. The active field, instead, is private and can only be accessed from within the same module or one of its submodules.

Exercise

The exercise for this section is located in 03_ticket_v1/04_visibility