Module visibility in Rust

Introduction

Module visibility controls which parts of your code can be seen and used by other parts. It helps keep your code organized and safe.

When you want to hide helper functions inside a module so others can't use them directly.

When you want to share some functions or data with other parts of your program.

When you want to organize your code into parts and control access between them.

When you want to prevent accidental changes to important parts of your code.

When you want to make your code easier to understand by showing only what is needed.

Syntax

Rust

mod my_module {
    pub fn public_function() {
        // code
    }

    fn private_function() {
        // code
    }
}

pub means the item is public and can be used outside the module.

Without pub, items are private and only usable inside the module.

Examples

cook is public and can be called from outside kitchen. clean is private and only usable inside kitchen.

Rust

mod kitchen {
    pub fn cook() {
        println!("Cooking food");
    }

    fn clean() {
        println!("Cleaning kitchen");
    }
}

The module tools and its function hammer are public, so they can be accessed from outside garage. The function open_door is private.

Rust

mod garage {
    pub mod tools {
        pub fn hammer() {
            println!("Using hammer");
        }
    }

    fn open_door() {
        println!("Opening garage door");
    }
}

Sample Program

This program shows a module house with a public submodule living_room. The function watch_tv is public and can be called from main. The other functions are private and cannot be called from outside their modules.

Rust

mod house {
    pub mod living_room {
        pub fn watch_tv() {
            println!("Watching TV in the living room");
        }

        fn clean_room() {
            println!("Cleaning the living room");
        }
    }

    fn lock_doors() {
        println!("Locking all doors");
    }
}

fn main() {
    house::living_room::watch_tv();
    // house::living_room::clean_room(); // This will cause an error because clean_room is private
    // house::lock_doors(); // This will cause an error because lock_doors is private
}

Important Notes

Use pub(crate) to make items visible only inside the current crate (project).

Modules can be nested, and visibility rules apply at each level.

Private items help keep your code safe by hiding details that should not be changed directly.

Summary

Module visibility controls what parts of code can be used outside their module.

pub makes items public; without it, items are private.

Use visibility to organize code and protect important parts.

Practice

1. What does the pub keyword do in Rust module visibility?

easy

A. Makes a function or item accessible outside its module

B. Makes a function private to its module

C. Deletes the function from the module

D. Renames the function for external use

Solution

1. Step 1: Understand the role of pub

   The pub keyword in Rust is used to make functions, structs, or other items accessible from outside the module they are defined in.

2. Step 2: Contrast with default visibility

   By default, items are private and cannot be accessed outside their module unless marked with pub.

3. Final Answer:

   Makes a function or item accessible outside its module -> Option A

4. Quick Check:

   pub = public access [OK]

Hint: Remember: pub means public, so others can use it [OK]

Common Mistakes:

• Thinking pub hides the item
• Confusing pub with renaming
• Assuming pub deletes the item

2. Which of the following is the correct way to make a struct public in Rust?

easy

A. struct MyStruct { pub field: i32 }

B. struct pub MyStruct { field: i32 }

C. pub struct MyStruct { field: i32 }

D. struct MyStruct pub { field: i32 }

Solution

1. Step 1: Recall correct syntax for public structs

   In Rust, to make a struct public, the pub keyword must come before the struct keyword.

2. Step 2: Check placements of pub

   pub struct MyStruct { field: i32 } correctly places pub before struct. Placing pub inside the braces (on field), writing struct pub MyStruct, or struct MyStruct pub {} are incorrect.

3. Final Answer:

   pub struct MyStruct { field: i32 } -> Option C

4. Quick Check:

   pub struct is correct syntax [OK]

Hint: pub goes before struct, not inside or after [OK]

Common Mistakes:

• Placing pub inside struct braces
• Putting pub after struct name
• Writing pub in the wrong order

3. Given the code:

pub mod outer {
    pub mod inner {
        pub fn greet() { println!("Hello!"); }
    }
}

fn main() {
    outer::inner::greet();
}

What will be the output when this code runs?

medium

A. No output

B. Error: greet is private

C. Error: inner module is private

D. Hello!

Solution

1. Step 1: Check visibility of modules and function

   The outer module is public, containing pub mod inner with pub fn greet(), making them all accessible from main.

2. Step 2: Confirm function call and output

   The call outer::inner::greet() is valid and will print "Hello!" to the console.

3. Final Answer:

   Hello! -> Option D

4. Quick Check:

   pub modules and functions allow access [OK]

Hint: pub on modules and functions means accessible outside [OK]

Common Mistakes:

• Overlooking that the outer module needs pub
• Thinking greet is private by default
• Ignoring nested module visibility

4. What is wrong with this Rust code?

mod my_mod {
    fn secret() {
        println!("Secret function");
    }
}

fn main() {
    my_mod::secret();
}

medium

A. secret function is private and cannot be called from main

B. Missing semicolon after println!

C. Module my_mod is not declared public

D. Function secret must have pub keyword inside main

Solution

1. Step 1: Check function visibility

   The function secret is not marked pub, so it is private to my_mod.

2. Step 2: Understand call from main

   Since main is outside my_mod, it cannot call a private function, causing a compile error.

3. Final Answer:

   secret function is private and cannot be called from main -> Option A

4. Quick Check:

   Private functions can't be accessed outside module [OK]

Hint: Private functions can't be called outside their module [OK]

Common Mistakes:

• Thinking module must be pub to call function
• Assuming missing semicolon causes error
• Believing pub inside main is needed

5. You have a private struct Config inside a module, but want to allow other modules to create instances with limited access to its fields. Which visibility setup is correct?

hard

A. Make struct private and fields public

B. Make struct pub but keep fields private

C. Make both struct and fields private

D. Keep struct private and make fields pub

Solution

1. Step 1: Understand struct and field visibility

   Making the struct pub allows other modules to create instances, but keeping fields private restricts direct access to internal data.

2. Step 2: Evaluate other approaches

   Keeping the struct private (with or without pub fields) prevents other modules from constructing instances. Making everything private blocks access entirely.

3. Final Answer:

   Make struct pub but keep fields private -> Option B

4. Quick Check:

   pub struct + private fields = controlled access [OK]

Hint: pub struct with private fields controls access well [OK]

Common Mistakes:

• Making fields pub but struct private (no instance creation)
• Making everything private (no access)
• Exposing fields unintentionally