Module visibility in Rust - Mini Project: Build & Apply

Understanding Module Visibility in Rust

📖 Scenario: You are building a small Rust project that organizes code into modules. You want to control which parts of your code are visible outside their modules, just like how you might decide which rooms in a house are open to guests.

🎯 Goal: Learn how to use pub to make modules and functions visible outside their module, and how to keep some parts private.

📋 What You'll Learn

Create a module named kitchen with a private function

Create a module named living_room with a public function

Use pub keyword to control visibility

Call the public function from living_room in main

Try to call the private function from kitchen in main and observe the error

💡 Why This Matters

🌍 Real World

In real Rust projects, controlling module visibility helps keep code clean and prevents accidental use of internal details.

💼 Career

Understanding module visibility is essential for writing maintainable Rust code in professional software development.

1

Create the kitchen module with a private function

Create a module called kitchen with a function named cook_food that prints "Cooking food...". Do NOT use pub before the function, so it stays private.

Rust

// Create the kitchen module with a private function cook_food
mod kitchen {
    // Your code here
}

Hint

Remember, functions are private by default in Rust modules unless you add pub.

2

Create the living_room module with a public function

Add a module called living_room with a function named watch_tv that prints "Watching TV...". Make this function public by adding pub before it.

Rust

mod kitchen {
    fn cook_food() {
        println!("Cooking food...");
    }
}

// Create the living_room module with a public function watch_tv
mod living_room {
    // Your code here
}

Hint

Use pub fn to make the function visible outside the module.

3

Call the public function from living_room in main

Write a main function that calls the public function watch_tv from the living_room module.

Rust

mod kitchen {
    fn cook_food() {
        println!("Cooking food...");
    }
}

mod living_room {
    pub fn watch_tv() {
        println!("Watching TV...");
    }
}

// Write main function to call living_room::watch_tv
fn main() {
    // Your code here
}

Hint

Use the full path living_room::watch_tv() to call the public function.

4

Try to call the private function from kitchen in main and observe the error

In the main function, add a call to kitchen::cook_food(). Run the program and observe the error message about visibility.

Rust

mod kitchen {
    fn cook_food() {
        println!("Cooking food...");
    }
}

mod living_room {
    pub fn watch_tv() {
        println!("Watching TV...");
    }
}

fn main() {
    living_room::watch_tv();
    // Try to call kitchen::cook_food() below
    // Your code here
}

Hint

This will cause a compile error because cook_food is private. This shows how Rust controls module visibility.

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