Activity 20: Design Your Own Structs and Methods

Overview

In groups of 5-6, you'll design a struct-based system for a real-world scenario. Focus on:

  • What fields belong in your structs
  • What enums represent choices in your domain
  • What methods you need and what type of self parameter each uses
  • How structs and enums work together

Time:

  • 10 minutes: Group design work
  • 10 minutes: Group presentations

Groups 1+2: Front of the room, Task A Groups 3+4: In the seats, Task B Groups 5+6: Outside the doors, Task C Groups 7+8: Lobby area, Task D

Instructions

  1. Add everyone's name to the sheet
  2. Design your system on paper:
    • List the main struct(s) you need with their fields and types
    • List any enums that represent choices or states
    • Write any impl blocks you need and method signatures (including whether it takes self, &self, or &mut self) - but feel free to leave the inside of each method unimplemented()
  3. Be ready to present:
    • Choose one person who will come to the front to explain your design
    • We'll go by task, so we'll hear two approaches to each problem

Task A: Smart Home Thermostat & Lights

Design a smart home system with thermostats and light bulbs.

Consider:

  • A Thermostat struct - what fields does it need?
    • Current temperature? Target temperature? Current mode?
  • A SmartLight struct - what makes a smart light?
    • Brightness level? Color? On/off state?
  • What enums represent choices?
    • Thermostat modes (Heat/Cool/Auto/Off)?
    • Light colors or temperature?

Methods to think about:

  • How do you read the current temperature? (&self?)
  • How do you change the target temperature? (&mut self?)
  • How do you turn a light on/off? (&mut self?)
  • What about a factory reset that returns the device to defaults? (self?)

Bonus: How might you store multiple lights in a home?

Task B: Coffee Shop Order System

Design a system for taking and managing coffee shop orders.

Consider:

  • An Order struct - what's in an order?
    • Customer name?
    • Items ordered?
    • Total price?
    • Order status?
  • A Drink struct - what defines a drink?
    • Type (latte, cappuccino, etc.)?
    • Size?
    • Customizations (extra shot, oat milk, etc.)?
  • What enums would help?
    • DrinkSize (Small/Medium/Large)?
    • OrderStatus (Pending/InProgress/Ready/Completed)?
    • MilkType (Whole/Skim/Oat/Almond)?

Methods to think about:

  • How do you calculate the total price? (&self?)
  • How do you add an item to an order? (&mut self?)
  • How do you mark an order as ready? (&mut self?)
  • What about completing/closing an order so it can't be modified? (self to consume it?)

Bonus: How do you handle customizations? Separate struct? Enum? Vec of options?

Task C: Music Streaming Playlist

Design a music streaming app's playlist system.

Consider:

  • A Playlist struct - what data does a playlist have?
    • Name, creator?
    • List of songs?
    • Play count? Duration?
    • Public or private?
  • A Song struct - what identifies a song?
    • Title, artist, album?
    • Duration in seconds?
    • Genre?
  • What enums fit?
    • Genre (Rock/Pop/Jazz/Classical/...)?
    • PlaylistVisibility (Public/Private/Unlisted)?

Methods to think about:

  • How do you get the total playlist duration? (&self?)
  • How do you add/remove songs from a playlist? (&mut self?)
  • How do you shuffle the playlist? (&mut self?)
  • What about converting a playlist to a "mix" that can't be edited? (self to consume?)

Bonus: Should shuffle return a new playlist or modify the existing one?

Task D: RPG Game Character

Design a role-playing game character system.

Consider:

  • A Character struct - what defines a character?
    • Name, level?
    • Health points (current and max)?
    • Inventory of items?
    • Character class?
  • An Item struct - what's in the inventory?
    • Name, description?
    • Item type (weapon, armor, potion)?
    • Value or power?
  • What enums make sense?
    • CharacterClass (Warrior/Mage/Rogue/Healer)?
    • ItemType (Weapon/Armor/Potion/Quest)?

Methods to think about:

  • How do you check if a character is alive? (&self?)
  • How do you take damage or heal? (&mut self?)
  • How do you level up? (&mut self?)
  • What about "retiring" a character and getting their final stats? (self to consume?)

Bonus: How does a character "use" an item from inventory?

Presentation Guidelines

When presenting (2 minutes per group):

  1. Introduce your scenario (15 seconds)

    • "We designed a [system name]"

  2. Show your main struct(s) (45 seconds)

    • "Our main struct is [Name] with fields: ..."
    • "We also have [other structs]"

  3. Show your enums (30 seconds)

    • "We used enums for [choices]: ..."

  4. Highlight interesting method (30 seconds)

    • Pick ONE interesting method
    • Explain why you chose &self, &mut self, or self
    • "We made X take &mut self because it needs to change..."

Discussion Questions (After Presentations)

  • Which groups had similar design decisions?
  • Did anyone use self (consuming) methods? When and why?
  • What made you choose an enum vs adding a field to a struct?
  • Did any groups nest structs inside other structs?
  • How did you decide what should be a separate struct vs just a field?

Key Concepts

As you work, remember:

  • &self: Use when you just need to READ data

    • Getting values, calculations, checking status
    • The struct is still usable afterward

  • &mut self: Use when you need to CHANGE data

    • Updating fields, adding to collections, state changes
    • The struct is still usable afterward

  • self: Use when you CONSUME the struct

    • Converting to something else, finalizing, deleting
    • The struct is NOT usable afterward

  • Enums: Use for CHOICES (one of several alternatives)

    • "This OR that" relationships
    • Statuses, modes, types

  • Structs: Use for GROUPING related data

    • "This AND that" relationships
    • Data that belongs together