Exercises · T12 classes objects

Covers: class definition, private data, public interface, constructors, getters/setters, invariants, const member functions, composition, classes managing collections, struct→class refactor. Lectures L31–L32. Outcomes PF-16.1–16.4.

21 exercises · ladder 🟢 → 🔴. The invariant is the grading axis: any public path that can corrupt state fails the exercise.

PF-E-132 · First Class: Counter

Difficulty: Beginner · Lecture: L31 · Outcomes: PF-16.1 Prerequisites: E-114 Problem: Define class Counter with private int count_ (starts 0) and public void increment(), int value() const. Demonstrate three increments and print the value. Input: none · Output: the value (3). Sample: — → 3 Hints: constructor or default member init sets count_{0}.

PF-E-133 · Private Data Enforcement

Difficulty: Beginner · Lecture: L31 · Outcomes: PF-16.1 Prerequisites: E-132 Problem: Using E-132's Counter, attempt c.count_ = 99; from main — record the exact compiler error in a comment. Then provide a legal path (increment()) and explain why the error is desirable. Input: none · Output: working program + quoted error in comment. Sample: — → compiles only after removing the illegal line. Hints: submit the version with the comment and without the illegal line.

PF-E-134 · Temperature Class

Difficulty: Beginner · Lecture: L31 · Outcomes: PF-16.2, PF-16.3 Prerequisites: E-133 Problem: class Temperature: private double celsius_; public constructor defaulting to 0; double celsius() const; void setCelsius(double) rejecting below −273.15 (print warning, keep old value); double fahrenheit() const (derived). Demonstrate a rejected set and a conversion. Input: none · Output: demonstration lines. Sample: — → setCelsius(-300) rejected / fahrenheit: 98.6 for 37. Hints: the invariant is celsius ≥ −273.15; fahrenheit() is computed, not stored.

PF-E-135 · Rectangle Class (two constructors)

Difficulty: Foundational · Lecture: L31 · Outcomes: PF-16.2 Prerequisites: E-134 Problem: class Rect: private w_, h_ (positive invariant). Default constructor makes a 1×1; parameterized constructor validates and falls back to 1×1 for non-positive inputs. Public area() const, perimeter() const, void scale(double k) (k must be positive). Demonstrate both constructors and a rejected scale. Input: none · Output: demonstration lines. Sample: — → area: 12 / scale(-2) rejected / area after scale(2): 24 Hints: route all initialization through the validation helper.

PF-E-136 · Bank Account Class (the invariant star)

Difficulty: Foundational · Lecture: L31 · Outcomes: PF-16.2, PF-16.3 Prerequisites: E-135 Problem: class Account: private owner_ (string), balanceCents_ (long long, ≥ 0 invariant). Constructor takes owner and opening balance (reject negative → 0). Public bool deposit(long long), bool withdraw(long long) (reject non-positive; reject overdraft), long long balance() const, void print() const. Drive a scripted scenario printing each verdict. Input: none · Output: transaction log + final balance. Sample: — → withdraw 900: rejected (insufficient funds) / balance: 500 Hints: refactor of E-118 — the same rules, now enforced inside the type.

PF-E-137 · Getters, Setters, and const-correctness

Difficulty: Foundational · Lecture: L31 · Outcomes: PF-16.3 Prerequisites: E-136 Problem: Add std::string owner() const and void rename(const std::string&) to Account. Write void audit(const Account& a) taking the account by const reference and calling owner() and balance() — demonstrate that removing const from either member function breaks audit. Quote the compiler error in a comment. Input: none · Output: audit output + commentary. Sample: — → audit: Ayesha holds 50000 cents Hints: const flows outward: const reference → const member functions required.

PF-E-138 · Student Class (struct→class refactor)

Difficulty: Intermediate · Lecture: L32 · Outcomes: PF-16.4 Prerequisites: E-137 Problem: Take E-113's Student struct and refactor to class Student: private members, constructor validating gpa ∈ [0, 4], setters that validate, const getters, print() const. Demonstrate: a rejected gpa at construction and via setter; a successful update. Input: none · Output: demonstration lines. Sample: — → set_gpa(9.0) rejected / gpa: 3.75 Hints: five-step refactor checklist (L32): privatize → construct → validate setters → const getters → move helpers.

PF-E-139 · Playlist Class (class managing a collection)

Difficulty: Intermediate · Lecture: L31 · Outcomes: PF-16.2, PF-16.4 Prerequisites: E-138 Problem: class Playlist: private std::vector<std::string> songs_ and a name. Public: add(const std::string&) (reject empty, reject duplicates — return bool), std::size_t size() const, void print() const, bool remove(const std::string&). Drive a scenario with duplicate-add and remove attempts. Input: none · Output: scenario log. Sample: — → add "Song A": ok / add "Song A": rejected (duplicate) / after remove: size 1 Hints: the vector is private — all mutation flows through validated methods.

PF-E-140 · Time Class with Wrapping Invariant

Difficulty: Intermediate · Lecture: L31 · Outcomes: PF-16.2 Prerequisites: E-139 Problem: class Clock: private h_, m_ (0–23, 0–59). Constructor validates; void tick() advances one minute with correct wrap (23:59 → 00:00); void addMinutes(int n) loops or multiplies; print() const as HH:MM. Demonstrate a wrap sequence and a rejected construction (25:00 → clamp to 23:59 with a warning, or to 00:00 — document your choice). Input: none · Output: clock states. Sample: — → 23:58 → tick → 23:59 → tick → 00:00 Hints: tick = m_ + 1; if (m_ == 60) { m_ = 0; ++h_; if (h_ == 24) h_ = 0; }.

PF-E-141 · Fraction Class (reduced automatically)

Difficulty: Intermediate · Lecture: L31 · Outcomes: PF-16.2 Prerequisites: E-140 Problem: class Fraction: private num_, den_ (den > 0 invariant — sign lives in the numerator). Constructor reduces via GCD (reuse E-046 as a helper function); rejects den 0 (→ 1 with warning). Public Fraction add(const Fraction&) const returning a new reduced Fraction; print() const as a/b. Demonstrate 1/6 + 1/3 = 1/2 and a rejection. Input: none · Output: fraction results. Sample: — → 1/6 + 1/3 = 1/2 / Fraction(3, 0): denominator rejected, set to 1 Hints: add: a/b + c/d = (ad + cb)/(bd) then reduce; invariant holds after every operation.

PF-E-142 · Inventory Class with Search

Difficulty: Intermediate · Lecture: L31 · Outcomes: PF-16.2, PF-16.4 Prerequisites: E-141 Problem: class Inventory: private std::vector<Item> where struct Item { std::string sku; int qty; } is public (data carrier). Public: bool restock(sku, qty) (qty > 0; adds item if unknown), bool sell(sku, qty) (reject overdraft / unknown sku), int qtyOf(sku) const (−1 if unknown), void print() const. Script: restock, sell, oversell, unknown-sku sell. Input: none · Output: scenario log. Sample: — → sell SKU1 x3: ok / sell SKU1 x99: rejected (only 7) / qtyOf(NOPE): -1 Hints: the public struct is a plain record; the class guards the vector.

PF-E-143 · Composition: Car has-an Engine

Difficulty: Intermediate · Lecture: L31 · Outcomes: PF-16.4 Prerequisites: E-142 Problem: class Engine: private rpm (0–8000 invariant), start(), stop(), bool running() const, int rpm() const. class Car: has a private Engine engine_; plus drive(int minutes) that requires the engine running (prints engine off otherwise) and accumulates private km_ (speed fixed at 60 km/h). Demonstrate: drive-while-off, then start/drive/stop; expose km() const. Input: none · Output: scenario log. Sample: — → drive: engine off / started / drove 20 km / km: 20 Hints: composition = a member object; Car delegates to Engine — no inheritance (not in course scope).

PF-E-144 · Operator Overload: Money class

Difficulty: Advanced Introductory · Lecture: L32 · Outcomes: PF-16.2 Prerequisites: E-143 Problem: class Money: private cents_ (long long, may be negative for debt). Public constructor; Money operator+(const Money&) const; and Money operator-() const; (negation). Print via print() const as $12.34 / -$3.50 (negative before the dollar sign). Demonstrate an expense ledger: income + income − expense + negation. Input: none · Output: ledger lines. Sample: — → $10.00 + $25.50 = $35.50 / −$12.75 / negated: $12.75 Hints: operators are member functions; keep cents as long long — never floats for money.

PF-E-145 · Class with Static Member: ID Generator

Difficulty: Advanced Introductory · Lecture: L32 · Outcomes: PF-16.2 Prerequisites: E-144 Problem: class Ticket: private static int nextId_ = 100; and private int id_. Constructor assigns id_ = nextId_++. Public int id() const. Demonstrate constructing 3 tickets and printing IDs. In comments: (1) why the counter is shared across all objects, (2) why a per-object counter would be wrong, (3) the testability cost of hidden static state (link to the globals policy). Input: none · Output: three IDs + commentary. Sample: — → 100 / 101 / 102 Hints: static members are declared in-class, defined outside the class — show that line.

PF-E-146 · const-object and const-member-function Drill

Difficulty: Advanced Introductory · Lecture: L32 · Outcomes: PF-16.3 Prerequisites: E-145 Problem: Make a const Account (E-136). Demonstrate: calling balance() works; calling deposit() fails (quote the error in a comment); calling a non-const getter fails until you add const. Deliver a two-sentence rule (in comments) for deciding which member functions are const. Input: none · Output: working program + documented experiments. Sample: — → compiles with all getters const; the illegal calls live in comments. Hints: const object → only const member functions callable — that's the whole feature.

PF-E-147 · Vector of Objects: Roster Class

Difficulty: Advanced Introductory · Lecture: L32 · Outcomes: PF-16.4, PF-14.4 Prerequisites: E-146 Problem: class Roster: private std::vector<Student> (E-138's class). Public: bool add(const Student&) (reject duplicate id), const Student* findById(int id) const (pointer or nullptr), double averageGpa() const (empty roster → 0.0 with a printed note), void printAll() const. Script: add 3, duplicate attempt, find hit/miss, average. Input: none · Output: scenario log. Sample: — → add id 101: ok / add id 101: rejected (duplicate) / findById(999): not found Hints: returning const Student* keeps the collection read-only through the pointer.

PF-E-148 · Stack Class (bounded, array-backed)

Difficulty: Advanced Introductory · Lecture: L32 · Outcomes: PF-16.2, PF-16.4 Prerequisites: E-147 Problem: class IntStack: private fixed int data_[32]; int top_{0};. Public: bool push(int) (full → false), bool pop(int& out) (empty → false), int size() const, bool empty() const. Demonstrate: fill to overflow attempt, drain to underflow attempt, LIFO order printout. Input: none · Output: scenario log. Sample: — → push 33: rejected (full) / pop: 32 / pop on empty: rejected Hints: top_ is both the size and the next free index; the invariant is 0 <= top_ <= 32.

PF-E-149 · Queue Class (circular buffer)

Difficulty: Advanced Introductory · Lecture: L32 · Outcomes: PF-16.2, PF-16.4 Prerequisites: E-148 Problem: class IntQueue: private fixed array of 8, plus head_, count_. Public: bool enqueue(int), bool dequeue(int& out), int size() const. Wrap-around must work: enqueue 8, dequeue 3, enqueue 3 more (indices wrap). Demonstrate FIFO order and both rejection cases. Input: none · Output: scenario log. Sample: — → enqueue 9th: rejected (full) / dequeue order: 1 2 3 … (FIFO) Hints: tail_ = (head_ + count_) % 8 — the modulo is the circle; trace the indices.

PF-E-150 · Money-Ledger Program (capstone-scale class design)

Difficulty: Advanced Introductory · Lecture: L32 · Outcomes: PF-16.4, PF-15.3 Prerequisites: E-149 Problem: Combine E-144's Money and E-118/E-136 ideas into class Ledger: records dated entries (entry number, Money amount, description) in a private vector; bool record(Money, const std::string&) rejects zero amounts; Money net() const; void printStatement() const. Script 5 entries including a rejection; print the statement and the net. Input: none · Output: statement + net. Sample: — → #1 +$1200.00 salary … / net: $977.25 Hints: the Ledger delegates arithmetic to Money — each class does one job.

PF-E-151 · Encapsulation Siege (adversarial review)

Difficulty: Advanced Introductory · Lecture: L32 · Outcomes: PF-16.1, PF-16.4 Prerequisites: E-150 Problem: You are given (in comments) a class Score with two encapsulation holes: a public data member and a setter missing its range check. (1) Show the exact attack code for each hole (as comments). (2) Fix the class. (3) Prove the fix: re-attempt both attacks and quote the compiler errors / rejection behavior you now observe. Input: none · Output: fixed class + documented attacks. Sample: — → attack 1: now a compile error (private) / attack 2: rejected by validation Hints: L31's siege game in written form — every public path must defend the invariant.

PF-E-152 · Course Synthesis: Library System

Difficulty: Advanced Introductory · Lecture: L32 · Outcomes: PF-16.4, PF-14.4, PF-15.3 Prerequisites: E-151 Problem: Build class Library managing class Book { title, author, copies } (Book public struct; Library the guard). Operations: add (reject duplicate title), loan (reject no copies), return (increment), findByAuthor(author) printing matches, and printCatalog(). Drive a full scenario: 3 adds, duplicate, 2 loans, 1 failed loan, 1 return, author search, catalog print. Every class validates its own state; no public data. Input: none · Output: full scenario log. Sample: — → loan "Dune": ok / loan "Dune": rejected (0 copies) / by "Ada": 2 found Hints: this is the L32 synthesis in miniature: structs as data carriers, classes as guardians, vectors as collections, every invariant defended.