Practice Questions — OOP - Classes, Objects, and Constructors

#include <iostream>
#include <string>
using namespace std;

class Student {
    string name;
    double marks;
public:
    Student() : name("Unknown"), marks(0) {}
    Student(string n, double m) : name(n), marks(m) {}
    void display() const {
        cout << name << " - " << marks << endl;
    }
};

int main() {
    Student s1;
    Student s2("Rahul", 85.5);
    Student s3("Priya", 92.3);
    s1.display();
    s2.display();
    s3.display();
    return 0;
}

#include <iostream>
using namespace std;

class Counter {
    static int totalCount;
public:
    Counter() { totalCount++; }
    ~Counter() { totalCount--; }
    static int getCount() { return totalCount; }
};
int Counter::totalCount = 0;

int main() {
    Counter a, b;
    cout << "Count: " << Counter::getCount() << endl;  // 2
    {
        Counter c;
        cout << "Count: " << Counter::getCount() << endl;  // 3
    }
    cout << "Count: " << Counter::getCount() << endl;  // 2
    return 0;
    // After main, a and b are destroyed
}
// Final count would be 0

#include <iostream>
#include <string>
using namespace std;

class BankAccount {
    string holder;
    double balance;
    static int totalAccounts;
public:
    BankAccount(string h, double b) : holder(h), balance(b) {
        totalAccounts++;
    }
    void deposit(double amount) {
        if (amount <= 0) { cout << "Invalid amount!" << endl; return; }
        balance += amount;
        cout << "Deposited Rs." << amount << ". New balance: Rs." << balance << endl;
    }
    void withdraw(double amount) {
        if (amount <= 0) { cout << "Invalid amount!" << endl; return; }
        if (amount > balance) { cout << "Insufficient balance!" << endl; return; }
        balance -= amount;
        cout << "Withdrew Rs." << amount << ". New balance: Rs." << balance << endl;
    }
    void display() const {
        cout << holder << ": Rs." << balance << endl;
    }
    static int getTotalAccounts() { return totalAccounts; }
};
int BankAccount::totalAccounts = 0;

int main() {
    BankAccount acc1("Arjun", 10000);
    BankAccount acc2("Meera", 5000);
    acc1.display();
    acc1.deposit(5000);
    acc1.withdraw(3000);
    acc1.withdraw(20000);
    cout << "Total accounts: " << BankAccount::getTotalAccounts() << endl;
    return 0;
}

#include <iostream>
using namespace std;

class IntArray {
    int* data;
    int size;
public:
    IntArray(int n) : size(n), data(new int[n]()) {}
    IntArray(const IntArray& other) : size(other.size), data(new int[other.size]) {
        for (int i = 0; i < size; i++) data[i] = other.data[i];
    }
    ~IntArray() { delete[] data; }
    void set(int i, int val) { data[i] = val; }
    int get(int i) const { return data[i]; }
    void display() const {
        for (int i = 0; i < size; i++) cout << data[i] << " ";
        cout << endl;
    }
};

int main() {
    IntArray original(3);
    original.set(0, 10);
    original.set(1, 20);
    original.set(2, 30);
    cout << "Original: ";
    original.display();

    IntArray copy = original;  // deep copy
    cout << "Copy: ";
    copy.display();

    copy.set(1, 99);
    cout << "After modifying copy:" << endl;
    cout << "Original: ";
    original.display();
    cout << "Copy: ";
    copy.display();
    return 0;
}

#include <iostream>
#include <string>
using namespace std;

class QueryBuilder {
    string query;
public:
    QueryBuilder& select(const string& cols) {
        query = "SELECT " + cols;
        return *this;
    }
    QueryBuilder& from(const string& table) {
        query += " FROM " + table;
        return *this;
    }
    QueryBuilder& where(const string& condition) {
        query += " WHERE " + condition;
        return *this;
    }
    QueryBuilder& orderBy(const string& col) {
        query += " ORDER BY " + col;
        return *this;
    }
    string build() const { return query; }
};

int main() {
    QueryBuilder qb;
    string query = qb.select("name, marks")
                     .from("students")
                     .where("marks > 80")
                     .orderBy("marks")
                     .build();
    cout << query << endl;
    return 0;
}

#include <iostream>
using namespace std;

class Matrix {
    int data[2][2];
public:
    Matrix(int a, int b, int c, int d) {
        data[0][0] = a; data[0][1] = b;
        data[1][0] = c; data[1][1] = d;
    }
    Matrix(const Matrix& other) {
        for (int i = 0; i < 2; i++)
            for (int j = 0; j < 2; j++)
                data[i][j] = other.data[i][j];
    }
    Matrix add(const Matrix& other) const {
        return Matrix(
            data[0][0] + other.data[0][0],
            data[0][1] + other.data[0][1],
            data[1][0] + other.data[1][0],
            data[1][1] + other.data[1][1]
        );
    }
    static Matrix identity() {
        return Matrix(1, 0, 0, 1);
    }
    void display() const {
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 2; j++)
                cout << data[i][j] << " ";
            cout << endl;
        }
    }
};

int main() {
    Matrix a(1, 2, 3, 4);
    Matrix b(5, 6, 7, 8);
    cout << "Matrix A:" << endl;
    a.display();
    cout << "Matrix B:" << endl;
    b.display();
    Matrix c = a.add(b);
    cout << "A + B:" << endl;
    c.display();
    cout << "Identity:" << endl;
    Matrix::identity().display();
    return 0;
}