C++ Abstraction

What is Abstraction?

Abstraction hides complex implementation details and exposes only what is necessary. In C++, abstract classes with pure virtual functions define an interface.

#include <iostream>
using namespace std;

// Abstract class (interface)
class IDatabase {
  public:
    virtual void connect()    = 0;   // pure virtual
    virtual void disconnect() = 0;   // pure virtual
    virtual void query(string sql) = 0;
    virtual ~IDatabase() { }
};

class MySQLDatabase : public IDatabase {
  public:
    void connect()    override { cout << "MySQL: Connected" << endl; }
    void disconnect() override { cout << "MySQL: Disconnected" << endl; }
    void query(string sql) override {
        cout << "MySQL executing: " << sql << endl;
    }
};

class PostgresDatabase : public IDatabase {
  public:
    void connect()    override { cout << "PostgreSQL: Connected" << endl; }
    void disconnect() override { cout << "PostgreSQL: Disconnected" << endl; }
    void query(string sql) override {
        cout << "PostgreSQL executing: " << sql << endl;
    }
};

int main() {
    IDatabase* db = new MySQLDatabase();
    db->connect();
    db->query("SELECT * FROM users");
    db->disconnect();
    delete db;
    return 0;
}

Output:

MySQL: Connected
MySQL executing: SELECT * FROM users
MySQL: Disconnected

#include <iostream>
using namespace std;

class Notification {
  public:
    virtual void send(string msg) = 0;
    virtual ~Notification() { }
};

class EmailNotification : public Notification {
  public:
    void send(string msg) override {
        cout << "Email: " << msg << endl;
    }
};

class SMSNotification : public Notification {
  public:
    void send(string msg) override {
        cout << "SMS: " << msg << endl;
    }
};

int main() {
    Notification* n1 = new EmailNotification();
    Notification* n2 = new SMSNotification();
    n1->send("Welcome!");
    n2->send("Your code is 1234");
    delete n1; delete n2;
    return 0;
}

Summary

• Abstract classes define an interface with pure virtual functions (= 0).
• Cannot instantiate abstract classes — only concrete subclasses.
• Abstraction separates "what" from "how" — a key OOP principle.