C++ Classes — Why Missing Constructors Corrupt Production

#include <iostream>
#include <string>

// CLASS DEFINITION — this is the blueprint, not a real account yet.
// No memory is allocated here. We're just describing the shape of a BankAccount.
class BankAccount {
private:
    // Private members: only methods inside this class can read or change these.
    // This prevents external code from setting balance to -999999 directly.
    std::string ownerName;
    double balance;
    int accountNumber;

public:
    // Constructor: called automatically when an object is created.
    // Its job is to guarantee the object starts in a valid, known state.
    BankAccount(std::string name, int accNum, double initialDeposit) {
        ownerName = name;
        accountNumber = accNum;
        // Enforce a business rule right at construction time.
        // An account can't start with negative money.
        balance = (initialDeposit >= 0) ? initialDeposit : 0.0;
    }

    // Public method: the only official way to add money to this account.
    // Notice we validate INSIDE the method — the class enforces its own rules.
    void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            std::cout << "Deposited $" << amount << " into " << ownerName << "'s account.\n";
        } else {
            std::cout << "Deposit amount must be positive.\n";
        }
    }

    // Public method: withdraw only if funds are available.
    bool withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            std::cout << "Withdrew $" << amount << " from " << ownerName << "'s account.\n";
            return true;
        }
        std::cout << "Withdrawal of $" << amount << " failed — insufficient funds.\n";
        return false;
    }

    // Getter: read-only access to balance. Caller sees the value but can't change it.
    double getBalance() const {  // 'const' means this method won't modify any member variables
        return balance;
    }

    void printSummary() const {
        std::cout << "Account #" << accountNumber
                  << " | Owner: " << ownerName
                  << " | Balance: $" << balance << "\n";
    }
};

int main() {
    // OBJECT CREATION — NOW memory is allocated. Two separate objects from one blueprint.
    BankAccount aliceAccount("Alice", 1001, 500.0);
    BankAccount bobAccount("Bob", 1002, 150.0);

    aliceAccount.deposit(200.0);        // Alice's balance becomes 700
    bobAccount.withdraw(200.0);         // Bob only has 150 — this should fail
    aliceAccount.withdraw(100.0);       // Alice withdraws 100 — succeeds

    std::cout << "\n--- Account Summaries ---\n";
    aliceAccount.printSummary();
    bobAccount.printSummary();

    return 0;
}

#include <iostream>
#include <string>

// Simulates a hardware sensor that allocates a data buffer on the heap.
// This example shows WHY destructors matter and how the member initializer list works.
class SensorDevice {
private:
    std::string sensorId;
    int samplingRateHz;
    double* readingsBuffer;   // Raw pointer — we own this heap memory
    int bufferCapacity;

public:
    // MEMBER INITIALIZER LIST (colon syntax): members are initialized here,
    // before the constructor body runs. More efficient than assignment inside the body.
    SensorDevice(std::string id, int rateHz, int capacity)
        : sensorId(id),
          samplingRateHz(rateHz),
          bufferCapacity(capacity) {

        // Heap allocation: we must free this in the destructor.
        readingsBuffer = new double[bufferCapacity];

        // Initialize all readings to zero so no garbage data exists in the buffer.
        for (int i = 0; i < bufferCapacity; ++i) {
            readingsBuffer[i] = 0.0;
        }
        std::cout << "[" << sensorId << "] Sensor online. Buffer of "
                  << bufferCapacity << " slots allocated.\n";
    }

    // DESTRUCTOR: runs when this object's lifetime ends (scope exit or delete).
    // The tilde (~) prefix is the destructor's calling card.
    ~SensorDevice() {
        delete[] readingsBuffer;   // CRITICAL: release heap memory we allocated
        readingsBuffer = nullptr;  // Defensive: prevent dangling pointer use-after-free
        std::cout << "[" << sensorId << "] Sensor offline. Buffer freed.\n";
    }

    // Record a reading at a specific slot index.
    void recordReading(int slot, double value) {
        if (slot >= 0 && slot < bufferCapacity) {
            readingsBuffer[slot] = value;
        } else {
            std::cout << "Slot " << slot << " is out of range.\n";
        }
    }

    void printReadings() const {
        std::cout << "[" << sensorId << "] Readings at " << samplingRateHz << "Hz: ";
        for (int i = 0; i < bufferCapacity; ++i) {
            std::cout << readingsBuffer[i];
            if (i < bufferCapacity - 1) std::cout << ", ";
        }
        std::cout << "\n";
    }
};

int main() {
    // Object created on the stack — destructor fires automatically when main() exits.
    SensorDevice tempSensor("TEMP-01", 100, 4);

    tempSensor.recordReading(0, 21.5);
    tempSensor.recordReading(1, 22.1);
    tempSensor.recordReading(2, 21.9);
    tempSensor.recordReading(3, 22.4);
    tempSensor.recordReading(5, 99.9);  // Out of range — should warn us

    tempSensor.printReadings();

    std::cout << "\nEnd of main() — destructor fires automatically below this line.\n";
    return 0;
    // ~SensorDevice() is called here automatically — no manual cleanup needed
}

#include <iostream>
#include <string>

// Models a smart thermostat. Temperature has real constraints — this class enforces them.
class TemperatureController {
private:
    // Private: external code cannot set these directly.
    // This means we can guarantee setpointCelsius is always within safe bounds.
    double setpointCelsius;
    double currentTempCelsius;
    bool heatingActive;
    std::string roomName;

    // Private HELPER method — internal logic, not part of the public interface.
    // We might completely rewrite this logic later without any callers knowing.
    void updateHeatingState() {
        heatingActive = (currentTempCelsius < setpointCelsius - 0.5);
    }

public:
    TemperatureController(std::string room, double initialSetpoint)
        : roomName(room),
          currentTempCelsius(18.0),
          heatingActive(false) {
        // Use the setter even in the constructor — reuses the validation logic.
        setSetpoint(initialSetpoint);
    }

    // SETTER with validation: the only way to change the setpoint.
    // Business rule: setpoints below 5°C or above 35°C are invalid.
    void setSetpoint(double newSetpoint) {
        if (newSetpoint < 5.0 || newSetpoint > 35.0) {
            std::cout << "[" << roomName << "] Invalid setpoint " << newSetpoint
                      << "°C. Must be between 5 and 35°C. Ignoring.\n";
            return;
        }
        setpointCelsius = newSetpoint;
        updateHeatingState();  // Recalculate after any setpoint change
        std::cout << "[" << roomName << "] Setpoint updated to " << setpointCelsius << "°C\n";
    }

    // GETTER: read-only access. Returns a copy of the value, not a reference.
    // Caller can read but never mutate through this.
    double getSetpoint() const {
        return setpointCelsius;
    }

    // Simulates a temperature sensor update arriving from hardware.
    void reportCurrentTemperature(double measuredTemp) {
        currentTempCelsius = measuredTemp;
        updateHeatingState();
    }

    void printStatus() const {
        std::cout << "[" << roomName << "] "
                  << "Current: " << currentTempCelsius << "°C | "
                  << "Setpoint: " << setpointCelsius << "°C | "
                  << "Heating: " << (heatingActive ? "ON" : "OFF") << "\n";
    }
};

int main() {
    TemperatureController livingRoom("Living Room", 21.0);
    TemperatureController bedroom("Bedroom", 18.0);

    // Simulate current sensor readings from the rooms.
    livingRoom.reportCurrentTemperature(19.2);
    bedroom.reportCurrentTemperature(17.8);

    livingRoom.printStatus();
    bedroom.printStatus();

    // Attempt an invalid setpoint — the class protects itself.
    livingRoom.setSetpoint(80.0);

    // Valid update
    bedroom.setSetpoint(20.0);
    bedroom.reportCurrentTemperature(17.8);
    bedroom.printStatus();

    return 0;
}

#include <iostream>
#include <memory>   // Required for unique_ptr and shared_ptr
#include <string>
#include <vector>

class NetworkConnection {
private:
    std::string hostAddress;
    int portNumber;
    bool isOpen;

public:
    NetworkConnection(std::string host, int port)
        : hostAddress(host), portNumber(port), isOpen(true) {
        std::cout << "Connected to " << hostAddress << ":" << portNumber << "\n";
    }

    ~NetworkConnection() {
        isOpen = false;
        std::cout << "Connection to " << hostAddress << ":" << portNumber << " closed.\n";
    }

    void sendData(const std::string& payload) const {
        if (isOpen) {
            std::cout << "Sending to " << hostAddress << ": \"" << payload << "\"\n";
        }
    }
};

void demonstrateStackLifetime() {
    std::cout << "--- Stack Object Demo ---\n";
    // Stack object: destructor fires automatically when this function returns.
    NetworkConnection localConn("192.168.1.10", 8080);
    localConn.sendData("Hello from stack");
    // No cleanup needed — ~NetworkConnection() fires right after this line.
}

void demonstrateSmartPointer() {
    std::cout << "\n--- Smart Pointer (Heap) Demo ---\n";
    // unique_ptr: heap allocation, but AUTOMATIC cleanup. No manual delete.
    // Make the intent explicit: this pointer OWNS the connection.
    std::unique_ptr<NetworkConnection> managedConn =
        std::make_unique<NetworkConnection>("10.0.0.1", 443);

    managedConn->sendData("Hello from heap via unique_ptr");
    // ~NetworkConnection() fires automatically when managedConn goes out of scope.
}

void demonstrateDynamicCollection() {
    std::cout << "\n--- Dynamic Collection Demo ---\n";
    // You don't know at compile time how many connections you'll need.
    // A vector of unique_ptrs is the modern, safe approach.
    std::vector<std::unique_ptr<NetworkConnection>> connectionPool;

    connectionPool.push_back(std::make_unique<NetworkConnection>("server-a.local", 5000));
    connectionPool.push_back(std::make_unique<NetworkConnection>("server-b.local", 5001));
    connectionPool.push_back(std::make_unique<NetworkConnection>("server-c.local", 5002));

    for (const auto& conn : connectionPool) {
        conn->sendData("Health check ping");
    }
    // All three destructors fire automatically when connectionPool goes out of scope.
    std::cout << "Exiting function — all connections will close automatically.\n";
}

int main() {
    demonstrateStackLifetime();
    demonstrateSmartPointer();
    demonstrateDynamicCollection();

    std::cout << "\nmain() finished.\n";
    return 0;
}

#include <iostream>
#include <string>

class SensorDevice {
private:
    std::string sensorId;
    double* readingsBuffer;
    int bufferCapacity;

    // Static member declaration (one copy across all instances)
    static int activeSensorCount;

public:
    SensorDevice(std::string id, int capacity)
        : sensorId(id), bufferCapacity(capacity) {
        readingsBuffer = new double[capacity];
        ++activeSensorCount;  // Increment global counter
        std::cout << "[" << sensorId << "] Online. Active sensors: " << activeSensorCount << "\n";
    }

    ~SensorDevice() {
        delete[] readingsBuffer;
        --activeSensorCount;
        std::cout << "[" << sensorId << "] Offline. Active sensors: " << activeSensorCount << "\n";
    }

    // Static member function: can be called without an object
    static int getActiveSensorCount() {
        return activeSensorCount;
    }

    void print() const {
        std::cout << "Sensor: " << sensorId << "\n";
    }
};

// Definition of static member — required in exactly one translation unit
int SensorDevice::activeSensorCount = 0;

int main() {
    std::cout << "Initial active sensors: " << SensorDevice::getActiveSensorCount() << "\n";

    SensorDevice temp("TEMP-01", 4);
    SensorDevice press("PRESS-02", 2);

    std::cout << "Active sensors: " << SensorDevice::getActiveSensorCount() << "\n";

    // When main ends, both destructors fire, counter goes to 0
    return 0;
}