134. Private Inheritance

1. Basic Declaration

Use the : symbol followed by the access mode (usually public).

// Base Class (Parent)
class SerialPort {
public:
    void open(int baud) { /* Generic open logic */ }
    void close() { /* Generic close logic */ }
};

// Derived Class (Child)
// Syntax: class Child : access_specifier Parent
class UART : public SerialPort {
public:
    // UART inherits open() and close() automatically.
    
    // Adds new specific functionality
    void set_parity(int p) { /* ... */ }
};

UART u;
u.open(9600);    // Calls Base function
u.set_parity(1); // Calls Derived function

2. Access Specifiers (protected)

Inheritance introduces a new access level: protected.

• private: Visible only to the Base class. (Derived classes cannot see it).
• protected: Visible to the Base class and Derived classes. (Outsiders cannot see it).
• public: Visible to everyone.

class Sensor {
protected:
    int raw_adc_value; // Children can access this directly
private:
    int secret_key;    // Children CANNOT access this
};

class TempSensor : public Sensor {
public:
    void read() {
        raw_adc_value = HW_Read(); // ✅ Allowed (protected)
        // secret_key = 0;         // ❌ Error (private)
    }
};

3. Constructor Execution Order

When you create a Derived object, the Base constructor runs first, then the Derived constructor.

When destroyed, the order is reversed (Derived destructor first, then Base).

class Base {
public:
    Base(int x) { /* Init Base */ }
};

class Derived : public Base {
public:
    // Must explicit call Base constructor in initializer list
    Derived(int x, int y) : Base(x) { 
        /* Init Derived */ 
    }
};

A Derived class object is essentially the Base class object with the new fields "glued" to the end of it. It forms a single contiguous block of memory.

1. Hardware Abstraction (HAL)

This is the standard architecture for portable drivers.

• Base Class: Display (Defines generic drawPixel, drawRect, clear).
• Derived Class: ILI9341_Display (Implements drawPixel for specific hardware).
• Application: Writes to Display*. It doesn't care which screen is connected.

2. Generic Protocol Handling

If you have multiple communication packets (Command Packet, Data Packet, Ack Packet) that all share a Header (ID, Length) and CRC, you create a BasePacket class.

• BasePacket handles CRC calculation and Header parsing.
• DataPacket adds the payload buffer. This saves Flash memory by not duplicating the CRC logic 3 times.