C++ Beginner · Lesson 15 of 22

Inheritance

Base and derived classes, protected members, and the 'is-a' rule.

Estimated time · 45–75 min

Learning goals

What you will understand

Explain inheritance in your own words
Use it in a small robotics example
Verify the result instead of assuming it worked

Understand the idea first

The mental model

Inheritance lets a derived class reuse and extend a base class: class ServoMotor : public Motor. Use it only for a true 'is-a' relationship, where a ServoMotor can be used anywhere a Motor is expected.

The derived constructor must call the base constructor first. Members marked protected are visible to derived classes but not to outsiders. Prefer composition (a member object) when the relationship is 'has-a'.

Guided lesson

Work through these steps

1.
Read the inheritance example and identify what each line owns or changes.
2.
Create a small source file, compile with -std=c++17 and warnings enabled, and run it.
3.
Change one input, predict the result before running, then compare the output.
4.
Record one compiler or runtime error and explain the correction in your own words.

Worked example

Read it before you run it

class Motor {
// The base class with behaviour every motor shares.
public:
// Public interface.
  explicit Motor(int id) : id_{id} {}
// Base constructor stores the ID.
  int id() const { return id_; }
// Shared getter.
protected:
// Visible to derived classes, hidden from everyone else.
  int id_;
// The motor number.
};
// End of the base class.
class ServoMotor : public Motor {
// ServoMotor IS-A Motor and adds servo features.
public:
// Public interface of the derived class.
  ServoMotor(int id, double gain) : Motor{id}, gain_{gain} {}
// Call the base constructor first, then set the extra member.
  double gain() const { return gain_; }
// A feature only servo motors have.
private:
// Hidden data.
  double gain_;
// Servo position gain.
};
// End of the derived class.

What the example is doing

1.Read declarations from right to left: identify each value's type, name, and initial value.
2.Follow the data through each expression before considering syntax shortcuts.
3.Compile with warnings enabled; a clean build is part of the result, not an optional polish step.

Watch it run · Values in memory

See what this code does

The terminal types this lesson's practice solution and runs it. The scene shows the kind of robot behaviour that code drives. It plays by itself and loops.

Practice code

Now type it yourself

Copy the starter into practice.cpp and replace every TODO with your own code. Build and run it: g++ -std=c++17 -Wall -Wextra -Wpedantic practice.cpp -o practice -pthread && ./practice

#include <iostream>

class Motor {
 public:
  explicit Motor(int id) : id_{id} {}
  int id() const { return id_; }

 protected:
  int id_;
};

// TODO: derive ServoMotor from Motor (public); its constructor calls Motor{id} and stores gain

int main() {
  ServoMotor s{2, 0.8};
  std::cout << "servo " << s.id() << " gain " << s.gain() << '\n';
  return 0;
}

Expected output

servo 2 gain 0.8

Stuck? Show the full solution (try it yourself first)

#include <iostream>

class Motor {
 public:
  explicit Motor(int id) : id_{id} {}
  int id() const { return id_; }

 protected:
  int id_;
};

class ServoMotor : public Motor {
 public:
  ServoMotor(int id, double gain) : Motor{id}, gain_{gain} {}
  double gain() const { return gain_; }

 private:
  double gain_;
};

int main() {
  ServoMotor s{2, 0.8};
  std::cout << "servo " << s.id() << " gain " << s.gain() << '\n';
  return 0;
}

Expected result

How you know it worked

After adding the shown fragment to the guided exercise with its listed headers and surrounding program, the finished program builds with -Wall -Wextra -Wpedantic.
Changing the stated input produces the result you predicted.

Practical exercise

Apply the lesson

Create Sensor as a base class (name, read()) and derive TemperatureSensor and EncoderSensor. Explain whether each relationship is is-a or has-a.

Before you continue

The program builds with -std=c++17 and no warnings
You can explain the key rule without reading the example
The normal and edge cases both have recorded results

If something goes wrong

The compiler reports that a name or type is unknown.

Start at the first error. Check spelling, required #include lines, and whether the declaration appears before use.

The program runs but the value is wrong.

Print the inputs and intermediate values, then check conversions, units, and collection order.