ProgramGuru

Model an Is-a Relationship

Model an Is-a Relationship

Public inheritance models a subtype usable where its base is expected.

A Graduate can inherit an accessible student label rather than duplicating the same behavior.

Before this lesson, review Store Shared Class Data when needed.

What you’ll learn

  • Use inheritance to model an Is-a Relationship.
  • Explain how the complete example produces Student.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

A Graduate can inherit an accessible student label rather than duplicating the same behavior.

Read the declarations above main before tracing its statements. Each declared name supplies the value, type or operation used later. The std:: prefix selects a name from the standard library, and cout << sends a value to the output stream.

C++17 · main.cpp · complete program

#include <iostream>
#include <string>

class Learner  {
  public: std::string label() const  {
    return "Student";
  }
}
;
class Graduate : public Learner  {
}
;
int main()  {
  Graduate student;
  std::cout << student.label() << '\n';
  return 0;
}

Expected output

Student
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <string>

class Learner  {
  public: std::string label() const  {
    return "Student";
  }
}
;
class Graduate : public Learner  {
}
;
int main()  {
  Graduate student;
  std::cout << student.label() << '\n';
  return 0;
}

Expected output

Student

Public inheritance models a subtype usable where its base is expected. A Graduate can inherit an accessible student label rather than duplicating the same behavior.

Trace the result: Read the complete program below from its starting values to the output operations. Each std::cout statement displays the value supplied at that point; \n begins the next line. The resulting lines, in order, are Student. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

Inheritance is not the best way to reuse every implementation. Prefer composition when the relationship is has-a rather than is-a.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <string>

class Learner  {
  public: std::string label() const  {
    return "Student";
  }
}
;
class Graduate : public Learner  {
}
;
int main()  {
  Graduate student;
  std::cout << student.label() << '\n';
  return 0;
}

Your task: Change the base label to Learner. Before running your edited program, predict the new output and explain which stored value, condition or object changes. Keep the rest of the example intact.

Use the complete source repeated here; this task does not depend on remembering code from another section.

Worked answer and reasoning

Worked variation: Change the base label to Learner. The inherited call returns Learner.

Public inheritance models a subtype usable where its base is expected. A Graduate can inherit an accessible student label rather than duplicating the same behavior.

Inheritance is not the best way to reuse every implementation. Prefer composition when the relationship is has-a rather than is-a.

For a baseline run, save this source as main.cpp and use clang++ -std=c++17 -Wall -Wextra -pedantic main.cpp -o school, or equivalent g++ flags. Recompile after editing so that you run the updated executable.

The source and output below belong to the edited program. Compare them with the original example to see the consequence of this single change.

C++17 · worked variation · complete program

#include <iostream>
#include <string>

class Learner  {
  public: std::string label() const  {
    return "Learner";
  }
}
;
class Graduate : public Learner  {
}
;
int main()  {
  Graduate student;
  std::cout << student.label() << '\n';
  return 0;
}

Expected output

Learner
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <string>

class Learner  {
  public: std::string label() const  {
    return "Student";
  }
}
;
class Graduate : public Learner  {
}
;
int main()  {
  Graduate student;
  std::cout << student.label() << '\n';
  return 0;
}
Before making the practice edit, what does the original C++17 program print?
Choose one answer. You can try again.

Answer and explanation

Student

Public inheritance models a subtype usable where its base is expected. A Graduate can inherit an accessible student label rather than duplicating the same behavior. Inheritance is not the best way to reuse every implementation. Prefer composition when the relationship is has-a rather than is-a.

What to remember

Public inheritance models a subtype usable where its base is expected. Inheritance is not the best way to reuse every implementation. Prefer composition when the relationship is has-a rather than is-a.

Useful connections: Store Shared Class Data, Give a Task a Name.

Language and design guidance: C++ Core Guidelines and C++ working draft. The draft describes an evolving language; these examples deliberately target C++17.

Reaching the recap and completing the quizzes are separate milestones.

    Next lesson: Choose Behavior at Runtime →