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Define a Required Capability

Define a Required Capability

A pure virtual function describes behavior a concrete derived class must supply.

A school report can depend on the Person interface while each role implements its own label.

Before this lesson, review Choose Behavior at Runtime when needed.

What you’ll learn

  • Use abstract classes to define a Required Capability.
  • Explain how the complete example produces Teacher.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

A school report can depend on the Person interface while each role implements its own label.

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 Person  {
  public: virtual std::string role() const = 0;
  virtual ~Person() = default;
}
;
class Teacher : public Person  {
  public: std::string role() const override  {
    return "Teacher";
  }
}
;
int main()  {
  Teacher teacher;
  std::cout << teacher.role() << '\n';
  return 0;
}

Expected output

Teacher
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <string>

class Person  {
  public: virtual std::string role() const = 0;
  virtual ~Person() = default;
}
;
class Teacher : public Person  {
  public: std::string role() const override  {
    return "Teacher";
  }
}
;
int main()  {
  Teacher teacher;
  std::cout << teacher.role() << '\n';
  return 0;
}

Expected output

Teacher

A pure virtual function describes behavior a concrete derived class must supply. A school report can depend on the Person interface while each role implements its own label.

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 Teacher. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

An abstract class cannot be instantiated directly. It can still contain state, constructors and implemented methods.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <string>

class Person  {
  public: virtual std::string role() const = 0;
  virtual ~Person() = default;
}
;
class Teacher : public Person  {
  public: std::string role() const override  {
    return "Teacher";
  }
}
;
int main()  {
  Teacher teacher;
  std::cout << teacher.role() << '\n';
  return 0;
}

Your task: Replace Teacher’s role text with Tutor. 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: Replace Teacher’s role text with Tutor. The concrete implementation returns Tutor.

A pure virtual function describes behavior a concrete derived class must supply. A school report can depend on the Person interface while each role implements its own label.

An abstract class cannot be instantiated directly. It can still contain state, constructors and implemented methods.

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 Person  {
  public: virtual std::string role() const = 0;
  virtual ~Person() = default;
}
;
class Teacher : public Person  {
  public: std::string role() const override  {
    return "Tutor";
  }
}
;
int main()  {
  Teacher teacher;
  std::cout << teacher.role() << '\n';
  return 0;
}

Expected output

Tutor
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <string>

class Person  {
  public: virtual std::string role() const = 0;
  virtual ~Person() = default;
}
;
class Teacher : public Person  {
  public: std::string role() const override  {
    return "Teacher";
  }
}
;
int main()  {
  Teacher teacher;
  std::cout << teacher.role() << '\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

Teacher

A pure virtual function describes behavior a concrete derived class must supply. A school report can depend on the Person interface while each role implements its own label. An abstract class cannot be instantiated directly. It can still contain state, constructors and implemented methods.

What to remember

A pure virtual function describes behavior a concrete derived class must supply. An abstract class cannot be instantiated directly. It can still contain state, constructors and implemented methods.

Useful connections: Choose Behavior at Runtime, 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: Keep Custom Operations Predictable →