ProgramGuru

Choose Behavior at Runtime

Choose Behavior at Runtime

A virtual instance function allows a base reference to dispatch to an actual object’s overriding implementation.

The report can ask for a role without knowing the concrete subtype.

Before this lesson, review Model an Is-a Relationship when needed.

What you’ll learn

  • Use virtual functions to choose Behavior at Runtime.
  • 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

The report can ask for a role without knowing the concrete subtype.

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;
  Person& person = teacher;
  std::cout << person.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;
  Person& person = teacher;
  std::cout << person.role() << '\n';
  return 0;
}

Expected output

Teacher

A virtual instance function allows a base reference to dispatch to an actual object’s overriding implementation. The report can ask for a role without knowing the concrete subtype.

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

Use override to catch signature mismatches. A polymorphic base used for deletion through a base pointer needs a suitable virtual destructor.

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;
  Person& person = teacher;
  std::cout << person.role() << '\n';
  return 0;
}

Your task: Change Teacher’s role text to 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: Change Teacher’s role text to Tutor. The same base-reference call returns Tutor.

A virtual instance function allows a base reference to dispatch to an actual object’s overriding implementation. The report can ask for a role without knowing the concrete subtype.

Use override to catch signature mismatches. A polymorphic base used for deletion through a base pointer needs a suitable virtual destructor.

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;
  Person& person = teacher;
  std::cout << person.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;
  Person& person = teacher;
  std::cout << person.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 virtual instance function allows a base reference to dispatch to an actual object’s overriding implementation. The report can ask for a role without knowing the concrete subtype. Use override to catch signature mismatches. A polymorphic base used for deletion through a base pointer needs a suitable virtual destructor.

Check a common mistake

What allows a base reference to use a derived role implementation?
Choose one answer. You can try again.

Answer and explanation

A virtual member overridden in the actual derived object.

Virtual dispatch follows the actual object for the overridden member. Name reuse or file order alone does not provide it.

What to remember

A virtual instance function allows a base reference to dispatch to an actual object’s overriding implementation. Use override to catch signature mismatches. A polymorphic base used for deletion through a base pointer needs a suitable virtual destructor.

Useful connections: Model an Is-a Relationship, 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: Define a Required Capability →