ProgramGuru

Name a Fixed Set of States

Name a Fixed Set of States

An enum class represents a fixed set of named alternatives.

Attendance::present explains a school state more clearly than an unexplained numeric code.

Before this lesson, review Group Related Fields when needed.

What you’ll learn

  • Use enumerations to name a Fixed Set of States.
  • Explain how the complete example produces true.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Attendance::present explains a school state more clearly than an unexplained numeric code.

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>

enum class Attendance  {
  present, absent
}
;
int main()  {
  Attendance state = Attendance::present;
  std::cout << std::boolalpha << (state == Attendance::present) << '\n';
  return 0;
}

Expected output

true
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

enum class Attendance  {
  present, absent
}
;
int main()  {
  Attendance state = Attendance::present;
  std::cout << std::boolalpha << (state == Attendance::present) << '\n';
  return 0;
}

Expected output

true

An enum class represents a fixed set of named alternatives. Attendance::present explains a school state more clearly than an unexplained numeric code.

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

Important mistakes and boundaries

Scoped enumerations do not implicitly become arbitrary integers. Validate external input before mapping it to an allowed state.

Change the school example

C++17 · edit this complete example

#include <iostream>

enum class Attendance  {
  present, absent
}
;
int main()  {
  Attendance state = Attendance::present;
  std::cout << std::boolalpha << (state == Attendance::present) << '\n';
  return 0;
}

Your task: Use Attendance::absent. 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: Use Attendance::absent. The comparison becomes false.

An enum class represents a fixed set of named alternatives. Attendance::present explains a school state more clearly than an unexplained numeric code.

Scoped enumerations do not implicitly become arbitrary integers. Validate external input before mapping it to an allowed state.

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>

enum class Attendance  {
  present, absent
}
;
int main()  {
  Attendance state = Attendance::absent;
  std::cout << std::boolalpha << (state == Attendance::present) << '\n';
  return 0;
}

Expected output

false
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

enum class Attendance  {
  present, absent
}
;
int main()  {
  Attendance state = Attendance::present;
  std::cout << std::boolalpha << (state == Attendance::present) << '\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

true

An enum class represents a fixed set of named alternatives. Attendance::present explains a school state more clearly than an unexplained numeric code. Scoped enumerations do not implicitly become arbitrary integers. Validate external input before mapping it to an allowed state.

What to remember

An enum class represents a fixed set of named alternatives. Scoped enumerations do not implicitly become arbitrary integers. Validate external input before mapping it to an allowed state.

Useful connections: Group Related Fields, 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: Look Up Keys and Unique Values →