ProgramGuru

Store Shared Class Data

Store Shared Class Data

A static member belongs to the class rather than an individual object.

A shared school pass threshold can be read without constructing a student record.

Before this lesson, review Read an Object Without Mutation when needed.

What you’ll learn

  • Use static members to store Shared Class Data.
  • Explain how the complete example produces 50.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

A shared school pass threshold can be read without constructing a student record.

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>

struct SchoolRules  {
  inline static constexpr int passMark = 50;
}
;
int main()  {
  std::cout << SchoolRules::passMark << '\n';
  return 0;
}

Expected output

50
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

struct SchoolRules  {
  inline static constexpr int passMark = 50;
}
;
int main()  {
  std::cout << SchoolRules::passMark << '\n';
  return 0;
}

Expected output

50

A static member belongs to the class rather than an individual object. A shared school pass threshold can be read without constructing a student record.

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

Important mistakes and boundaries

Static state is shared state and can complicate concurrency if mutable. Keep each student’s own mark in an instance field.

Change the school example

C++17 · edit this complete example

#include <iostream>

struct SchoolRules  {
  inline static constexpr int passMark = 50;
}
;
int main()  {
  std::cout << SchoolRules::passMark << '\n';
  return 0;
}

Your task: Change the shared threshold to 60. 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 shared threshold to 60. The program prints 60.

A static member belongs to the class rather than an individual object. A shared school pass threshold can be read without constructing a student record.

Static state is shared state and can complicate concurrency if mutable. Keep each student’s own mark in an instance field.

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>

struct SchoolRules  {
  inline static constexpr int passMark = 60;
}
;
int main()  {
  std::cout << SchoolRules::passMark << '\n';
  return 0;
}

Expected output

60
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

struct SchoolRules  {
  inline static constexpr int passMark = 50;
}
;
int main()  {
  std::cout << SchoolRules::passMark << '\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

50

A static member belongs to the class rather than an individual object. A shared school pass threshold can be read without constructing a student record. Static state is shared state and can complicate concurrency if mutable. Keep each student’s own mark in an instance field.

What to remember

A static member belongs to the class rather than an individual object. Static state is shared state and can complicate concurrency if mutable. Keep each student’s own mark in an instance field.

Useful connections: Read an Object Without Mutation, 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: Model an Is-a Relationship →