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Protect Fixed Values

Protect Fixed Values

A const variable cannot be modified through that name.

constexpr additionally requires a suitable constant expression at initialization, making a fixed school pass threshold explicit.

Before this lesson, review Choose Data Types when needed.

What you’ll learn

  • Use constants and constexpr to protect Fixed Values.
  • 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

constexpr additionally requires a suitable constant expression at initialization, making a fixed school pass threshold explicit.

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>

int main()  {
  constexpr int passMark = 50;
  int marks = 72;
  std::cout << std::boolalpha << (marks >= passMark) << '\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>

int main()  {
  constexpr int passMark = 50;
  int marks = 72;
  std::cout << std::boolalpha << (marks >= passMark) << '\n';
  return 0;
}

Expected output

true

A const variable cannot be modified through that name. constexpr additionally requires a suitable constant expression at initialization, making a fixed school pass threshold explicit.

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

Const is not a promise that every referenced object is deeply immutable. constexpr and runtime const are related but different restrictions.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  constexpr int passMark = 50;
  int marks = 72;
  std::cout << std::boolalpha << (marks >= passMark) << '\n';
  return 0;
}

Your task: Change marks to 40. 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 marks to 40. The comparison prints false.

A const variable cannot be modified through that name. constexpr additionally requires a suitable constant expression at initialization, making a fixed school pass threshold explicit.

Const is not a promise that every referenced object is deeply immutable. constexpr and runtime const are related but different restrictions.

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>

int main()  {
  constexpr int passMark = 50;
  int marks = 40;
  std::cout << std::boolalpha << (marks >= passMark) << '\n';
  return 0;
}

Expected output

false
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  constexpr int passMark = 50;
  int marks = 72;
  std::cout << std::boolalpha << (marks >= 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

true

A const variable cannot be modified through that name. constexpr additionally requires a suitable constant expression at initialization, making a fixed school pass threshold explicit. Const is not a promise that every referenced object is deeply immutable. constexpr and runtime const are related but different restrictions.

What to remember

A const variable cannot be modified through that name. Const is not a promise that every referenced object is deeply immutable. constexpr and runtime const are related but different restrictions.

Useful connections: Choose Data Types, 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: Convert Values Deliberately →