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Organize Names and Declarations

Organize Names and Declarations

A namespace groups related names and avoids accidental collisions.

A header provides declarations that another source file can include, while definitions supply the actual behavior.

Before this lesson, review Solve a Smaller Version when needed.

What you’ll learn

  • Use namespaces and headers to organize Names and Declarations.
  • 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 header provides declarations that another source file can include, while definitions supply the actual behavior.

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>

namespace school  {
  constexpr int passMark = 50;
}
int main()  {
  std::cout << school::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>

namespace school  {
  constexpr int passMark = 50;
}
int main()  {
  std::cout << school::passMark << '\n';
  return 0;
}

Expected output

50

A namespace groups related names and avoids accidental collisions. A header provides declarations that another source file can include, while definitions supply the actual behavior.

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

Including a file is a preprocessing operation, not a runtime import. Keep non-inline definitions out of commonly included headers to avoid duplicate-definition errors.

Change the school example

C++17 · edit this complete example

#include <iostream>

namespace school  {
  constexpr int passMark = 50;
}
int main()  {
  std::cout << school::passMark << '\n';
  return 0;
}

Your task: Change school::passMark to 60 in its definition. 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 school::passMark to 60 in its definition. The program prints 60.

A namespace groups related names and avoids accidental collisions. A header provides declarations that another source file can include, while definitions supply the actual behavior.

Including a file is a preprocessing operation, not a runtime import. Keep non-inline definitions out of commonly included headers to avoid duplicate-definition errors.

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>

namespace school  {
  constexpr int passMark = 60;
}
int main()  {
  std::cout << school::passMark << '\n';
  return 0;
}

Expected output

60
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

namespace school  {
  constexpr int passMark = 50;
}
int main()  {
  std::cout << school::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 namespace groups related names and avoids accidental collisions. A header provides declarations that another source file can include, while definitions supply the actual behavior. Including a file is a preprocessing operation, not a runtime import. Keep non-inline definitions out of commonly included headers to avoid duplicate-definition errors.

What to remember

A namespace groups related names and avoids accidental collisions. Including a file is a preprocessing operation, not a runtime import. Keep non-inline definitions out of commonly included headers to avoid duplicate-definition errors.

Useful connections: Solve a Smaller Version, 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: Use a Fixed Collection →