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Separate Visibility from Lifetime

Separate Visibility from Lifetime

Scope describes where a name can be used.

Lifetime describes when its object exists. A block-local mark can hide another name without changing the outer object.

Before this lesson, review Read Without Copying or Changing when needed.

What you’ll learn

  • Use scope and lifetime to separate Visibility from Lifetime.
  • Explain how the complete example produces 80 → 70.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Lifetime describes when its object exists. A block-local mark can hide another name without changing the outer object.

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()  {
  int marks = 70;
  {
    int marks = 80;
    std::cout << marks << '\n';
  }
  std::cout << marks << '\n';
  return 0;
}

Expected output

80
70
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

int main()  {
  int marks = 70;
  {
    int marks = 80;
    std::cout << marks << '\n';
  }
  std::cout << marks << '\n';
  return 0;
}

Expected output

80
70

Scope describes where a name can be used. Lifetime describes when its object exists. A block-local mark can hide another name without changing the outer object.

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

Important mistakes and boundaries

A pointer or reference to an expired local object is dangling even if the address looks plausible. Shadowing can also hide mistakes.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  int marks = 70;
  {
    int marks = 80;
    std::cout << marks << '\n';
  }
  std::cout << marks << '\n';
  return 0;
}

Your task: Change only the inner mark to 90. 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 only the inner mark to 90. The lines become 90 and 70.

Scope describes where a name can be used. Lifetime describes when its object exists. A block-local mark can hide another name without changing the outer object.

A pointer or reference to an expired local object is dangling even if the address looks plausible. Shadowing can also hide mistakes.

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()  {
  int marks = 70;
  {
    int marks = 90;
    std::cout << marks << '\n';
  }
  std::cout << marks << '\n';
  return 0;
}

Expected output

90
70
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  int marks = 70;
  {
    int marks = 80;
    std::cout << marks << '\n';
  }
  std::cout << marks << '\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

80 70

Scope describes where a name can be used. Lifetime describes when its object exists. A block-local mark can hide another name without changing the outer object. A pointer or reference to an expired local object is dangling even if the address looks plausible. Shadowing can also hide mistakes.

What to remember

Scope describes where a name can be used. A pointer or reference to an expired local object is dangling even if the address looks plausible. Shadowing can also hide mistakes.

Useful connections: Read Without Copying or Changing, 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: Choose by Parameter Types →