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Share and Observe Ownership

Share and Observe Ownership

Std::shared_ptr counts participating owners, while weak_ptr observes without extending lifetime.

Lock a weak observer before use to obtain an owner if the object still exists.

Before this lesson, review Give One Owner Responsibility when needed.

What you’ll learn

  • Use shared ptr and weak ptr to share and Observe Ownership.
  • 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

Lock a weak observer before use to obtain an owner if the object still exists.

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>
#include <memory>

int main()  {
  auto owner = std::make_shared<int>(72);
  std::weak_ptr<int> observer = owner;
  owner.reset();
  std::cout << std::boolalpha << observer.expired() << '\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>
#include <memory>

int main()  {
  auto owner = std::make_shared<int>(72);
  std::weak_ptr<int> observer = owner;
  owner.reset();
  std::cout << std::boolalpha << observer.expired() << '\n';
  return 0;
}

Expected output

true

Std::shared_ptr counts participating owners, while weak_ptr observes without extending lifetime. Lock a weak observer before use to obtain an owner if the object still exists.

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

Shared ownership can form cycles; weak observers help break them. Reference counting does not make mutable object access automatically thread-safe.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <memory>

int main()  {
  auto owner = std::make_shared<int>(72);
  std::weak_ptr<int> observer = owner;
  owner.reset();
  std::cout << std::boolalpha << observer.expired() << '\n';
  return 0;
}

Your task: Keep owner alive instead of resetting it. 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: Keep owner alive instead of resetting it. The observer’s expired result is false.

Std::shared_ptr counts participating owners, while weak_ptr observes without extending lifetime. Lock a weak observer before use to obtain an owner if the object still exists.

Shared ownership can form cycles; weak observers help break them. Reference counting does not make mutable object access automatically thread-safe.

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>
#include <memory>

int main()  {
  auto owner = std::make_shared<int>(72);
  std::weak_ptr<int> observer = owner;
  
  std::cout << std::boolalpha << observer.expired() << '\n';
  return 0;
}

Expected output

false
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <memory>

int main()  {
  auto owner = std::make_shared<int>(72);
  std::weak_ptr<int> observer = owner;
  owner.reset();
  std::cout << std::boolalpha << observer.expired() << '\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

Std::shared_ptr counts participating owners, while weak_ptr observes without extending lifetime. Lock a weak observer before use to obtain an owner if the object still exists. Shared ownership can form cycles; weak observers help break them. Reference counting does not make mutable object access automatically thread-safe.

What to remember

Std::shared_ptr counts participating owners, while weak_ptr observes without extending lifetime. Shared ownership can form cycles; weak observers help break them. Reference counting does not make mutable object access automatically thread-safe.

Useful connections: Give One Owner Responsibility, 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: Distinguish Copying from Transfer →