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Give One Owner Responsibility

Give One Owner Responsibility

Std::unique_ptr owns an object and releases it when its lifetime ends.

It is movable rather than copyable, making transfer of ownership explicit.

Before this lesson, review Tie Cleanup to Lifetime when needed.

What you’ll learn

  • Use unique ptr to give One Owner Responsibility.
  • Explain how the complete example produces 72 → true.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

It is movable rather than copyable, making transfer of ownership 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>
#include <memory>
#include <utility>

int main()  {
  auto mark = std::make_unique<int>(72);
  auto owner = std::move(mark);
  std::cout << *owner << '\n' << std::boolalpha << (mark == nullptr) << '\n';
  return 0;
}

Expected output

72
true
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <memory>
#include <utility>

int main()  {
  auto mark = std::make_unique<int>(72);
  auto owner = std::move(mark);
  std::cout << *owner << '\n' << std::boolalpha << (mark == nullptr) << '\n';
  return 0;
}

Expected output

72
true

Std::unique_ptr owns an object and releases it when its lifetime ends. It is movable rather than copyable, making transfer of ownership 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 72 → true. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

Do not dereference the old unique_ptr after its ownership has moved. The new owner, not every raw observer, controls destruction.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <memory>
#include <utility>

int main()  {
  auto mark = std::make_unique<int>(72);
  auto owner = std::move(mark);
  std::cout << *owner << '\n' << std::boolalpha << (mark == nullptr) << '\n';
  return 0;
}

Your task: Create the owned mark as 80. 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: Create the owned mark as 80. The output becomes 80 and true.

Std::unique_ptr owns an object and releases it when its lifetime ends. It is movable rather than copyable, making transfer of ownership explicit.

Do not dereference the old unique_ptr after its ownership has moved. The new owner, not every raw observer, controls destruction.

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>
#include <utility>

int main()  {
  auto mark = std::make_unique<int>(80);
  auto owner = std::move(mark);
  std::cout << *owner << '\n' << std::boolalpha << (mark == nullptr) << '\n';
  return 0;
}

Expected output

80
true
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <memory>
#include <utility>

int main()  {
  auto mark = std::make_unique<int>(72);
  auto owner = std::move(mark);
  std::cout << *owner << '\n' << std::boolalpha << (mark == nullptr) << '\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

72 true

Std::unique_ptr owns an object and releases it when its lifetime ends. It is movable rather than copyable, making transfer of ownership explicit. Do not dereference the old unique_ptr after its ownership has moved. The new owner, not every raw observer, controls destruction.

Check a common mistake

Can two unique_ptr objects copy ownership of the same object?
Choose one answer. You can try again.

Answer and explanation

No. Transfer ownership by moving.

Unique ownership is noncopyable; const does not turn it into shared ownership.

What to remember

Std::unique_ptr owns an object and releases it when its lifetime ends. Do not dereference the old unique_ptr after its ownership has moved. The new owner, not every raw observer, controls destruction.

Useful connections: Tie Cleanup to Lifetime, 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: Share and Observe Ownership →