C++ 右值 shared_ptr 和右值 weak_ptr

Question

std::shared_ptr<std::string> test() {
  return std::make_shared<std::string>("sdsd");
}

cout << *test() << endl;

The above code works. Can someone please let me know where is the "sdsd" string stored. I was expecting it to error out as rvalue is a temporary object. Where is the rvalue copied to and stored in?

With weak_ptr

std::weak_ptr<std::string> test() {
  return std::make_shared<std::string>("sdsd");
}

  cout << *test().lock() << endl;

Interestingly the above code errors out. What's the difference?

Answer 1

In

std::shared_ptr<std::string> test() {
  return std::make_shared<std::string>("sdsd");
}

the constructed shared_ptr is returned and lives on as a temporary variable long enough to be used by the caller before going out of scope and freeing the string allocated by make_shared .

The "sdsd" string is stored in dynamic storage owned by the returned shared_ptr .

In

std::weak_ptr<std::string> test() {
  return std::make_shared<std::string>("sdsd");
} 

the shared_ptr wasn't returned and went out of scope, taking the string allocated by make_shared with it. Since this was the only existing copy of the shared_ptr this leaves the returned temporary weak_ptr variable connected to an expired share_ptr . If you test the return value of lock you'll see it's a default-constructed shared_ptr holding a null pointer.

Documentation for std::weak_ptr::lock .

The "sdsd" string isn't stored anywhere after test returns. It departed when the shared_ptr that owned it went out of scope.

Answer 2

The above code works. Can someone please let me know where is the "sdsd" string stored. I was expecting it to error out as rvalue is a temporary object. Where is the rvalue copied to and stored in?

The memory was allocated in the call to make_shared .

Interestingly the above code errors out. What's the difference?

The difference is that the lock operation on a weak_ptr can fail if there does not exist at least one shared_ptr to the object. That's the purpose of weak_ptr -- to allow the object to be freed and access it only if it still exists.