Std::weak_ptr::lock (3) Linux Manual Page

std::weak_ptr<T>::lock – std::weak_ptr<T>::lock

Synopsis

std::shared_ptr<T> lock() const noexcept; (since C++11)
Creates a new std::shared_ptr that shares ownership of the managed object. If there is no managed object, i.e. *this is empty, then the returned shared_ptr also is empty.
Effectively returns expired() ? shared_ptr<T>() : shared_ptr<T>(*this), executed atomically.

Parameters

(none)

Return value

A shared_ptr which shares ownership of the owned object if std::weak_ptr::expired returns false. Else returns default-constructed shared_ptr of type T.

Notes

Both this function and the constructor of std::shared_ptr may be used to acquire temporary ownership of the managed object referred to by a std::weak_ptr. The difference is that the constructor of std::shared_ptr throws an exception when its std::weak_ptr argument is empty, while std::weak_ptr<T>::lock() constructs an empty std::shared_ptr<T>.

Example

// Run this code

#include <iostream>

#include <memory>
void observe(std::weak_ptr<int> weak)

{
    if (auto observe = weak.lock()) {
        std::cout << "\tobserve() able to lock weak_ptr<>, value=" << *observe << "\n";
    } else {
        std::cout << "\tobserve() unable to lock weak_ptr<>\n";
    }
}
int main()

{
    std::weak_ptr<int> weak;
    std::cout << "weak_ptr<> not yet initialized\n";
    observe(weak);
    {
        auto shared = std::make_shared<int>(42);
        weak = shared;
        std::cout << "weak_ptr<> initialized with shared_ptr.\n";
        observe(weak);
    }
    std::cout << "shared_ptr<> has been destructed due to scope exit.\n";
    observe(weak);
}

Output:

  weak_ptr<> not yet initialized
          observe() unable to lock weak_ptr<>
  weak_ptr<> initialized with shared_ptr.
          observe() able to lock weak_ptr<>, value=42
  shared_ptr<> has been destructed due to scope exit.
          observe() unable to lock weak_ptr<>

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG_2316 C++11 lock() was not required to be atomic, but required to be noexcept, which led to a contradiction specified to be atomic

std::weak_ptr::lock (3) Linux Manual Page