NAME

std::weak_ptr::lock - std::weak_ptr::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
lock() was not required to be atomic, but
LWG 2316 C++11 required to be noexcept, which led to a specified to be atomic
contradiction