std::weak_ptr<T>::lock

Since C++11

std::shared_ptr<T> lock() const noexcept;

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

#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);
}

Result

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