std::weak_ptr<T>::lock

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.

# Declarations

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

(since C++11)

# 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::lock() constructs an empty std::shared_ptr.

# Example

#include <iostream>
#include <memory>
 
void observe(std::weak_ptr<int> weak)
{
    if (auto p = weak.lock())
        std::cout << "\tobserve() is able to lock weak_ptr<>, value=" << *p << '\n';
    else
        std::cout << "\tobserve() is unable to lock weak_ptr<>\n";
}
 
int main()
{
    std::weak_ptr<int> weak;
    std::cout << "weak_ptr<> is not yet initialized\n";
    observe(weak);
 
    {
        auto shared = std::make_shared<int>(42);
        weak = shared;
        std::cout << "weak_ptr<> is initialized with shared_ptr\n";
        observe(weak);
    }
 
    std::cout << "shared_ptr<> has been destructed due to scope exit\n";
    observe(weak);
}

# Defect reports

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

# See also

expired