std::weak_ptr<T>::lock
| 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
#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); }
Output:
weak_ptr<> is not yet initialized
observe() is unable to lock weak_ptr<>
weak_ptr<> is initialized with shared_ptr
observe() is able to lock weak_ptr<>, value=42
shared_ptr<> has been destructed due to scope exit
observe() is 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 |
See also
| checks whether the referenced object was already deleted (public member function) |