std::weak_ptr::lock - cppreference.com (original) (raw)

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.

[edit] 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.

[edit] 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.

[edit] Example

#include #include   void observe(std::weak_ptr 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 weak; std::cout << "weak_ptr<> is not yet initialized\n"; observe(weak);   { auto shared = std::make_shared(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<>

[edit] Defect reports

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

[edit] See also

| | checks whether the referenced object was already deleted (public member function) [edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |