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std::weak_ptr::lock

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< cpp‎ | memory‎ | weak ptr
Revision as of 20:48, 17 March 2014 by Cubbi (Talk | contribs)

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std::shared_ptr<T> lock() const
(since C++11)

Creates a new 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.

Contents

[edit] Parameters

(none)

[edit] Return value

A shared_ptr which shares ownership of the owned object.

[edit] Exceptions

noexcept specification:  
noexcept
  

[edit] Notes

Both this function and the constructor of shared_ptr may be used to acquire temporary ownership of the managed object. The difference is that the constructor of std::shared_ptr<T> throws an exception when the std::weak_ptr argument is empty, while std::weak_ptr<T>::lock() constructs an empty std::shared_ptr<T>.

[edit] Example

#include <iostream>
#include <memory>
#include <thread>
 
void observe(std::weak_ptr<int> weak) 
{
    std::shared_ptr<int> observe(weak.lock());
    if (observe) {
        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);
 
    {
        std::shared_ptr<int> shared(new 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<>

[edit] See also

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