std::weak_ptr

std::weak_ptr is a smart pointer that holds a non-owning ("weak") reference to an object that is managed by std::shared_ptr . It must be converted to std::shared_ptr in order to access the referenced object.

std::weak_ptr models temporary ownership: when an object needs to be accessed only if it exists, and it may be deleted at any time by someone else, std::weak_ptr is used to track the object, and it is converted to std::shared_ptr to acquire temporary ownership. If the original std::shared_ptr is destroyed at this time, the object's lifetime is extended until the temporary std::shared_ptr is destroyed as well.

Another use for std::weak_ptr is to break reference cycles formed by objects managed by std::shared_ptr . If such cycle is orphaned (i.e., there are no outside shared pointers into the cycle), the shared_ptr reference counts cannot reach zero and the memory is leaked. To prevent this, one of the pointers in the cycle can be made weak.

[edit] Member types

#include <iostream>
#include <memory>
 
std::weak_ptr<int> gw;
 
void observe()
{
 std::cout << "gw.use_count() == " << gw.use_count() << "; ";
 // we have to make a copy of shared pointer before usage:
 if (std::shared_ptr <int> spt = gw.lock())
 std::cout << "*spt == " << *spt << '\n';
 else
 std::cout << "gw is expired\n";
}
 
int main()
{
 {
 auto sp = std::make_shared <int>(42);
 gw = sp;
 
 observe();
 }
 
 observe();
}

gw.use_count() == 1; *spt == 42
gw.use_count() == 0; gw is expired

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