#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();
}

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