性病：弱[医]PTR：：LOCK

#include <iostream>
#include <memory>
 
void observe(std::weak_ptr<int> weak) 
{
    if (auto observe = weak.lock()) {
        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
 
    {
        auto shared = std::make_shared<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
}

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<>

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