i want to write a copyOnwrite thread-safe vector like below, if i not use weak_ptr, it works fine. it seems shared_ptr reset call vector copy constructor and then weak_ptr's. the weak_ptr ctor not behaves as expect..

so how can i fix this issue.. my code just like below

class B {
public:
     void print() {
     }
 };
 class A {
    typedef std::vector<std::weak_ptr<B>> TargetList;
    std::shared_ptr<TargetList> _targets;
    std::mutex  _targetsMutex;

public:
    A() {
        _targets = std::make_shared<TargetList>();
    }
    void addTarget(std::shared_ptr<B> newTarget) {
        std::lock_guard<std::mutex> lock(_targetsMutex);

        if (!_targets.unique()) {
            _targets.reset(new TargetList(*_targets));
        }
        assert(_targets.unique());
        _targets->push_back(newTarget);
    }
    void traverse() {
        std::shared_ptr<TargetList> targets;
        {
            std::lock_guard<std::mutex> lock(_targetsMutex);
            targets = _targets;
            assert(!_targets.unique());
        }

        assert(!targets.unique());

        for ( auto & it : *targets) {
            auto outp = it.lock();
            if(outp) {
                outp->print();
            }
        }
    }
};

some kind of test code here..

static void spawnB(A *a) {
    std::shared_ptr<B> b[20000];
    for (int i = 100; i< 20000; i++) {
        b[i] = std::make_shared<B>();
        a->addTarget(b[i]);
    }
    std::cout<<a->getCount()<<std::endl;
}

int main() {
    A *a = new A();
    std::shared_ptr<B> b = std::make_shared<B>();
    a->addTarget(b);

    std::thread t1(&spawnB, a);
    std::thread t2(&A::traverse1000, a);
    b.reset();
    t1.join();
    t2.join();      
    delete a;
    return 0;
}