template <class T>
class ObjectPool
{
public:
    using DeleterType = std::function<void(T*)>;

    void add(std::unique_ptr<T> t)
    {
        std::lock_guard<std::mutex> locker(mutex_);
        pool_.push_back(std::move(t));
    }

    std::unique_ptr<T, DeleterType> get
 
()
    {
        std::lock_guard<std::mutex> locker(mutex_);
        if (pool_.empty()) {
            cout << "no more object" << endl;
            return nullptr;
        }

        //every time add custom deleter for default unique_ptr
        std::unique_ptr<T, DeleterType> ptr(pool_.back().release(), [this
 
](T* t)
        {
            pool_.push_back(std::unique_ptr<T>(t));
        });

        pool_.pop_back();
        return std::move(ptr);
    }

    bool empty()
    {
        std::lock_guard<std::mutex> locker(mutex_);
        return pool_.empty();
    }

    size_t size()
    {
        std::lock_guard
 
<std::mutex> locker(mutex_);
        return pool_.size();
    }

private:
    std::mutex mutex_;
    std::vector<std::unique_ptr<T>> pool_;
};

c++ 對象池的創建