0.前言

C++11為我們帶來了std::shared_ptr菲茬，方便我們在動態(tài)分配內(nèi)存的時候發(fā)生memory leak来庭，他是基于引用計數(shù)實現(xiàn)的，下面給出一個簡化版本承疲。

1.實現(xiàn)

namespace smart_ptr {
    class counter {
    public:
        counter(unsigned cnt):count_(cnt) {

        }

        unsigned& operator++() {
            ++count_;
            return count_;
        }

        unsigned operator++(int) {
            unsigned count = count_;
            ++count_;
            return count;
        }

        unsigned& operator--() {
            --count_;
            return count_;
        }

        unsigned operator--(int) {
            unsigned count = count_;
            --count_;
            return count;
        }
        unsigned getCount()const {
            return count_;
        }

    private:
        unsigned count_ = 0;
    };

    template<typename T>
    class shared_ptr {
    public:
        using pointer=T *;
        using reference=T &;
        shared_ptr(pointer ptr) {
            counter_ = new counter(1);
            data_ = ptr;
        }
        ~shared_ptr() {
            --(*counter_);
            if (counter_->getCount() == 0) {
                delete data_;
                data_ = nullptr;
                delete counter_;
                counter_ = nullptr;
            }
        }

        shared_ptr(const shared_ptr<T>& s_ptr) {
            counter_ = s_ptr.counter_;
            ++(*counter_);
            data_ = s_ptr.data_;
        }

        pointer get()const {
            return data_;
        }

        unsigned use_count()const {
            return counter_->count_;
        }
        pointer operator->() {
            return data_;
        }

        reference operator*() {
            return *data_;
        }
    private:
        counter* counter_;
        pointer data_;
    };
}

在這里我們聲明了一個類counter專門用于計算指向動態(tài)內(nèi)存的對象數(shù)量记焊，在我們撞見shared_ptr對象時逸月，動態(tài)分配一個內(nèi)存給成員counter_，當(dāng)對象發(fā)生拷貝時遍膜，使用同一個counter增加引用計數(shù)碗硬。在析構(gòu)時，我們將counter進行自減瓢颅，如果自減后counter為零恩尾，那么就釋放動態(tài)分配的內(nèi)存。也就是說挽懦，如果我們想要像new操作符一樣手動析構(gòu)翰意，那么我們只需要手動調(diào)用~shared_ptr()即可。手動調(diào)用析構(gòu)函數(shù)只是銷毀了對象信柿，并沒有釋放這段內(nèi)存冀偶。