Interview problem: Implement shared pointer 

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template<typename T>
class SharedPtr {
public:
    explicit SharedPtr(T *ptr=nullptr): ptr_(ptr), count_(new long(0)) {
        if (ptr_) {
            ++(*count_);
        }
    }

    ~SharedPtr() {
        if (count_) {
            --(*count_);
            if (*count_ == 0) {
                delete count_;
                delete ptr_;
            }
        }
    }

    SharedPtr(const SharedPtr& other): ptr_(other.ptr_), count_(other.count_) {
        ++(*count_);
    }

    SharedPtr& operator=(const SharedPtr& other) {
        SharedPtr tmp(other);
        tmp.swap(*this);
        return *this;
    }
    
    SharedPtr(SharedPtr&& other) noexcept: ptr_(other.ptr_), count_(other.count_) {
        other.ptr_ = nullptr;
        other.count_ = nullptr;
    }
    
    SharedPtr& operator=(SharedPtr&& other) noexcept {
        other.swap(*this);
        return *this;
    }
    
    T& operator*() const {
        return *ptr_;
    }

    T* operator->() const {
        return ptr_;
    }

    explicit operator bool() const {
        return ptr_;
    }
    
    void swap(SharedPtr &other) noexcept {
        std::swap(ptr_, other.ptr_);
        std::swap(count_, other.count_);
    }

private:
    T* ptr_;
    long *count_;
};
Location: 美国加利福尼亚州帕罗奥图

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