【重学C++】03 | 手撸C++智能指针实战教程

class AutoIntPtr {
public:
    AutoIntPtr(int* p = nullptr) : ptr(p) {}
    ~AutoIntPtr() { delete ptr; }

    int& operator*() const { return *ptr; }
    int* operator->() const { return ptr; }

private:
    int* ptr;
};

template <typename T>
class smart_ptr {
public:
	explicit smart_ptr(T* ptr = nullptr): ptr_(ptr) {}
	~smart_ptr() {
		delete ptr_;
	}
	T& operator*() const { return *ptr_; }
	T* operator->() const { return ptr_; }
private:
	T* ptr_;
}

void foo(smart_ptr<int> int_ptr) {
    // ...
}

int main() {
    int* raw_ptr = new int(42);
    foo(raw_ptr);  // 隐式转换为 smart_ptr<int>
    std::cout << *raw_ptr << std::endl;   // error: raw_ptr已经被回收了
    // ...
}

smart_ptr<int> ptr1{new int(10)};
smart_ptr<int> ptr2 = ptr1;

template <typename T>
class unique_smart_ptr {
public:
	explicit unique_smart_ptr(T* ptr = nullptr): ptr_(ptr) {}

	~unique_smart_ptr() {
		delete ptr_;
	}

	// 1. 自定义移动构造函数
	unique_smart_ptr(unique_smart_ptr&& other) {
		// 1.1 把other.ptr_ 赋值到this->ptr_
		ptr_ = other.ptr_;
		// 1.2 把other.ptr_指为nullptr，other不再拥有资源指针
		other.ptr_ = nullptr;
	}

	// 2. 自定义赋值行为
	unique_smart_ptr& operator = (unique_smart_ptr rhs) {
		// 2.1 交换rhs.ptr_和this->ptr_
		std::swap(rhs.ptr_, this->ptr_);
		return *this;
	}

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

private:
	T* ptr_;
};

unique_smart_ptr<int> ptr1{new int(10)};
unique_smart_ptr<int> ptr2 = ptr1; // error
unique_smart_ptr<int> ptr3 = std::move(ptr1); // ok

unique_smart_ptr<int> ptr4{ptr1} // error
unique_smart_ptr<int> ptr5{std::move(ptr1)} // ok

template <typename T>
class shared_smart_ptr {
public:
	// 构造函数
	explicit shared_smart_ptr(T* ptr = nullptr)
	// 析构函数
	~shared_smart_ptr()
	// 移动构造函数
	shared_smart_ptr(shared_smart_ptr&& other)
	// 拷贝构造函数
	shared_smart_ptr(const shared_smart_ptr& other)
	// 赋值函数
	shared_smart_ptr& operator = (shared_smart_ptr rhs)
	// 返回当前引用次数
	int use_count() const { return *count_; }

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

private:
	T* ptr_;
	int* count_;
}

explicit shared_smart_ptr(T* ptr = nullptr){
	ptr_ = ptr;
	if (ptr_) {
		count_ = new int(1);
	}
}

~shared_smart_ptr() {
	// ptr_为nullptr，不需要做任何处理
	if (ptr_) {
		return;
	}
	// 计数器减一
	--(*count_);
	// 计数器减为0，回收对象
	if (*count_ == 0) {
		delete ptr_;
		delete count_;
		return;
	}

}

shared_smart_ptr(shared_smart_ptr&& other) {
	ptr_ = other.ptr_;
	count_ = other.count_;

	other.ptr_ = nullptr;
	other.count_ = nullptr;
}

shared_smart_ptr& operator = (shared_smart_ptr rhs) {
	std::swap(rhs.ptr_, this->ptr_);
	std::swap(rhs.count_, this->count_);
	return *this;
}

shared_smart_ptr(const shared_smart_ptr& other) {
	ptr_ = other.ptr_;
	count_ = other.count_;

	if (ptr_) {
		(*count_)++;
	}
}

template <typename T>
class shared_smart_ptr {
public:
	explicit shared_smart_ptr(T* ptr = nullptr){
		ptr_ = ptr;
		if (ptr_) {
			count_ = new int(1);
		}
	}

	~shared_smart_ptr() {
		// ptr_为nullptr，不需要做任何处理
		if (ptr_ == nullptr) {
			return;
		}

		// 计数器减一
		--(*count_);

		// 计数器减为0，回收对象
		if (*count_ == 0) {
			delete ptr_;
			delete count_;
		}
	}

	shared_smart_ptr(shared_smart_ptr&& other) {
		ptr_ = other.ptr_;
		count_ = other.count_;
		
		other.ptr_ = nullptr;
		other.count_ = nullptr;
	}

	shared_smart_ptr(const shared_smart_ptr& other) {
		ptr_ = other.ptr_;
		count_ = other.count_;
		if (ptr_) {
			(*count_)++;
		}
	}

	shared_smart_ptr& operator = (shared_smart_ptr rhs) {
		std::swap(rhs.ptr_, this->ptr_);
		std::swap(rhs.count_, this->count_);
		return *this;
	}

	int use_count() const { return *count_; };

	T& operator*() const { return *ptr_; };

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

private:
	T* ptr_;
	int* count_;
};

int main(int argc, const char** argv) {
	shared_smart_ptr<int> ptr1(new int(1));
	std::cout << "[初始化ptr1] use count of ptr1: " << ptr1.use_count() << std::endl;
	{
		// 赋值使用拷贝构造函数
		shared_smart_ptr<int> ptr2 = ptr1;
		std::cout << "[使用拷贝构造函数将ptr1赋值给ptr2] use count of ptr1: " << ptr1.use_count() << std::endl;

		// 赋值使用移动构造函数
		shared_smart_ptr<int> ptr3 = std::move(ptr2);
		std::cout << "[使用移动构造函数将ptr2赋值给ptr3] use count of ptr1: " << ptr1.use_count() << std::endl;
	}
	std::cout << "[ptr2和ptr3析构后] use count of ptr1: " << ptr1.use_count() << std::endl;
}

[初始化ptr1] use count of ptr1: 1
[使用拷贝构造函数将ptr1赋值给ptr2] use count of ptr1: 2
[使用移动构造函数将ptr2赋值给ptr3] use count of ptr1: 2
[ptr2和ptr3析构后] use count of ptr1: 1