119 lines
3.6 KiB
C++
119 lines
3.6 KiB
C++
//
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// Created by light on 19-12-9.
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//
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#include <iostream>
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// copy and swap : https://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom
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// ADL : https://stackoverflow.com/questions/8111677/what-is-argument-dependent-lookup-aka-adl-or-koenig-lookup
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namespace A {
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template<typename T>
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class smart_ptr {
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public:
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smart_ptr() noexcept : ptr_(new T()) {
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}
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smart_ptr(const T &ptr) noexcept : ptr_(new T(ptr)) {
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}
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smart_ptr(smart_ptr &rhs) noexcept {
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std::cout << "copy ctor" << std::endl;
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ptr_ = rhs.release(); // 释放所有权,此时rhs的ptr_指针为nullptr
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}
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// 方法1：为了避免自赋值,通常采用下面写法 不好! 不具备异常安全,只具备自我赋值安全性
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// smart_ptr &operator=(const smart_ptr &rhs) {
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// if (*this != rhs) {
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// delete ptr_;
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// ptr_ = new T(rhs.ptr_); // 当new 发生异常,此时ptr_指向的而是一块被删除区域,而不是被赋值对象的区域
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// return *this;
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// }
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// return *this;
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// }
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// 方法2：如果new出现异常,ptr_会保持原装! 也可以处理自我赋值! 还是不够好!
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// smart_ptr &operator=(const smart_ptr &rhs) {
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// T *origin = ptr_;
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// ptr_ = new T(rhs.ptr_);
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// delete origin;
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// return *this;
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// }
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// 方法3：copy and swap 很好!
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// smart_ptr &operator=(smart_ptr &rhs) noexcept {
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// smart_ptr tmp(rhs);
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// swap(tmp);
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// return *this;
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// }
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// 方法4：同方法3,改为传值
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// 既适用于copy ctor也适用于 move ctor
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smart_ptr &operator=(smart_ptr rhs) noexcept {
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swap(rhs);
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return *this;
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}
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// move ctor
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smart_ptr(smart_ptr &&rhs) noexcept {
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std::cout << "move ctor" << std::endl;
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ptr_ = rhs.ptr_;
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if (ptr_)
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rhs.ptr_ = nullptr;
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}
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// move assignment
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// smart_ptr &operator=(smart_ptr &&rhs) noexcept {
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// std::cout << "move assignment" << std::endl;
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// smart_ptr tmp(rhs);
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// swap(rhs);
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// return *this;
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// }
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void swap(smart_ptr &rhs) noexcept { // noexcept == throw() 保证不抛出异常
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using std::swap;
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swap(ptr_, rhs.ptr_);
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}
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T *release() noexcept {
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T *ptr = ptr_;
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ptr_ = nullptr;
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return ptr;
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}
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T *get() const noexcept {
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return ptr_;
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}
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private:
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T *ptr_;
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};
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// 提供一个非成员swap函数for ADL(Argument Dependent Lookup)
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template<typename T>
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void swap(A::smart_ptr<T> &lhs, A::smart_ptr<T> &rhs) noexcept {
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lhs.swap(rhs);
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}
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}
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// 注释开启,会引发ADL冲突
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//namespace std {
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// // 提供一个非成员swap函数for ADL(Argument Dependent Lookup)
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// template<typename T>
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// void swap(A::smart_ptr<T> &lhs, A::smart_ptr<T> &rhs) noexcept {
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// lhs.swap(rhs);
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// }
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//
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//}
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int main() {
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using std::swap;
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A::smart_ptr<std::string> s1("hello"), s2("world");
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// 交换前
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std::cout << *s1.get() << " " << *s2.get() << std::endl;
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swap(s1, s2); // 这里swap 能够通过Koenig搜索或者说ADL根据s1与s2的命名空间来查找swap函数
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// 交换后
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std::cout << *s1.get() << " " << *s2.get() << std::endl;
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// s1 = s2;
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A::smart_ptr<std::string> s3 = s1;
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A::smart_ptr<std::string> s4 = std::move(s1);
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} |