一、迭代器

C++ 迭代器是指向容器内元素的对象。它们可用于循环访问该容器的对象。我们知道迭代器的一个示例是指针。指针可用于循环访问 C 样式数组.

二、代码

自己实现一个迭代器

// C++ iterators are objects that point to an element inside a container.
// They can be used to iterate through the objects of that container.
// One example of an iterator that you know is a pointer. A pointer
// can be used to iterate through a C style array. Take the following
// C-style code snippet:
// C++ 迭代器是指向容器内元素的对象。它们可用于循环访问该容器的对象。
// 我们知道迭代器的一个示例是指针。指针可用于循环访问 C 样式数组.// int *array = malloc(sizeof(int) * 10);
// int *iter = array;
// int zero_elem = *iter;
// iter++;
// int first_elem = *iter;// As we can see, the ++ operator can be used to iterate through the
// C style array, and the derefence operator returns the value at the
// iterator.// 正如我们所看到的，++ 运算符可用于遍历 C 样式数组，而引用运算符在迭代器处返回值。// The main components of a C++ iterator are its main two operators. The
// dereference operator (*) on an iterator should return the value of the
// element at the current position of the iterator. The ++ (postfix increment)
// operator should increment the iterator's position by 1. As you can see, this
// is true with the pointer being used to iterate through a C style array.
// C++迭代器的主要组件是其主要的两个运算符。迭代器上的取引用运算符(*),应返回迭代器当前位置处的元素值。++（后缀增量）运算符应将迭代器的位置递增1。和c语言的用法一样。// There are a few examples about how to use iterators to access elements
// in C++ STL containers in vectors.cpp, sets.cpp, unordered_maps.cpp, 
// and auto.cpp. This is because using iterators in C++ to access and modify
// elements in C++ STL containers is considered good style, and worth
// mentioning in these files. // 有几个示例 vectors.cpp、sets.cpp、unordered_maps.cpp 和 auto.cpp 介绍了如何使用迭代器访问C++STL 容器中的元素。
// 在 C++ 中使用迭代器来访问和修改 C++ STL 容器中的元素被认为是很好的风格，在这些文件中值得一提。// This file will mainly focus on the implementation of iterators. In this
// file, we demonstrate implementing C++ iterators by writing a basic doubly
// linked list (DLL) iterator.// Includes std::cout (printing) for demo purposes.
#include <iostream>// This is the definition of the Node struct, used in our DLL.
struct Node {Node(int val) : next_(nullptr), prev_(nullptr), value_(val) {}Node* next_;Node* prev_;int value_;
};// This class implements a C++ style iterator for the doubly linked list class 
// DLL. This class's constructor takes in a node that marks the start of the
// iterating. It also implements several operators that increment the iterator
// (i.e. accessing the next element in the DLL) and test for equality between
// two different iterators by comparing their curr_ pointers.
class DLLIterator {public:DLLIterator(Node* head) : curr_(head) {}// Implementing a prefix increment operator (++iter).DLLIterator& operator++() {curr_ = curr_->next_;return *this;}// Implementing a postfix increment operator (iter++). The difference// between a prefix and postfix increment operator is the return value// of the operator. The prefix operator returns the result of the// increment, while the postfix operator returns the iterator before// the increment.DLLIterator operator++(int) {DLLIterator temp = *this;++*this;return temp;}// This is the equality operator for the DLLIterator class. It// tests that the current pointers are the same.bool operator==(const DLLIterator &itr) const {return itr.curr_ == this->curr_;}// This is the inequality operator for the DLLIterator class. It// tests that the current pointers are not the same.bool operator!=(const DLLIterator &itr) const {return itr.curr_ != this->curr_;}// This is the dereference operator for the DLLIterator class. It// returns the value of the element at the current position of the// iterator. The current position of the iterator is marked by curr_,// and we can access the value of curr_ by accessing its value field.int operator*() {return curr_->value_;}private:Node* curr_;
};// This is a basic implementation of a doubly linked list. It also includes
// iterator functions Begin and End, which return DLLIterators that can be
// used to iterate through this DLL instance.
class DLL {public:// DLL class constructor.DLL() : head_(nullptr), size_(0) {}// Destructor should delete all the nodes by iterating through them.~DLL() {Node *current = head_;while(current != nullptr) {Node *next = current->next_;delete current;current = next;}head_ = nullptr;}// Function for inserting val at the head of the DLL.void InsertAtHead(int val) {Node *new_node = new Node(val);new_node->next_ = head_;if (head_ != nullptr) {head_->prev_ = new_node;}head_ = new_node;size_ += 1;}// The Begin() function returns an iterator to the head of the DLL,// which is the first element to access when iterating through.DLLIterator Begin() {return DLLIterator(head_);}// The End() function returns an iterator that marks the one-past-the-last// element of the iterator. In this case, this would be an iterator with// its current pointer set to nullptr.DLLIterator End() {return DLLIterator(nullptr);}Node* head_{nullptr};size_t size_;
};// The main function shows the usage of the DLL iterator.
int main() {// Creating a DLL and inserting elements into it.DLL dll;dll.InsertAtHead(6);dll.InsertAtHead(5);dll.InsertAtHead(4);dll.InsertAtHead(3);dll.InsertAtHead(2);dll.InsertAtHead(1);// We can iterate through our DLL via both our prefix and postfix operators.std::cout << "Printing elements of the DLL dll via prefix increment operator\n";for (DLLIterator iter = dll.Begin(); iter != dll.End(); ++iter) {std::cout << *iter << " ";}std::cout << std::endl;std::cout << "Printing elements of the DLL dll via postfix increment operator\n";for (DLLIterator iter = dll.Begin(); iter != dll.End(); iter++) {std::cout << *iter << " ";}std::cout << std::endl;return 0;
}

三、运行结果