手撸一个C++迭代器
先撸一个简单版本
C++语言的特点就是少了一个符号都会造成非常不同的结果。以下代码有个很致命的错误，不知道能否一眼看出来。
// ranges_iterators.cpp : 此文件包含 "main" 函数。程序执行将在此处开始并结束。
//#include <iostream>
#include <map>
#include <string>
#include <algorithm>
#include <iterator>template <typename Rng, size_t I>
class elements_view
{using base_range = std::remove_reference_t<Rng>;using base_iterator = decltype(std::declval<Rng>().begin());using base_value_type = typename std::iterator_traits<base_iterator>::value_type;public:class iterator {public:using difference_type = std::ptrdiff_t; // 迭代器是“高级”的指针，用起来应该跟指针相似，迭代器相减得到的结果的类型应该与指针相减得到的结果类型一样。using value_type = std::tuple_element_t<I, base_value_type>;using pointer = decltype(&std::get<I>(*std::declval<Rng>().begin())); // 是否与 value_type * 相等？using reference = decltype(std::get<I>(*std::declval<Rng>().begin())); // 是否与 value_type & 相等？using iterator_catalog = std::input_iterator_tag;iterator() = default;explicit iterator(const base_iterator& it) : it_(it) {}reference operator *() const { return std::get<I>(*it_); }pointer operator -> () const { return &std::get<I>(*it_); }iterator& operator ++() {++it_;return *this;}iterator operator ++(int) {iterator temp{ *this };++it_;return temp;}friend bool operator ==(const iterator& lhs, const iterator& rhs) {return lhs.it_ == rhs.it_;}friend bool operator !=(const iterator& lhs, const iterator& rhs) {return lhs.it_ != rhs.it_;}private:base_iterator it_{};};elements_view(Rng& rng) : rng_(&rng) {}iterator begin() const {// return iterator { rng_->begin() }auto iter = rng_->begin();return iterator{ iter };}iterator end() const {// return iterator{ rng_->end() };auto iter = rng_->end();return iterator{ iter };}private:base_range* rng_{};
};template <size_t I, typename Rng>
auto elements(Rng r) {return elements_view<Rng, I>(r);
}int main()
{std::map<int, std::string> m{ {1,"aaa"}, {2, "bbb"} };auto v = elements<1>(m);//for (auto it = v.begin(); it != v.end(); ++it)// std::cout << *it << '\n';auto out = std::ostream_iterator<std::string>(std::cout, ", ");std::copy(v.begin(), v.end(), out);std::cout << "Hello World!\n";
}
再撸一个可以跟ranges配合的版本
#include <iostream>
#include <map>
#include <string>
#include <algorithm>
#include <iterator>
#include <ranges>namespace impl2 {template <typename Rng, size_t I>class elements_view{using base_range = Rng;using base_iterator = std::ranges::iterator_t<base_range>;using base_sentinel = std::ranges::sentinel_t<base_range>;using base_value_type = std::ranges::range_value_t<base_range>;public:class sentinel;class iterator {public:using difference_type = std::ptrdiff_t; // 迭代器是“高级”的指针，用起来应该跟指针相似，迭代器相减得到的结果的类型应该与指针相减得到的结果类型一样。using value_type = std::tuple_element_t<I, base_value_type>;using pointer = decltype(&std::get<I>(*std::declval<base_iterator>())); // 是否与 value_type * 相等？using reference = decltype(std::get<I>(*std::declval<base_iterator>())); // 是否与 value_type & 相等？using iterator_catalog = std::input_iterator_tag;iterator() = default;explicit iterator(const base_iterator& it) : it_(it) {}reference operator *() const { return std::get<I>(*it_); }pointer operator -> () const { return &std::get<I>(*it_); }iterator& operator ++() {++it_;return *this;}iterator operator ++(int) {iterator temp{ *this };++it_;return temp;}friend bool operator ==(const iterator&, const sentinel&);friend bool operator ==(const iterator& lhs, const iterator& rhs) {return lhs.it_ == rhs.it_;}private:base_iterator it_{};};class sentinel {public:sentinel() = default;explicit sentinel(base_sentinel iter) : iter_(iter) {}friend bool operator ==(const iterator& lhs, const sentinel& rhs) {return lhs.it_ == rhs.iter_;}private:base_sentinel iter_;};elements_view(Rng rng) : rng_(std::move(rng)) {}iterator begin() const { return iterator{ std::ranges::begin(rng_) }; }sentinel end() const { return sentinel{ std::ranges::end(rng_) }; }private:base_range rng_;};template <size_t I, typename Rng>auto elements(Rng&& r)requires std::ranges::view<std::remove_cvref_t<Rng>>{return elements_view<std::remove_cvref_t<Rng>, I>(std::forward<Rng>(r));}template <size_t I, typename Rng>auto elements(Rng&& r){if constexpr (std::is_lvalue_reference_v<Rng>){return elements<I>(std::views::all(std::forward<Rng>(r)));}else{return elements<I>(std::ranges::owning_view(std::forward<Rng>(r)));}}}
