undefined

先看看概念

实现一个数入行迭代器

#include <string>
#include <iostream>

using namespace std;

class istream_line_reader {
public:
    class iterator {  // 实现 InputIterator
    public:
        typedef ptrdiff_t difference_type;
        typedef string value_type;
        typedef const value_type* pointer;
        typedef const value_type& reference;
        typedef input_iterator_tag iterator_category;

        iterator() noexcept : stream_(nullptr) {}

        explicit iterator(istream& is) : stream_(&is) {
            ++*this;
        }

        reference operator*() const noexcept {
            return line_;
        }

        pointer operator->() const noexcept {
            return &line_;
        }

        iterator& operator++() {
            getline(*stream_, line_);
            if (!*stream_) {
                stream_ = nullptr;
            }
            return *this;
        }

        iterator operator++(int) {
            iterator temp(*this);
            ++*this;
            return temp;
        }

        bool operator==(const iterator& rhs) const noexcept {
            return stream_ == rhs.stream_;
        }

        bool operator!=(const iterator& rhs) const noexcept {
            return !operator==(rhs);
        }

    private:
        istream* stream_;
        string line_;
    };

    istream_line_reader() noexcept : stream_(nullptr) {}

    explicit istream_line_reader(istream& is) noexcept : stream_(&is) {}

    iterator begin() {
        return iterator(*stream_);
    }

    iterator end() const noexcept {
        return iterator();
    }


private:
    istream* stream_;
};

int main() {
    for (const string& line : istream_line_reader(cin)) {
        cout << line << endl;
}}