如何在 C++ 中逐行迭代 cin？

Question

I want to iterate over std::cin , line by line, addressing each line as a std::string . Which is better:

string line;
while (getline(cin, line))
{
    // process line
}

or

for (string line; getline(cin, line); )
{
    // process line
}

? What is the normal way to do this?

Answer 1

Since UncleBen brought up his LineInputIterator, I thought I'd add a couple more alternative methods. First up, a really simple class that acts as a string proxy:

class line {
    std::string data;
public:
    friend std::istream &operator>>(std::istream &is, line &l) {
        std::getline(is, l.data);
        return is;
    }
    operator std::string() const { return data; }    
};

With this, you'd still read using a normal istream_iterator. For example, to read all the lines in a file into a vector of strings, you could use something like:

std::vector<std::string> lines;

std::copy(std::istream_iterator<line>(std::cin), 
          std::istream_iterator<line>(),
          std::back_inserter(lines));

The crucial point is that when you're reading something, you specify a line -- but otherwise, you just have strings.

Another possibility uses a part of the standard library most people barely even know exists, not to mention being of much real use. When you read a string using operator>>, the stream returns a string of characters up to whatever that stream's locale says is a white space character. Especially if you're doing a lot of work that's all line-oriented, it can be convenient to create a locale with a ctype facet that only classifies new-line as white-space:

struct line_reader: std::ctype<char> {
    line_reader(): std::ctype<char>(get_table()) {}
    static std::ctype_base::mask const* get_table() {
        static std::vector<std::ctype_base::mask> 
            rc(table_size, std::ctype_base::mask());

        rc['\n'] = std::ctype_base::space;
        return &rc[0];
    }
};  

To use this, you imbue the stream you're going to read from with a locale using that facet, then just read strings normally, and operator>> for a string always reads a whole line. For example, if we wanted to read in lines, and write out unique lines in sorted order, we could use code like this:

int main() {
    std::set<std::string> lines;

    // Tell the stream to use our facet, so only '\n' is treated as a space.
    std::cin.imbue(std::locale(std::locale(), new line_reader()));

    std::copy(std::istream_iterator<std::string>(std::cin), 
        std::istream_iterator<std::string>(), 
        std::inserter(lines, lines.end()));

    std::copy(lines.begin(), lines.end(), 
        std::ostream_iterator<std::string>(std::cout, "\n"));
    return 0;
}

Keep in mind that this affects all input from the stream. Using this pretty much rules out mixing line-oriented input with other input (eg reading a number from the stream using stream>>my_integer would normally fail).

Answer 2

What I have (written as an exercise, but perhaps turns out useful one day), is LineInputIterator:

#ifndef UB_LINEINPUT_ITERATOR_H
#define UB_LINEINPUT_ITERATOR_H

#include <iterator>
#include <istream>
#include <string>
#include <cassert>

namespace ub {

template <class StringT = std::string>
class LineInputIterator :
    public std::iterator<std::input_iterator_tag, StringT, std::ptrdiff_t, const StringT*, const StringT&>
{
public:
    typedef typename StringT::value_type char_type;
    typedef typename StringT::traits_type traits_type;
    typedef std::basic_istream<char_type, traits_type> istream_type;

    LineInputIterator(): is(0) {}
    LineInputIterator(istream_type& is): is(&is) {}
    const StringT& operator*() const { return value; }
    const StringT* operator->() const { return &value; }
    LineInputIterator<StringT>& operator++()
    {
        assert(is != NULL);
        if (is && !getline(*is, value)) {
            is = NULL;
        }
        return *this;
    }
    LineInputIterator<StringT> operator++(int)
    {
        LineInputIterator<StringT> prev(*this);
        ++*this;
        return prev;
    }
    bool operator!=(const LineInputIterator<StringT>& other) const
    {
        return is != other.is;
    }
    bool operator==(const LineInputIterator<StringT>& other) const
    {
        return !(*this != other);
    }
private:
    istream_type* is;
    StringT value;
};

} // end ub
#endif

So your loop could be replaced with an algorithm (another recommended practice in C++):

for_each(LineInputIterator<>(cin), LineInputIterator<>(), do_stuff);

Perhaps a common task is to store every line in a container:

vector<string> lines((LineInputIterator<>(stream)), LineInputIterator<>());

Answer 3

The first one.

Both do the same, but the first one is much more readable, plus you get to keep the string variable after the loop is done (in the 2nd option, its enclosed in the for loop scope)

Answer 4

Go with the while statement.

See Chapter 16.2 (specifically pages 374 and 375) of Code Complete 2 by Steve McConell.

To quote:

Don't use a for loop when a while loop is more appropriate. A common abuse of the flexible for loop structure in C++, C# and Java is haphazardly cramming the contents of a while loop into a for loop header.

.

C++ Example of a while loop abusively Crammed into a for Loop Header

for (inputFile.MoveToStart(), recordCount = 0; !inputFile.EndOfFile(); recordCount++) {
    inputFile.GetRecord();
}

C++ Example of appropriate use of a while loop

inputFile.MoveToStart();
recordCount = 0;
while (!InputFile.EndOfFile()) {
    inputFile.getRecord();
    recordCount++;
}

I've omitted some parts in the middle but hopefully that gives you a good idea.

Answer 5

This is based on Jerry Coffin's answer. I wanted to show c++20's std::ranges::istream_view . I also added a line number to the class. I did this on godbolt, so I could see what happened. This version of the line class still works with std::input_iterator .

https://en.cppreference.com/w/cpp/ranges/basic_istream_view

https://www.godbolt.org/z/94Khjz

class line {
    std::string data{};
    std::intmax_t line_number{-1};
public:
    friend std::istream &operator>>(std::istream &is, line &l) {
        std::getline(is, l.data);
        ++l.line_number;
        return is;
    }
    explicit operator std::string() const { return data; }
    explicit operator std::string_view() const noexcept { return data; }
    constexpr explicit operator std::intmax_t() const noexcept { return line_number; }    
};
int main()
{
    std::string l("a\nb\nc\nd\ne\nf\ng");
    std::stringstream ss(l);
    for(const auto & x : std::ranges::istream_view<line>(ss))
    {
        std::cout << std::intmax_t(x) << " " << std::string_view(x) << std::endl;
    }
}

prints out:

0 a
1 b
2 c
3 d
4 e
5 f
6 g