c++ - 检查流是否以换行符结尾

Question

我想检查流(实际上是 ifstream)是否以换行符结尾。我想出了这个:

bool StreamEndsWithNewline(std::basic_istream<char> & the_stream)
{
    if (the_stream.peek() == EOF) {
        the_stream.clear(); //clear flags set by peek()
        return false;
    }
    std::string line = "blah";
    while (std::getline(the_stream, line)) {
       // ...
    }
    return line.empty();
}

想法是，如果流的最后一行有一个 \n 结束字符，while 循环将进行一次额外的迭代(因为尚未达到 eof)，其中空字符串将被分配给行参数。

必须单独处理“空”流的特殊情况。

它似乎适用于 Windows (vs2010)。我一般可以这样做吗？

Answer 1

您的代码有效。

但是，您可以尝试查找流并仅测试最后一个字符或丢弃读取的字符:

#include <cassert>
#include <iostream>
#include <limits>
#include <sstream>

bool StreamEndsWithNewline(std::basic_istream<char>& stream) {
    const auto Unlimited = std::numeric_limits<std::streamsize>::max();
    bool result = false;
    if(stream) {
        if(std::basic_ios<char>::traits_type::eof() != stream.peek()) {
            if(stream.seekg(-1, std::ios::end)) {
                char c;
                result = (stream.get(c) && c == '\n');
                stream.ignore(Unlimited);
            }
            else {
                stream.clear();
                while(stream && stream.ignore(Unlimited, '\n')) {}
                result = (stream.gcount() == 0);
            }
        }
        stream.clear();
    }
    return result;
}

int main() {
    std::cout << "empty\n";
    std::istringstream empty;
    assert(StreamEndsWithNewline(empty) == false);

    std::cout << "empty_line\n";
    std::istringstream empty_line("\n");
    assert(StreamEndsWithNewline(empty_line) == true);

    std::cout << "line\n";
    std::istringstream line("Line\n");
    assert(StreamEndsWithNewline(line) == true);

    std::cout << "unterminated_line\n";
    std::istringstream unterminated_line("Line");
    assert(StreamEndsWithNewline(unterminated_line) == false);

    std::cout << "Please enter ctrl-D: (ctrl-Z on Windows)";
    std::cout.flush();
    assert(StreamEndsWithNewline(std::cin) == false);
    std::cout << '\n';

    std::cout << "Please enter Return and ctrl-D (ctrl-Z on Windows): ";
    std::cout.flush();
    assert(StreamEndsWithNewline(std::cin) == true);
    std::cout << '\n';

    return 0;
}