C++取txt文本置顶行内容

Question

有一个txt文档 里面有多行数据
怎么获取指定行的内容 函数带一个int参数 取第几行的内容
如readtxtline(int line)

Answer 1

string ReadLine(string filename,int line)
{
    int l,i=0;
    string temp;
    fstream file;
    file.open(filename.c_str());
 
    if(line<=0)
    {
        return "Error 1: 行数错误，不能为0或负数。";
    }
    if(file.fail())
    {
        return "Error 2: 文件打开错误。";
    }

    while(getline(file,temp) && i < line)
    {
        i++;
    }
    if(i<line){
        temp="Error 3: 行数超出文件长度。"
    }
    file.close();
    return temp;
}

Answer 2

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <optional>
#include <numeric>

class FileReader {
    constexpr static size_t read_current = std::numeric_limits<size_t>::max();

public:
    FileReader(std::string filename)
        : line_no_{ 0 },
          filename_{ std::move(filename) },
          stream_{ filename_ },
          positions_{ stream_.tellg() } {}

    ~FileReader() { stream_.close(); }

    std::optional<std::string> read_line(size_t index = read_current) {
        if (index == read_current) {
            return _read_forward();
        }
        else {
            if (_seek_line(index)) {
                return _read_forward();
            }
            else {
                return std::nullopt;
            }
        }
    }

    std::optional<std::string> _read_forward() {
        std::string res;

        if (std::getline(stream_, res)) {
            ++line_no_;
            if (positions_.size() <= line_no_)
                positions_.push_back(stream_.tellg());

            return res;
        }
        else {
            _seek_line(line_no_);
            stream_.clear(std::ios_base::eofbit);

            return std::nullopt;
        }
    }

    bool _seek_line(size_t i) {
        if (i < positions_.size()) {
            line_no_ = i;
            stream_.seekg(positions_[i]);

            return true;
        }
        else {
            _seek_line(positions_.size() - 1);

            while (_read_forward() && line_no_ < i)
                ;

            return i == line_no_;
        }
    }

public:
    size_t line_no_;
    std::string filename_;
    std::ifstream stream_;
    std::vector<std::streampos> positions_;
};

int main() {
     FileReader reader(R"(test.txt)");

     for (auto i : { 0, 1, 3, 2, 7, 14, 12, 486, 487, 488, 489, 1 }) {
         auto line = reader.read_line(i);
         std::cout << i << ' ' << (line ? line.value() : std::string{ "read to end" }) << '\n';
     }
}