c++ - std::getline 返回同一行的倍数

Question

我正在解析一个 .txt 文件以制作一个 Exp 计算器，但我遇到了一个奇怪的错误，它会将流中超过一行的任何行加倍。我正在使用此代码打开和关闭文件以检查更改并仅打印更改。

这是文本文件的样子..

[11:58:18] Mind increased by 0.000013 to 28.160389
[11:58:18] Mind logic increased by 0.000015 to 33.213428
[11:58:18] Miscellaneous items increased by 0.000061 to 59.381138
[11:58:18] Repairing increased by 0.000782 to 35.52212
[11:58:19] Mind increased by 0.000015 to 28.160404

当我添加 sleep 以减慢速度时，它确实逐行获取每一行并将它们打印到控制台，但在完成后它继续打印流中超过 1 行的每一行..

#include <windows.h>
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
using namespace std;

int main()
{
    ios::streampos file_end;
    ifstream file ("C://Users//Matthew//wurm//players//Maximillian//logs//_Skills.2013-03.txt");
    if(file.is_open())
    {
        file.seekg( ios::beg, ios::end );
        file_end = file.tellg();
        file.close();
    }else{
        cout << "Unable to open file";
    }

    while(true)
    {
        ifstream file ("C://Users//Matthew//wurm//players//Maximillian//logs//_Skills.2013-03.txt");
        if (file.is_open())
        {
            file.seekg( ios::beg, ios::end );
            streampos new_end = file.tellg();

            if(new_end > file_end)
            {
                file.seekg(file_end);

                string line;
                while(getline(file, line))
                {
                    if(!line.empty())
                        cout << line << endl;
                    Sleep(1000);
                }

                file_end = new_end;
            }
            file.close();
        }else{
            cout << "Unable to open file";
        }
    }
    return 0;
}

这是输出...记下时间戳。我不知道为什么要这样做......

[16:42:36] Mind logic increased by 0.000015 to 33.349262
[16:42:36] Miscellaneous items increased by 0.000053 to 59.777897
[16:42:36] Repairing increased by 0.000694 to 36.59152
[16:42:37] Mind increased by 0.000013 to 28.270370
[16:42:37] Mind logic increased by 0.000019 to 33.349281
[16:42:38] Mind increased by 0.000013 to 28.270384
[16:42:38] Mind logic increased by 0.000015 to 33.349297
[16:42:38] Repairing increased by 0.000694 to 36.59222
[16:42:39] Mind logic increased by 0.000015 to 33.349312
[16:42:40] Repairing increased by 0.000694 to 36.59291
[16:42:42] Mind increased by 0.000015 to 28.270399
[16:42:42] Mind logic increased by 0.000015 to 33.349327
[16:42:42] Repairing increased by 0.000694 to 36.59361
[16:42:43] Mind increased by 0.000013 to 28.270412
[16:42:43] Mind logic increased by 0.000015 to 33.349342
[16:42:44] Mind increased by 0.000013 to 28.270426
[16:42:44] Mind logic increased by 0.000019 to 33.349361
[16:42:44] Repairing increased by 0.000694 to 36.59430
//I added this afterwards to break up what I printed out first... why did it reprint all the times that had 2 or more lines at once?
[16:42:36] Miscellaneous items increased by 0.000053 to 59.777897
[16:42:36] Repairing increased by 0.000694 to 36.59152
[16:42:37] Mind increased by 0.000013 to 28.270370
[16:42:37] Mind logic increased by 0.000019 to 33.349281
[16:42:38] Mind increased by 0.000013 to 28.270384
[16:42:38] Mind logic increased by 0.000015 to 33.349297
[16:42:38] Repairing increased by 0.000694 to 36.59222
[16:42:39] Mind logic increased by 0.000015 to 33.349312
[16:42:40] Repairing increased by 0.000694 to 36.59291
[16:42:42] Mind increased by 0.000015 to 28.270399
[16:42:42] Mind logic increased by 0.000015 to 33.349327
[16:42:42] Repairing increased by 0.000694 to 36.59361
[16:42:43] Mind increased by 0.000013 to 28.270412
[16:42:43] Mind logic increased by 0.000015 to 33.349342
[16:42:44] Mind increased by 0.000013 to 28.270426
[16:42:44] Mind logic increased by 0.000019 to 33.349361
[16:42:44] Repairing increased by 0.000694 to 36.59430

Answer 1

在 getline() 循环之后，流中的读取位置不保证等于 new_end，因为有人可能在你之后写入文件已确定 new_end，但在您阅读它之前或期间。

这会导致一些行被打印出来，随后将 file_end 调整为一个表明这些行尚未被打印的值。这些行将在您的外层 while 循环的下一次迭代中再次打印。

通过再次调用 tellg() 更新 file_end，而不是通过从 new_end 分配。你必须调用clear()在进行更新之前在流上，因为调用 std::getline() 直到流上的 failbit 被设置，这导致 tellg() 返回 -1(即不是文件末尾的实际位置)。