在C ++中，在文本文件开头插入行的正确方法是什么？ [英] In C++, what is the proper way to insert a line at the beginning of a text file?

问题描述

我正在创建一个大文本文件，第一行是一个摘要，取决于内容。我目前插入一个摘要行，其中包含我期望的摘要是，并在创建文件后，我将其与实际写入的内容进行比较。如果信息不同，我将覆盖初始摘要。

I am creating a large text file, the first line is a summary which depends on the contents. I am currently inserting a summary line that contains what I expect the summary to be, and after creating the file I compare this with what was actually written. If the information is different I overwrite the initial summary.

代码如下。

  std::ofstream ofile(filename); 
  numOutSegInSynth-=computeSegmentsFromLevel(numLevelsInSynth-1,myInitCkt);

  //extra padding inserted, to prevent summary overwriting segments
  ofile<<numNodesInSynth<<" "<<numOutSegInSynth<<"  0"<<std::endl;
  ofile.close();
  for (long i=0; i<(numLevelsInSynth-1); i++) {  
    //at all
    long numSegOnLevel = computeSegmentsFromLevel(i,myInitCkt); 
    assert(numSegOnLevel); 
    numSegsCreated+=numSegOnLevel;
    buildSegmentsList(i,numSegOnLevel,filename,myInitCkt);   
  }
  if (numSegsCreated != numOutSegInSynth) {
  //update segment count
    std::ofstream ofile(filename, ios::in | ios::out );
    ofile.seekp(0, ios::beg);
    short pred = log10(numOutSegInSynth)+1;
    short act = log10(numSegsCreated)+1;
    if (act>pred) {
      ofile<<numNodesInSynth<<" "<<numSegsCreated<<" 0"<<std::endl;
    } else {
      ofile<<numNodesInSynth<<" "<<numSegsCreated<<"  0"<<std::endl;
    }
      ofile.close();
    }

我想知道是否可能只是追加到开始文件，要插入一个新行或单个字符
感谢

I would like to know if it is possible to just append to the start of the file, either to insert an new line or a single character Thanks

推荐答案

正如Ignacio说，大多数文件系统不支持在文件中插入，更不用说文本文件。而C ++标准库的一个文件的概念是一个简单的字节流。因此，在开始时，一个固定大小的摘要，因为它似乎是你的意图与你的代码，是一个实用的解决方案（注意：你的代码似乎没有达到这个固定的大小）。

As Ignacio says, most filesystems don't support insertion in files, not to mention text files. And the C++ standard library's notion of a file is a simple stream of bytes. And so a fixed size summary at the start, as it seems you were intending with your code, is one practical solution (note: your code doesn't appear to achieve this fixed size).

但是，一些常见的文件系统确实支持将额外的信息与文件关联。

However, some common filesystems do support associating extra information with files.

例如，Windows NTFS支持多个数据流文件：

For example, Windows NTFS supports multiple data streams per file:

C:\test> echo blah blah >data.txt

C:\test> type data.txt
blah blah

C:\test> echo some info >data.txt:summary

C:\test> type data.txt
blah blah

C:\test> more <data.txt:summary
some info

C:\test> _

例如，这是Windows资源管理器用于向文件添加摘要信息的机制。

E.g., this is the mechanism used by Windows Explorer for adding summary info to files.

所以，如果你为一个支持多个流的系统编程，并且不计划将程序或数据移植到其他系统，你可以考虑一个多流文件。

So, if you're programming for a system that supports multiple streams, and don't plan on porting the program or the data to other systems, you might consider a multiple-stream file.

第三种方法是通过拥有两个或多个关联文件来有效实现自己。例如。一个数据文件和一个摘要文件。使用例如一个连接它们的命名约定。

A third alternative is to effectively implement that yourself, by having two or more associated files. E.g. one data file, and one summary file. With e.g. a naming convention connecting them.

hth。，

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