如何在c ++中打开文件而又不删除文件内容且不附加文件? [英] How can I open a file in c++, without erasing its contents and not append it?

问题描述

我试图找到一种方法，可以在不读取整个文件的情况下编辑二进制文件中的内容.

I am trying to find a way using which I can Edit the contents in a binary file, without reading the whole file.

假设这是我的文件

abdde

我想做到

abcde

我尝试了以下操作:- 尝试1)

I tried following:- Attempt 1)

ofstream f("binfile", ios::binary);
if(f.is_open()){
  char d[]={'c'};
  f.seekp(2,ios::beg);
  f.write(d, 1);
  f.close();
}
//the file get erased

输出:

**c

尝试2)

ofstream f("binfile", ios::binary | ios::app);
if(f.is_open()){
  char d[]={'c'};
  f.seekp(2,ios::beg);
  f.write(d, 1);
  f.close();
}
//the file simple gets append seekp() does nothing

输出:

abddec

尝试3)

ofstream f("binfile", ios::binary | ios::app);
if(f.is_open()){
  char d[]={'c'};
  f.seekp(2);
  f.write(d, 1);
  f.close();
}
//same as before the file simple gets append seekp() does nothing

输出:

abddec

如果我只是尝试用'h'替换文件的第一个字节'a'

And if I just try to replace the 1st byte of the file, which is 'a' with 'h'

ofstream f("binfile", ios::binary);
if(f.is_open()){
  char d[]={'c'};
  f.seekp(ios::beg);
  f.write(d, 1);
  f.close();
}
//file is erased

输出:

h

我该怎么办?操作系统甚至可能允许程序在任何时候自行编辑文件吗?

What do I do? Is it even possible for the OS to allow a program to edit a file at any point own its own?

推荐答案

std::ios::app表示光标在每次写操作之前都放在文件的末尾.寻求没有效果.

std::ios::app means the cursor is put at the end of the file before every write. Seeking has no effect.

同时，对于输出流，std::ios::binary默认进入截断"模式.

Meanwhile, std::ios::binary goes into "truncation" mode by default for an output stream.

您都不想要这些.

我建议std::ios::out | std::ios::in，也许只是创建一个std::fstream fs(path, std::ios::binary)而不是使用std::ofstream.

I suggest std::ios::out | std::ios::in, perhaps by just creating a std::fstream fs(path, std::ios::binary) rather than using an std::ofstream.

是的，这有点令人困惑.

Yes, it's a bit confusing.

(参考)

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