打开没有截断的二进制输出文件流 [英] Opening a binary output file stream without truncation

问题描述

我试图打开一个二进制输出文件，我需要附加一些数据。我不能顺序地输出数据，所以我需要能够在文件流中搜索，不能使用 std :: ios :: app 标志。

I am trying to open a binary output file to which I need to append some data. I cannot output the data sequentially, so I need to be able to seek within the file stream and cannot use the std::ios::app flag.

不幸的是，当打开没有 std :: ios :: app 标志的输出文件流时，文件打开时会被截断。以下是一些示例代码：

Unfortunately, when opening an output file stream without the std::ios::app flag, the file gets truncated when it is opened. Here's some sample code:

#include <iostream>
#include <fstream>

int main() {
    std::ofstream file("output.bin", std::ios::binary | std::ios::ate);

    std::streamoff orig_offset = file.tellp();
    std::cout << "Offset after opening: " << orig_offset << std::endl;

    file.seekp(0, std::ios::end);
    std::streamoff end_offset = file.tellp();
    std::cout << "Offset at end: " << end_offset << std::endl;

    file << "Hello World" << std::endl;

    std::streamoff final_offset = file.tellp();
    std::cout << "Offset after writing: " << final_offset << std::endl;

    return 0;
}

我希望每次执行都将Hello World附加到文件。但是，该文件在打开时会被截断。

I would expect every execution to append "Hello World" to the file. However, the file is truncated as soon as it is opened.

我做错了什么？如果这是Visual Studio中的错误，是否有任何解决方法？

What am I doing wrong? If this is a bug in Visual Studio, are there any workarounds?

编辑：
每次程序运行时，无论文件是否存在内容，程序输出：

Every time the program runs, regardless of whether the file exists or already has contents, the program outputs this:

Offset after opening: 0
Offset at end: 0
Offset after writing: 12

推荐答案

输出和输入模式：

std::fstream file("output.bin", std::ios::in | std::ios::out | std::ios::binary | std::ios::ate);

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