我如何打开一个文件进行阅读&写，创建它，如果它不存在，不截断它？ [英] How can I open a file for reading & writing, creating it if it does not exist, without truncating it?

问题描述

std :: fstream 的I​​ / O标志的正确设置是什么，其中我想要从和

What is the proper set of I/O flags for a std::fstream, where I want to be able to read from and write to the file, without truncating the file if it exists, but creating it if it does not?

我试过了

std::ios::binary | std::ios::in | std::ios::out
std::ios::binary | std::ios::in | std::ios::out | std::ios::ate

但是如果文件不存在，

but neither of these create the file if it does not already exist.

我不想要 std :: ios :: app ，因为我也需要能够随意放置文件，同时放置 和放置游标。

I don't want std::ios::app, because I also need to be able to seek around the file at will, with both the get and put cursors.

一个解决方法，首先要实例化一个 std :: ofstream ，然后立即关闭它，并打开我真正想要的流，但是这似乎很乱，如果它可以避免与单个流对象。

One workaround, I suppose, would be to instantiate an std::ofstream first, then immediately close it and open the stream I really want, but that seems messy if it can be avoided with a single stream object.

推荐答案

现在，我的结论是 std :: ios :: in

At this time, I'm concluding that std::ios::in outright prevents this, and that I must use the workaround.

因此：

if (!std::ostream(path.c_str()))
   throw std::runtime_error("Could not create/open file");

std::fstream fs(path.c_str(), std::ios::binary | std::ios::in | std::ios::out);
if (!fs)
   throw std::runtime_error("Could not open file");

// ... use `fs`

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