C++ 从文件中读取，清除它，写入它
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Read from file, clear it, write to it
提问by Anonymous
I'm trying to read data from a text file, clear it, and then write to it, in that order using the fstreamclass.
我正在尝试从文本文件中读取数据，清除它，然后使用fstream类按顺序写入它。
My question is how to clear a file after reading from it. I know that I can open a file and clear it at the same time, but is there some function I can call on the stream to clear its contents?
我的问题是如何在读取文件后清除文件。我知道我可以打开一个文件并同时清除它，但是我可以在流上调用一些函数来清除它的内容吗？
回答by xian
You should open it, perform your input operations, and then close it and reopen it with the std::fstream::trunc flag set.
您应该打开它，执行您的输入操作，然后关闭它并使用 std::fstream::trunc 标志集重新打开它。
#include <fstream>
int main()
{
std::fstream f;
f.open("file", std::fstream::in);
// read data
f.close();
f.open("file", std::fstream::out | std::fstream::trunc);
// write data
f.close();
return 0;
}
回答by doron
If you want to be totally safe in the event of a crash or other disastrous event, you should do the write to a second, temporary file. Once finished, delete the first file and rename the temporary file to the first file. See the Boost Filesystemlibrary for help in doing this.
如果您想在发生崩溃或其他灾难性事件时完全安全，您应该写入第二个临时文件。完成后，删除第一个文件并将临时文件重命名为第一个文件。有关执行此操作的帮助，请参阅Boost 文件系统库。
