C++ How to check the last modified time of a file

问题

I'm caching some information from a file and I want to be able to check periodically if the file's content has been modified so that I can read the file again to get the new content if needed.

That's why I'm wondering if there is a way to get a file's last modified time in C++.

回答1:

There is no language-specific way to do this, however the OS provides the required functionality. In a unix system, the stat function is what you need. There is an equivalent _stat function provided for windows under Visual Studio.

So here is code that would work for both:

#include <sys/types.h>
#include <sys/stat.h>
#ifndef WIN32
#include <unistd.h>
#endif

#ifdef WIN32
#define stat _stat
#endif

auto filename = "/path/to/file";
struct stat result;
if(stat(filename.c_str(), &result)==0)
{
    auto mod_time = result.st_mtime;
    ...
}

回答2:

You can use boost's last_write_time for that. Boost is cross platform.

Here's the tutorial link for that.

Boost has the advantage that it works for all kinds of file names, so it takes care of non-ASCII file names.

回答3:

since the time of this post, c++17 has been released, and it includes a filesystem library based on the boost filesystem library:

https://en.cppreference.com/w/cpp/experimental/fs

which includes a way to get the last modification time:

https://en.cppreference.com/w/cpp/filesystem/last_write_time

回答4:

Please note that there are some limitations:

... The [time] resolution is as low as one hour on some filesystems... During program execution, the system clock may be set to a new value by some other, possibly automatic, process ...

来源：https://stackoverflow.com/questions/40504281/c-how-to-check-the-last-modified-time-of-a-file

标签

c++

file-io

last-modified