How can I get a file's size in C++? [duplicate]

Let's create a complementary question to this one. What is the most common way to get the file size in C++? Before answering, make sure it is portable (may be executed on Unix, Mac and Windows), reliable, easy to understand and without library dependencies (no boost or qt, but for instance glib is ok since it is portable library).

#include <fstream>

std::ifstream::pos_type filesize(const char* filename)
{
    std::ifstream in(filename, std::ifstream::ate | std::ifstream::binary);
    return in.tellg(); 
}

See http://www.cplusplus.com/doc/tutorial/files/ for more information on files in C++.

edit: this answer is not correct since tellg() does not necessarily return the right value. See http://stackoverflow.com/a/22986486/1835769

While not necessarily the most popular method, I've heard that the ftell, fseek method may not always give accurate results in some circumstances. Specifically, if an already opened file is used and the size needs to be worked out on that and it happens to be opened as a text file, then it's going to give out wrong answers.

The following methods should always work as stat is part of the c runtime library on Windows, Mac and Linux.

#include <sys/stat.h>

long GetFileSize(std::string filename)
{
    struct stat stat_buf;
    int rc = stat(filename.c_str(), &stat_buf);
    return rc == 0 ? stat_buf.st_size : -1;
}

or 

long FdGetFileSize(int fd)
{
    struct stat stat_buf;
    int rc = fstat(fd, &stat_buf);
    return rc == 0 ? stat_buf.st_size : -1;
}

On some systems there is also a stat64/fstat64. So if you need this for very large files you may want to look at using those.

Using the C++ filesystem library:

#include <filesystem>

int main(int argc, char *argv[]) {
  std::filesystem::path p{argv[1]};

  std::cout << "The size of " << p.u8string() << " is " <<
      std::filesystem::file_size(p) << " bytes.\n";
}