std::filesystem::file_size(3)

NAME
std::filesystem::file_size - std::filesystem::file_size

Synopsis
   Defined in header <filesystem>
   std::uintmax_t file_size( const std::filesystem::path& p ); (1) (since C++17)
   std::uintmax_t file_size( const std::filesystem::path& p, (2) (since C++17)
                        std::error_code& ec ) noexcept;
   If p does not exist, reports an error.
   For a regular file p, returns the size determined as if by reading the st_size
   member of the structure obtained by POSIX stat (symlinks are followed).
   The result of attempting to determine the size of a directory (as well as any other
   file that is not a regular file or a symlink) is implementation-defined.
   The non-throwing overload returns static_cast<std::uintmax_t>(-1) on errors.

Parameters
   p - path to examine
   ec - out-parameter for error reporting in the non-throwing overload

Return value
   The size of the file, in bytes.

Exceptions
   Any overload not marked noexcept may throw std::bad_alloc if memory allocation
   fails.
   1) Throws std::filesystem::filesystem_error on underlying OS API errors, constructed
   with p as the first path argument and the OS error code as the error code argument.
   2) Sets a std::error_code& parameter to the OS API error code if an OS API call
   fails, and executes ec.clear() if no errors occur.

Example
// Run this code
 #include <cmath>
 #include <filesystem>
 #include <fstream>
 #include <iostream>
 namespace fs = std::filesystem;
 struct HumanReadable
 {
     std::uintmax_t size{};
 private:
     friend std::ostream& operator<<(std::ostream& os, HumanReadable hr)
     {
         int o{};
         double mantissa = hr.size;
         for (; mantissa >= 1024.; mantissa /= 1024., ++o);
         os << std::ceil(mantissa * 10.) / 10. << "BKMGTPE"[o];
         return o ? os << "B (" << hr.size << ')' : os;
     }
 };
 int main(int, char const* argv[])
 {
     fs::path example = "example.bin";
     fs::path p = fs::current_path() / example;
     std::ofstream(p).put('a'); // create file of size 1
     std::cout << example << " size = " << fs::file_size(p) << '\n';
     fs::remove(p);
     p = argv[0];
     std::cout << p << " size = " << HumanReadable{fs::file_size(p)} << '\n';
     try
     {
         std::cout << "Attempt to get size of a directory:\n";
         [[maybe_unused]] auto x_x = fs::file_size("/dev");
     }
     catch (fs::filesystem_error& e)
     {
         std::cout << e.what() << '\n';
     }
     for (std::error_code ec; fs::path bin : {"cat", "mouse"})
     {
         bin = "/bin"/bin;
         if (const std::uintmax_t size = fs::file_size(bin, ec); ec)
             std::cout << bin << " : " << ec.message() << '\n';
         else
             std::cout << bin << " size = " << HumanReadable{size} << '\n';
     }
 }

Possible output:
 "example.bin" size = 1
 "./a.out" size = 22KB (22512)
 Attempt to get size of a directory:
 filesystem error: cannot get file size: Is a directory [/dev]
 "/bin/cat" size = 50.9KB (52080)
 "/bin/mouse" : No such file or directory

See also
   resize_file changes the size of a regular file by truncation or zero-fill
   (C++17) (function)
   space determines available free space on the file system
   (C++17) (function)
   file_size returns the size of the file to which the directory entry refers
               (public member function of std::filesystem::directory_entry)