std::filesystem::directory_entry::file_size(3)

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

Synopsis
   std::uintmax_t file_size() const; (1) (since C++17)
   std::uintmax_t file_size( std::error_code& ec ) const noexcept; (2) (since C++17)
   If the file size is cached in this directory_entry, returns the cached value.
   Otherwise, returns:
   1) std::filesystem::file_size(path()),
   2) std::filesystem::file_size(path(), ec).

Parameters
   ec - out-parameter for error reporting in the non-throwing overload

Return value
   The size of the referred-to filesystem object.

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
   Prints the list of files in a given directory alongside with their sizes in human
   readable form. // Run this code
 #include <cmath>
 #include <cstdint>
 #include <filesystem>
 #include <iostream>
 struct HumanReadable
 {
     std::uintmax_t size{};
     template<typename Os> friend Os& operator<<(Os& os, HumanReadable hr)
     {
         int i{};
         double mantissa = hr.size;
         for (; mantissa >= 1024.0; mantissa /= 1024.0, ++i)
         {}
         os << std::ceil(mantissa * 10.0) / 10.0 << i["BKMGTPE"];
         return i ? os << "B (" << hr.size << ')' : os;
     }
 };
 int main(int argc, const char* argv[])
 {
     const auto dir = argc == 2 ? std::filesystem::path{argv[1]}
                        : std::filesystem::current_path();
     for (std::filesystem::directory_entry const& entry :
          std::filesystem::directory_iterator(dir))
         if (entry.is_regular_file())
             std::cout << entry.path().filename() << " size: "
                       << HumanReadable{entry.file_size()} << '\n';
 }

Possible output:
 "boost_1_73_0.tar.bz2" size: 104.2MB (109247910)
 "CppCon 2018 - Jon Kalb “Copy Elision”.mp4" size: 15.7MB (16411990)
 "cppreference-doc-20190607.tar.xz" size: 6.3MB (6531336)
 "hana.hpp" size: 6.7KB (6807)

See also
   file_size returns the size of a file
   (C++17) (function)