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:


[edit] Parameters

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

[edit] Return value

The size of the referred-to filesystem object.

[edit] 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.

[edit] Example

Prints the list of files in a given directory alongside with their sizes in human readable form.

#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 : 
        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)

[edit] See also

returns the size of a file
(function) [edit]