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,
                          std::error_code& ec ) noexcept;
(2) (since C++17)

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.


[edit] Parameters

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

[edit] Return value

The size of the file, in bytes.

[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

#include <cmath>
#include <filesystem>
#include <fstream>
#include <iostream>
namespace fs = std::filesystem;
struct HumanReadable
    std::uintmax_t size{};
    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';
    p = argv[0];
    std::cout << p << " size = " << HumanReadable{fs::file_size(p)} << '\n';
        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';
            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

[edit] See also

changes the size of a regular file by truncation or zero-fill
(function) [edit]
determines available free space on the file system
(function) [edit]
returns the size of the file to which the directory entry refers
(public member function of std::filesystem::directory_entry) [edit]