[fs.op.file.size]

uintmax_t filesystem::file_size(const path& p);
uintmax_t filesystem::file_size(const path& p, error_code& ec) noexcept;

Returns:

(2.1)
If is_regular_file(p), the size in bytes of the file p resolves to, determined as if by the value of the POSIX stat class member st_size obtained as if by POSIX stat .
(2.2)
Otherwise, the result is implementation-defined.

The signature with argument ec returns static_cast<uintmax_t>(-1) if an error occurs.

Throws: As specified in [fs.err.report].

31 Input/output library [input.output]