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std::filesystem::file_size(3) C++ Standard Libarystd::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  fol-
       lowed).

	  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-de-
       fined.

	  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 er-
       rors, 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  en-
       try refers
		      (public	member	 function  of  std::filesystem::direc-
       tory_entry)

http://cppreference.com		  2024.06.10	 std::filesystem::file_size(3)

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