Std::filesystem::file_size (3) Linux Manual Page

std::filesystem::file_size – std::filesystem::file_size

Synopsis

Defined in header<filesystem>

    std::uintmax_t file_size(const std::filesystem::path &p);

std::uintmax_t file_size(const std::filesystem::path &p, (1)(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 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 -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

The overload that does not take a std::error_code& parameter throws 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. The overload taking a std::error_code& parameter sets it to the OS API error code if an OS API call fails, and executes ec.clear() if no errors occur. Any overload not marked noexcept may throw std::bad_alloc if memory allocation fails.

Example

// Run this code

#include <iostream>

#include <fstream>

#include <filesystem>

namespace fs = std::filesystem;

int main()

{
    fs::path p = fs::current_path() / "example.bin";
    std::ofstream(p).put('a'); // create file of size 1
    std::cout << "File size = " << fs::file_size(p) << '\n';
    fs::remove(p);
    try {
        fs::file_size("/dev"); // attempt to get size of a directory
    } catch (fs::filesystem_error &e) {
        std::cout << e.what() << '\n';
    }
}

Possible output:

File size = 1
filesystem error: cannot get file size: Is a directory [/dev]

std::filesystem::file_size (3) Linux Manual Page