Get a file's size with C++17

By: Priya Philip 2 months, 2 weeks ago

std:: experimental:: filesystem:: file_size

<filesystem> (added in C++17) makes this straightforward.

#include <cstdint>
#include <filesystem>

// ...

std::uintmax_t size = std::filesystem::file_size("c:\\foo\\bar.txt");

Returns the size of the regular file p, determined as if by reading the st_size member of the structure obtained by POSIX stat (symlinks are followed)

Attempting to determine the size of a directory (as well as any other file that is not a regular file or a symlink) is treated as an error.

The non-throwing overload returns returns -1 on errors.


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

Return value

The size of the file, in bytes.


The overload that does not take a error_code& parameter throws fileystem_error on underlying OS API errors, constructed with p as the first argument and the OS error code as the error code argument. std::bad_alloc may be thrown if memory allocation fails. The overload taking a 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. This overload has noexcept specification: noexcept


#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';
    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]


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