为什么 std::filesystem::file_size(file) 与实际文件大小不同？

Question

#include<iostream>
#include<random>
#include<fstream>
#include<filesystem>

int main(){
    std::default_random_engine dre;
    std::uniform_int_distribution<> uid{ 1, 100'000 };

    std::filesystem::path file("./test.txt");

    std::unique_ptr<int[]> D{ std::make_unique<int[]>(10'000) };
    for (int i = 0; i < 10'000; ++i)
        D[i] = uid(dre);

    std::ofstream out(file, std::ios::out | std::ios::binary);
    out.write((char*)D.get(), sizeof(int) * 10'000);

    std::cout << sizeof(int) * 10'000 << std::endl;
    std::cout << std::filesystem::file_size(file) << std::endl;
    // should be same.
}

I expected the result will be same. But the result is

40000
36864

When I checked the test.txt file in window, the size is 40'000. that is what I expected. But while runtime, something I can't understand is happening. why std::filesystem::file_size(file) is less than real file size?

Answer 1

std::ofstream buffers its output, so it is possible (even likely) that by the time you are calling file_size() , there are still bytes in the buffer that ofstream has not written to the actual file yet.

Close the ofstream first:

std::ofstream out(file, std::ios::out | std::ios::binary);
out.write((char*)D.get(), sizeof(int) * 10'000);
out.close();

std::cout << std::filesystem::file_size(file) << std::endl;

Or, at least call flush() on it:

std::ofstream out(file, std::ios::out | std::ios::binary);
out.write((char*)D.get(), sizeof(int) * 10'000);
out.flush();

std::cout << std::filesystem::file_size(file) << std::endl;