std::ifstream read into vector results in 0 sized vector

Asked Dec 21 '22 at 18:57

Active Dec 21 '22 at 22:21

Viewed 33 times

I'm aware that there are other ways to accomplish the read of data. But I am confused, why in this code does the size print as 0? file.txt is a random text file with more than 4 characters.

#include <iostream>
#include <fstream>
#include <vector>

int main() {
  std::ifstream ifs("file.txt");
  std::vector<char> file_vec{};
  file_vec.reserve(4);
  ifs.read(file_vec.data(), 4);
  std::cout << file_vec.size() << std::endl;
  ifs.close();
}

edited Dec 21 '22 at 22:21

Remy Lebeau

555,201
31
458
770

asked Dec 21 '22 at 18:57

user3472

3

`ifs.read` does not change the `size` of the vector, because it has no way of doing that when all you pass is a pointer to the underlying data – UnholySheep Dec 21 '22 at 19:00
5

Replace `file_vec.reserve(4);` with `file_vec.resize(4);` – Kevin Dec 21 '22 at 19:03
1

`reserve` allocates storage for `vector`, it sets the `vector`s capacity, but doesn't tell `vector` to use it yet, it doesn't set the size. You want to look at `resize`. – user4581301 Dec 21 '22 at 19:04
`reserve` does not do what you think it does. – Sam Varshavchik Dec 21 '22 at 19:10
Better to utilize those other methods. – sweenish Dec 21 '22 at 19:11
The comments already address the underlying issue, I'd also encourage you to [take a look at `std::istream_iterator`](https://en.cppreference.com/w/cpp/iterator/istream_iterator) – Ryan Haining Dec 21 '22 at 22:25

std::ifstream read into vector results in 0 sized vector

0 Answers0