Writing binary data to fstream in c++

Question

Question

I have a few structures I want to write to a binary file. They consist of integers from cstdint, for example uint64_t. Is there a way to write those to a binary file that does not involve me manually splitting them into arrays of char and using the fstream.write() functions?

What I've tried

My naive idea was that c++ would figure out that I have a file in binary mode and << would write the integers to that binary file. So I tried this:

#include <iostream>
#include <fstream>
#include <cstdint>
 
using namespace std;
 
int main() {
  fstream file;
  uint64_t myuint = 0xFFFF;
  file.open("test.bin", ios::app | ios::binary);
  file << myuint;
  file.close();
  return 0;
}

However, this wrote the string "65535" to the file.

Can I somehow tell the fstream to switch to binary mode, like how I can change the display format with << std::hex?

Failing all that above I'd need a function that turns arbitrary cstdint types into char arrays.

I'm not really concerned about endianness, as I'd use the same program to also read those (in a next step), so it would cancel out.

See e.g. [this answer](https://stackoverflow.com/questions/1559254/are-there-binary-memory-streams-in-c) — Jodocus, Mar 26 '18 at 13:12

YSC · Accepted Answer · 2022-11-30T18:44:31.823

16

Yes you can, this is what std::fstream::write is for:

#include <iostream>
#include <fstream>
#include <cstdint>

int main() {
  std::fstream file;
  uint64_t myuint = 0xFFFF;
  file.open("test.bin", std::ios::app | std::ios::binary);
  file.write(reinterpret_cast<char*>(&myuint), sizeof(myuint)); // or with recent C++: reinterpret_cast<std::byte*>
}

edited Nov 30 '22 at 18:44

answered Mar 26 '18 at 13:10

YSC

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That solves part of my problem. I'd like to be able to just use `<<` with my custom fstream. Can I somehow overload `<<` for all (or a set of) cstdint types? Like a template that accepts a predefined list of types? – youR.Fate Mar 26 '18 at 13:29
Yes, just define a new type using `std::fstream` in its implementation, with a templated `operator<<`. – YSC Mar 26 '18 at 13:36
Yes, but can I tell that template to only accept, for example uint64_t, uint16_t and uint8_t, but not for example, std::string, or bool? – youR.Fate Mar 26 '18 at 13:38
Yes. Yes you can. – lmat - Reinstate Monica Dec 03 '20 at 20:54
why should i memcpy it to a char buffer? – wowonline Nov 30 '22 at 18:18
@wowonline edited out, that was a mistake – YSC Nov 30 '22 at 18:44

Writing binary data to fstream in c++

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What I've tried

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