CODEWITHSUNDEEP
c++integerbinaryfilesfwrite
Suganthi Selvaraj
Suganthi Selvaraj

Reputation: 85

Convert fwrite to C++ type code to write binary files

I am trying to convert C style file IO into C++ style file IO. I need to write the number 42 as a 4-byte signed integer.

Here's an example of what C type IO that I tried that worked

#include <stdio.h>
#include <string>
#include <fstream>

using namespace std;

int main()
{
  FILE *myFile;
  myFile = fopen ("input_file.dat", "wb");

  int magicn = 42;
  fwrite (&magicn, sizeof(magicn), 1, myFile);

  fclose (myFile);
  return 0;
}

I am trying to convert the above code to C++ type IO based on a suggestion from a different question I asked (How to write a string with padding to binary file using fwrite?).

Here's my attempt:

#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

int main()
{
  ofstream myFile ("input_file.dat", ios::binary);
  int magicn = 42;
  myFile << setw(sizeof(magicn)) << magicn;
  myFile.close();
  return 0;
}

However, the output I am expecting when I use the 'xxd -b input_file.dat' command is not the same.

The output I am expecting is (generated with C type IO code)

0000000: 00101010 00000000 00000000 00000000 *...

But I see (generated with my attempt of C++ type IO code)

0000000: 00100000 00100000 00110100 00110010 42

Looking for a solution. Appreciate the help!

Upvotes: 0

Views: 563

Answers (2)

KamilCuk
KamilCuk

Reputation: 141900

Use ostream::write.

myFile.write(reinterpret_cast<char*>(&magicn), sizeof(magicn));

Upvotes: 1

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385385

Your current approach is more like fprintf(myFile, "%d", magicn). That is, it performs formatted insertion to the stream, so you end up with the ASCII code for the 42nd ASCII character.

The analogue of fwrite is ostream::write. Just look at the available members of ostream to find out what you can do with it.

Upvotes: 2

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