Writing integer to binary file using C++?

Question

I have a very simple question, which happens to be hard for me since this is the first time I tried working with binary files, and I don't quite understand them. All I want to do is write an integer to a binary file.

Here is how I did it:

#include <fstream>
using namespace std;
int main () {
    int num=162;
    ofstream file ("file.bin", ios::binary);
    file.write ((char *)&num, sizeof(num));
    file.close (); 
    return 0;
}

Could you please tell me if I did something wrong, and what?

The part that is giving me trouble is line with file.write, I don't understand it.

Thank you in advance.

Nitpick advice: 1) say reinterpret_cast<const char *>(&num), and omit the file.close() and the return 0, as all those happen automatically. — Kerrek SB
– Kerrek SB, Commented Feb 11, 2012 at 22:29
@KerrekSB Could you please explain what reinterpret_cast<const char *>(&num) does? — eqagunn
– eqagunn, Commented Feb 11, 2012 at 22:32
It casts the type of &num to const char *, similar to your current cast, but it's a more deliberate way of writing what you intend than the crude C cast. It's just a matter of style and finesse, but you should get used to saying precisely what you want in C++. — Kerrek SB
– Kerrek SB, Commented Feb 11, 2012 at 22:33
@KerrekSB: Yeah, yeah. That's what happens when you type without thinking. To my defense you can rephrase the sentence like that: "..., as to the best of my knowledge only some compilers...", and the marked phrase is synonym to 'as far as I know'. — Asaf
– Asaf, Commented Feb 12, 2012 at 10:38

xorover · Accepted Answer · 2020-05-31 00:12:35Z

38

The part that is giving me trouble is line with file.write, I don't understand it.

If you read the documentation of ofstream.write() method, you'll see that it requests two arguments:

a pointer to a block of data with the content to be written;
an integer value representing the size, in bytes, of this block.

This statement just gives these two pieces of information to ofstream.write():

file.write(reinterpret_cast<const char *>(&num), sizeof(num));

&num is the address of the block of data (in this case just an integer variable), sizeof(num) is the size of this block (e.g. 4 bytes on 32-bit platforms).

edited May 31, 2020 at 0:12

xorover

1399 bronze badges

answered Feb 11, 2012 at 23:23

user1149224

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3 Comments

Mikolasan Over a year ago

Important notice here: it writes data from the memory as it is stored there and resulting binary file will be not readable on machines with different endianness.

thatGuy Over a year ago

@Mikolasan how can i make it so that the binary file is readable on different machines?

Mikolasan Over a year ago

@thatGuy your code should check what endianness on the running machine, if it differs from binary file format, then convert the data after reading or before writing, otherwise read/write and use as it is.

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