Reading and writing int to a binary file in C++

c++, file-io, visual-c++

Solution

`o.write` does not write character, it writes bytes (if flagged with ios::binary). The char-pointer is used because a char has length 1 Byte.

o.write((char*)a,sizeof(a));

`(char*) a` is the adress of what `o.write` should write. Then it writes `sizeof(a)` bytes to a file. There are no characters stored, just bytes.

If you open the file in a Hex-Editor you would see something like this if a is `int i = 10`: `0A 00 00 00` (4 Byte, on x64).

Reading is analogue.

Here is a working example:

#include <iostream>
#include <fstream>
#include <string>

using namespace std;


int main (int argc, char* argv[]){
    const char* FILENAM = "a.txt";
    int toStore = 10;
    ofstream o(FILENAM,ios::binary);

    o.write((char*)&toStore,sizeof(toStore));
    o.close();

    int toRestore=0;
    ifstream i(FILENAM,ios::binary);
    i.read((char*)&toRestore,sizeof(toRestore));

    cout << toRestore << endl;


    return 0;
}

Problem

I am unclear about how reading long integers work. If I say ``` long int a[1]={666666} ofstream o("ex",ios::binary); o.write((char*)a,sizeof(a)); ``` to store values to a file and want to read them back as it is ``` long int stor[1]; ifstream i("ex",ios::binary); i.read((char*)stor,sizeof(stor)); ``` how will I be able to display the same number as stored using the information stored in multiple bytes of character array?

Original source