c++ - writing into binary files

Question

#include <iostream>
#include <fstream>

using namespace std;

class info {

private:
    char name[15];
    char surname[15];
    int age;
public:
    void input(){
        cout<<"Your name:"<<endl;
            cin.getline(name,15);
        cout<<"Your surname:"<<endl;
        cin.getline(surname,15);
        cout<<"Your age:"<<endl;
        cin>>age;
        to_file(name,surname,age);
    }

    void to_file(char name[15], char surname[15], int age){
        fstream File ("example.bin", ios::out  | ios::binary | ios::app);
    // I doesn't know how to fill all variables(name,surname,age) in 1 variable (memblock) 
        //example File.write ( memory_block, size ); 

File.close();
    }

};

int main(){

info ob;
ob.input();

 return 0;
}

I don't know how to write more than 1 variable into a file, please help, I included an example ;) Maybe there are better ways to write to a file, please help me with this, it's to hard for me to solve.

Answer 1

For a text file, you could easily output one variable per line using a similar << to the one you use with std::cout.

For a binary file, you need to use std::ostream::write(), which writes a sequence of bytes. For your age attribute, you'll need to reinterpret_cast this to const char* and write as many bytes as is necessary to hold an int for your machine architecture. Note that if you intend to read this binary date on a different machine, you'll have to take word size and endianness into consideration. I also recommend that you zero the name and surname buffers before you use them lest you end up with artefacts of uninitialised memory in your binary file.

Also, there's no need to pass attributes of the class into the to_file() method.

#include <cstring>
#include <fstream>
#include <iostream>

class info
{
private:
    char name[15];
    char surname[15];
    int age;

public:
    info()
        :name()
        ,surname()
        ,age(0)
    {
        memset(name, 0, sizeof name);
        memset(surname, 0, sizeof surname);
    }

    void input()
    {
        std::cout << "Your name:" << std::endl;
        std::cin.getline(name, 15);

        std::cout << "Your surname:" << std::endl;
        std::cin.getline(surname, 15);

        std::cout << "Your age:" << std::endl;
        std::cin >> age;

        to_file();
    }

    void to_file()
    {
        std::ofstream fs("example.bin", std::ios::out | std::ios::binary | std::ios::app);
        fs.write(name, sizeof name);
        fs.write(surname, sizeof surname);
        fs.write(reinterpret_cast<const char*>(&age), sizeof age);
        fs.close();
    }
};

int main()
{
    info ob;
    ob.input();
}

A sample data file may look like this:

% xxd example.bin
0000000: 7573 6572 0000 0000 0000 0000 0000 0031  user...........1
0000010: 3036 3938 3734 0000 0000 0000 0000 2f00  069874......../.
0000020: 0000                                     ..