CODEWITHSUNDEEP
c++file-io
user3769571
user3769571

Reputation: 65

What is wrong with this code of file handling in c++?

I am new to c++ and specifically file handling. I made this code. 

#include <iostream>
#include <fstream>
using namespace std;
class student
{
    public:
    char s;
    int age;
};
int main (void)
{
    student a;
    a.s = 'a';
    a.age = 1;
    ofstream myfile;
    myfile.open("a1.txt",ios::app);
    myfile.write(reinterpret_cast<char*>(&a),sizeof(student));
    return 0;
}

When I opened the file a1.txt, it had the character a correctly in there, but for the integer it had some weird encoding and a message that if you continue to edit this file, it will be corrupted. Can't I write an object to a file containing an integer and a character or a string as well?

Upvotes: 0

Views: 94

Answers (3)

Sir Digby Chicken Caesar
Sir Digby Chicken Caesar

Reputation: 3133

I would define your own << operator that handles your custom type.

#include <iostream>
#include <fstream>
using namespace std;

class student
{
    char s;
    int age;
};

ostream& operator<<(ostream &output, const student &o) 
{
    output << o.s << "  " << o.age;
    return output;
}

int main (void)
{
    student a;
    a.s = 'a';
    a.age = 1;
    ofstream myfile;
    myfile.open("a1.txt",ofstream::app);
    ofstream << a;
    return 0;
}

When you called myfile.write(reinterpret_cast<char*>(&a),sizeof(student)); it was not converting your struct to a human-readable string then writing it to file. In reality, it was interpreting the memory of your struct as a series of characters then writing it to file.

Upvotes: 0

Mike Seymour
Mike Seymour

Reputation: 254721

You're writing the binary representation of student into the file. The character will come out as expected; but the int will be the bytes used to represent the value, not a readable number.

If you want the output to be formatted as readable text, use formatted output:

myfile << a.s << ' ' << a.age << '\n';

For convenience, you could overload the operator for your class:

ostream & operator<<(ostream & os, student const & a) {
    return os << a.s << ' ' << a.age;
}

myfile << a << '\n';

For more complex structures, you might consider the Boost.Serialization library. Or you might do what I tend to do, with tuples instead of plain structures, and variadic templates to read and write them, but that might be rather more fiddly than you'd like.

Upvotes: 1

deviantfan
deviantfan

Reputation: 11434

You actually did write the int to the file, but not readable (123 => '1' '2' '3')
but the 4 (or 8) byte of that int. Your program can read the value back too,
the only probem is that we humans can´t read that form well.

Concering Strings:Just writing the whole struct will probably fail
(depending on the exact type of the string variable etc.), because the
"string" often stores only a pointer in the struct (which points to some
memory elsewhere, and this other memory isn´t written automatically to the file)
To be safe, write each variable of the struct explicitely (and handle different
var type appropiately) instead of writing them all together.

This way, things like different variable ordering and struct padding
can´t cause problems too. Other pitfalls to remember are different int
sizes and endianess on different computers... serialization isn´t trivial.

Upvotes: 0

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