Problem while write in txt file

[andrezc]

Hi, i'm having a problem while writing data in a txt file. My code is this:

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

using namespace std;
class students
{
    private:
    char _name[20];

    public:
    students();
    void sRegister();
};

students :: students()
{
    strcpy(_name, "");
}
void students ::  sRegister()
{
    cout << "Enter a student's name: " << endl;
    cin >> _name;

}

int main()
{
    ofstream fregistry ("students_list.txt");

    students reg1;

    reg1.sRegister();

    fregistry.write((char *)&reg1, sizeof(students));

    return 0;
}

The problem is:

after run my program, I input "Andre" with name, after this, I read the text file:

andre@wolfman:~/CPP$ cat students_list.txt
Andre$�������andre@wolfman:~/CPP$


What is " $������� "?

Thanks.

[PanGalactic]

Here's a hint: on line 36, you write out sizeof(students) bytes. How many bytes is that?

[andrezc]

Here's a hint: on line 36, you write out sizeof(students) bytes. How many bytes is that?

I used "sizeof" to inform a size of registry and after write in the text file.

[Null]

File IO works only with POD types, so you can't write whole class to file. You can store data in struct:

struct student_data
{
  char name[20];
  // int age
  // ...
};

class students
{
    private:

    public:
    student_data data; 
    students();
    void sRegister();
};

students :: students()
{
    strcpy(data.name, "");
}
void students ::  sRegister()
{
    cout << "Enter a student's name: " << endl;
    cin >> data.name;

}

int main()
{
    ofstream fregistry ("students_list.txt",ios::binary); /// don't forget ios::binary flag!

    students reg1;
    reg1.sRegister();
    fregistry.write((char *)&reg1.data, sizeof(reg1.data));
    fregistry.close();

// check the output
    ifstream fregistry_in;
    fregistry_in.open("students_list.txt",ios::in | ios::binary);

    students reg2;
    fregistry_in.read((char *)&reg2.data, sizeof(reg2.data));
    fregistry_in.close();
    cout <<reg2.data.name;

    return 0;
}

[PanGalactic]

File IO does not work well with POD types either. The size of "integer" "long" and "pointer" types, for example, vary across 8, 16, 32 and 64-bit platforms. The endianness of integral types varies across CPU families. And floating point types may be represented differently across platforms. All non-string types need to be explicitly serialized and de-serialized. And even strings are tricky -- you have to agree what character encoding is being used.

@andrezc: The garbage at the end of the file is because the full size of the _name member, all 20 bytes, are being written to the file, not just the 5 characters from the name "Andre". What you are seeing are the nul terminator and the uninitialized remnants of the _data array.