Write an array to binary file and read it back in

[cartpauj]

I have a class called Person
Then I have an array of Person's called db[100]. How do I write the entire array to a binary file, and then read it back in?

I'm currently using to write:
sFile.write((char *) db, (sizeof(Person) * 100));

And to read:
theFile.read((char *) db, (sizeof(Person) * 100));

It's writing stuff to the file and it's also reading stuff back in but if I try to display the list of Person's after reading it back in, it only shows up to Person 25 and then complete garbage after that.

Any Ideas?

[blackcoder41]

it only shows up to Person 25 and then complete garbage after that.

i think because you're multiply the size by 100.

post your code..

[Duthomhas]

i think because you're multiply the size by 100.

No, it is because he is trying to dump a class to file.

Google around "C++ serialization" for more help.

Specifically, your class should know how to read and write itself from and to file. I would make a method that does that. C++ provides you with the iostream operator overloads for that this very purpose:

class Person
  {
  ...
  friend ostream& operator << ( ostream&, const Person& );
  };

ostream& operator << ( ostream& outs, const Person& person )
  {
  // This method presumes a textual file. See the notes that follow.
  outs << "Person\n";
  outs << person.surname << "\n";
  outs << person.givenname << "\n";
  outs << person.age << "\n";
  outs << endl;
  return outs;
  }

As in this snippit, I recommend you stay away from binary files for schoolwork simply because they are more grief to handle than you think. (Unless you are required to manipulate a binary file.) The above would work with a file like:

Smith
Will
41

Andrews
Julie
74

Cusack
Sinéad
61

In order to write such a file, simply use a loop:

for (unsigned n = 0; n < 100; n++)
  sFile << people[ n ];

Likewise, to read such a file, use a loop. Here it is using a std::vector instead of an array:

vector <Person> people;
Person person;
while (sFile >> person)
  people.push_back( person );

Hope this helps.

[blackcoder41]

hmmm.. can you guess what type is db? he's trying to cast it as char *

[Duthomhas]

The ostream::write() method takes a char* as its first argument.

Having no other information that what he has given us, we can only presume that db is an array of Person:
    Person db[100];.

Again, I recommend that you stay away from direct arrays, and use a vector or a deque instead:
    deque <Person> db;

[blackcoder41]

@Duoas
i see.. thanks, i'll read about vectors tomorrow. it's already 1 am here. my head going to crack now.

@cartpauj
have you figured it out already? post it here.