Reading objects from file and getting error

Question

(This is a homework project.) I am trying to write objects from class A (Contains couple of strings and ints, also a list from objects from class B ) into file. Then I have to read back these objects from the file and display their content. I am using this code:

Writing:

ofstream ofs("filestorage.bin", ios::app|ios::binary);
A d;
ofs.write((char *)&d, sizeof(d));
ofs.close();

Reading:

ifstream ifs("filestorage.bin", ios::binary);
A e(1);

while(ifs.read((char *)&e, sizeof(e))) {    
    cout<<e;
}

ifs.close();

<< is already redefined.

It writes the data into the file, then reads it back, displays everything I want but in the end I'm getting a nice fat "Access Violation " Error. I also tried to write and read simple variables into the file (like ints). That works fine; but when I try to read an object or string I get "Access Violation". The writing seams to be OK because I get no errors.

Can you tell me why this is happening and how can I fix it? If it is necessary I can post my A and B classes too. Thanks!

There's so many things that could go wrong with this... Read/search about serialization in C++, you'll find lots of interesting stuff. As for your question, the definition of A could let someone tell you why this doesn't work. — Mat
– Mat, Commented Nov 6, 2012 at 19:07
Got to check for the eof() otherwise you'll read past the end of the file — MaximR
– MaximR, Commented Nov 6, 2012 at 19:13
When you get the access violation, what does the callstack say? That may lead you directly to the problem. — xtofl
– xtofl, Commented Nov 6, 2012 at 19:21
You have operator << defined already. Use it to write to the file like you did to cout and define operator >>. — andre
– andre, Commented Nov 6, 2012 at 19:39

andre · Accepted Answer · 2012-11-06 19:34:39Z

1

Implement the two operators << and >> for your class.

class A {
    int a;
    string s;
pubilc:
    friend ostream& operator<< (ostream& out, A& a ) {
        out << a.a << endl;
        out << a.s << endl;
    }
    friend istream& operator>> (istream& is, A& a ) {
        //check that the stream is valid and handle appropriately.
        is >> a.a; 
        is >> a.s;
    }
};

Write:

A b;
ofstream ofs("filestorage.bin", ios::app|ios::binary);
ofs << b;

Read:

fstream ifs("filestorage.bin", ios::binary);
A b;
ifs >> b;

edited Nov 6, 2012 at 19:34

answered Nov 6, 2012 at 19:10

andre

7,2996 gold badges50 silver badges80 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

andre Over a year ago

@user1803967 This would be easier if you did post your classes.

MaximR · Accepted Answer · 2012-11-06 19:17:52Z

0

You can try just checking the stream status

while(ifs.read((char *)&e, sizeof(e)).good()){
     cout<<e;
}

answered Nov 6, 2012 at 19:17

MaximR

2792 silver badges8 bronze badges

2 Comments

andre Over a year ago

You have to serialization the data. See Mat's comment.

MaximR Over a year ago

No doubt it is not the greatest way to go, but he reports that it reads the object fine, prints, and crashes afterward - this might be "good enough", especially if A doesn't have any pointer members. And even for your solution, if you read in a loop - still have to check for eof.

Collectives™ on Stack Overflow

Reading objects from file and getting error

2 Answers 2

1 Comment

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related