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Saving Classes to files...

Started by JoshG Dec 27, 2001 at 5:50 AM 6 replies 850+ views

Original Post

Dec 27, 2001 at 5:50 AM

Hello all, I am trying to write a program that needs to save information from a Tree. naturally, I''m sure most have come across this before. I have a Binary Tree, with each node being information about a User. so far, in the class I have: ----------------CODE--------- class BTreeNode{ public: BTreeNode *pLow; BTreeNode *pHigh; char *Name; __fastcall BTreeNode(); }; class BTUser: public BTreeNode{ public: char *Password; }; ----------------------------- so I have a Name, a Password, and pointers that are used to navigate the tree. really the pointers have no relevance i don''t think here, as it is the class that I wish to save. So far I am trying to save just the first node, to see if I can get it to work. Basically it isn''t, and it is MOST bizzare. I save it to a file like this... ---------------------CODE--------- FILE *savef = fopen(FileName.c_str(),"w+"); BTUser *save = (BTUser*)dlMain->Users->Head; BTUser temp; temp.pLow = temp.pHigh = NULL; strcpy(temp.Name,save->Name); fwrite(&temp,sizeof(BTUser),1,savef); fclose(savef); --------------------------------- ok... so that seems to work, then I load it, and it mucks up the name and the password!! I load it like this... ------------CODE---------------- -------------------------------- this code works however when I set the values of Password & Name like this... Name = "hello"; Password = "gbye"; Any tips that you have are all welcome!! even different approaches are welcome!! --Josh

JoshG

Author

JoshG

Dec 27, 2001 at 6:13 AM

sorry, the code for loading that I have used is...

FILE *fUsers;
fUsers = fopen(FileName.c_str(),"r");

BTUser *add;
add = new BTUser;

fread(add,sizeof(BTUser),1,fUsers);
ShowMessage(add->Name);

delete add;
fclose(fUsers);

Thanx
--Josh

SteveC

Dec 27, 2001 at 9:51 AM

The basic problem you have here is that you are trying to write out a data structure with variable length entries ( eg the strings )

besides that, a char* is just a pointer

also I think your write/read sizes, based on the size of the class, are also likely to cause problems.

To persist a class ( eg write it out ) what you should do is have some sort of methods for writing/reading the class itself.
( there are many different ways of doing this, such as overloading operator<< for streams )

And then, the class should write out the individual data members it needs in order to reconstruct itself correctly.

if a data node''s name and password list is what needs to be written one way to do so ( to a text file ) would be as follows:

1. write out the name
2. write a space
3. write out how many passwords there are
4. write a space
5. write out each password followed by a space

putting the spaces in there will make it easier to read in with iostreams.

Hope this helps

Dak Lozar

Dec 27, 2001 at 1:44 PM

You may want to read the article:
Why Pluggable Factories Rock my Multiplayer World.

I refer you to this article because it brifly talks about object serialization which may be what you want to do...

Dave "Dak Lozar" Loeser

Zipster

511

Zipster

Dec 28, 2001 at 1:13 AM

Another problem is rebuilding the tree when you load it. Assuming you use some sort of recursive save function with your tree, you will have a linear set of data in a file, but due to the nature of the recursive method, you have to write the save function so that it also records the information''s relative position in the tree, or else loading the data into the original tree form is impossible.

JoshG

Author

JoshG

Dec 28, 2001 at 1:19 AM

Yeah, I know that is a problem. however there are recursive functions that will return the nodes in either Pre-Order, in-order, and postorder.
One of those (i'm not sure which) is the order that the nodes were entered into the tree. So i can make the tree in exactly the same tree by adding the nodes in the same order.
The only thing wrong with this is that the tree could be unbalanced, and wouldn't every get balanced. But i'm not particularly worried about that at the moment, I'd really just like to be able to save and load it

I really enjoyed that article on pluggable factories, That has brought new ideas to my mind on ho to fix OTHER problems I have on other projects, but it didn't help so much with this problem.

Edited by - JoshG on December 28, 2001 2:21:45 AM

CaptainJester

101

CaptainJester

Dec 28, 2001 at 8:39 AM

Just use fstream. Once you have a file open on a stream, you can just ''cout << yourObject;'' This will save your object to a file. Then when you ''cin >> yourObject;'', the stream object takes care of everything to make sure it goes in the right spots. Also if you use string instead of char*, you will have an easier time with it.

---
Make it work.
Make it fast.

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