Thread: Another dumb question about file i/o

I don't see why not as long as you close the file before trying to open another with the same stream variable. Give it a try and see for yourself.
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-Prelude

o i c.. man i gotta start paying attention ;) otherwise ill never make a programmer. anyways, here's the story and what i want my program to do.
I have a program, which is sopposed to be a RPG engine - calm down its a very very simple one. All it does is. basically, what one of those choose your own adventure kind of books does; it reads you a piece of story, then gives you some choices (1 - 9, depending on the file), then you choose one, and it goes on to the next file (depending on your choice.) so basically, the data files are like pages in the book.
Anyway, i got all the basic code down, using somewhat liek i did above in my first post. however, there is a problem with what i did. so, i didn't want to post a "4 gazillion pages of code program" and ask everyone to fix it for me; long story short, I cut the program down to 57 lines, that still give the same error(i know its still big but sorry :() and id like to ask if anyone has any ideas, cause ive been bug hunting for quite some time now.
anyway heres the code:

in game.cpp:
--snip snip--
i messed up, go look further down for the proper code

It compiles nicely, but programs that work with files require me to create the files and I'm lazy. What is the problem that you're getting with the above code? Run-time error, just wacky output or something else?

-Prelude

The program works fine the first time through the loop. it loads up the dat variable, loads the file, and displays its contents; then lets you choose your option. however as soon as it attempts to run through the loop again, just when it gets to opening the new file, it crashes with a page fault in the exe.

GAME caused an invalid page fault in
module GAME.EXE at 0167:00403997.

and of course the rest which is

Registers:
EAX=7250794d CS=0167 EIP=00403997 EFLGS=00010246
EBX=0065fdf0 SS=016f ESP=0065fc40 EBP=0065fcc8
ECX=00780d00 DS=016f ESI=00000000 FS=2baf
EDX=00780e8c ES=016f EDI=0065fc54 GS=0000
Bytes at CS:EIP:
ff 50 04 84 c0 74 03 89 75 38 39 75 44 75 24 6a
Stack dump:
004250b2 0065fcc8 00000008 00420464 00780e30 007804c0 00780e61 00420464 004250b2 0065fdf0 0041dfa3 ffffffff 00403374 004272ac 004011b8 00780e61

and i have no idea what it means

The problems that I see are that you don't close the file inside your loop whereas you do try to open the file. You also said that you wanted different files to be opened and the loop doesn't change your currentfile string anywhere. So what is happening is the file is opened, read from, then the program attempts to open the same file again. This causes your page fault since the file is already open. Try moving the close command to the end of the loop and see if the error still shows up.

Code:

  for(;;) {
    thisdat.open(currentfile.c_str()); // Where do you change this?
    cout << "Opened " << currentfile << endl;
    for(;i<10;i++) {
      if (!(thisdat.eof())) {
      for(;;) {
        ch = thisdat.peek();
        if (ch == left) break;
        thisdat.get(ch);
        cout << ch;
      }
      if (ch == left) {
        thisdat.get();
        thisdat.get(buffer, 300, '>');
        dat[i] = buffer;
        thisdat.get();
        ch = thisdat.peek();
      }
    }
    // File close should be here too
  }
  buffer = "";
  thisdat.close(); // Shouldn't this be in the loop?

-Prelude

I see why you thought the file was closed outside the main loop.. sorry, i messed up the formatting

okay, heres an update. I messed around a bit and discovered that if I attempt to open menu.dat again (after the first time i open it), the game crashes. however, if i open any other file(s), in any prder, back and forth, open them twice, open them eight times in a row, it works. but menu.dat only works once.

???
hmm weird
here's the current code(43 lines right now and absolutely barebones :);

Code:

#include <iostream>
#include <string>
#include <fstream>

using namespace std;

int main()
{
	string dat[10], currentfile("menu.dat");
	char* buffer = "";
	char ch, input = ' ';
	int chtoint; //just giving things values
	ifstream thisdat;
	
	for(;;) {
		thisdat.open(currentfile.c_str());
		cout << "Opened " << currentfile.c_str() << endl;//for debugging
		for(;;) {
			thisdat.get(ch);
			if (ch == '<') break;
			cout << ch;
		}
		if (ch == '<') {
			thisdat.get(buffer, 300, '>');
			dat[1] = buffer;
		}
		buffer = "";
		thisdat.close();

		cout << "\nChoice: ";
		cin.get(input);
		cin.get();
		switch (input) {
		case '1':
			chtoint = input - 48;  //0-9 are 48-57 in ascii code, we want them to be 0-9 
			currentfile = dat[chtoint];
			break;
		default:
			break;
		}
	}
	return 1;
}

heres menu.dat:

Code:

Here's choices:

1. goto A<a.dat>

a.dat:

Code:

We're in A

1. goto B<b.dat>

b.dat:

Code:

We're in B

1. goto menu<menu.dat>

when the prog executes, it reads out all of menu.dat (except whats in the angle brackets, which goes into dat[1]);
i pick choice 1
it enters a.dat
i pick choice 1 again
it enters b.dat
i pick choice 1 again
it exits with an invalid page fault
any ideas on why i cant go back to menu.dat????

Well, folks, here we are. I got my code.. originally 110 lines.. down to 23 - yes, you read me right, 23 lines of code, which still gives me the same error as before. heres the code:

Code:

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

int main()
{
	string currentfile("data/menu.dat");
	char* buffer = "";
	ifstream thisdat;
	
	for(;;) {
		thisdat.open(currentfile.c_str());
		thisdat.get(buffer, 5000, '<');
		cout << buffer;
		thisdat.get();
		thisdat.get(buffer, 300, '>');
		currentfile = buffer;
		thisdat.close();
		cin.get(); 
	}
	return 1;
}

however.. if I change the line;
char* buffer = "";

to char* buffer = " ";

or char* buffer = "menu.dat";

or even char* buffer = "Hi how are you";

it works fine. obviously I don't understand something about pointers.. can anyone please tell me why this is??
BTW the contents of the currently working datafiles follow.

menu.dat:

Code:

Going to a.dat...
<a.dat>

a.dat:

Code:

This is A.dat; going back to menu.dat...
<menu.dat>

And the output when the program is run:

Code:

Going to a.dat...(<-here i press enter)

This is a.dat; going back to menu.dat...

which repeats as much as you want.

ps heres the datafiles, exe, and 23 lines of code

Ack! How embarrassing, I can't believe I missed that. You're buffer was initialized as a string literal, you can't assign data to a string literal so when you attempted a get, it resulted in an access violation. I changed
char *buffer = " ";
to
char buffer[5000] = " ";
and it worked fine.

Excuse me while I go whack myself with a book for a while for not seeing something so simple :P

-Prelude

>but it does work if I actually assign something to it
I don't see why it would, and it doesn't for me.

-Prelude

that's weird....
like I siad I guess I don't understand *something* about the way pointers work..

Just a thought.. does "" in your book equal " " ?
because in the code i had in the zip file, char* buffer was initialised to "" (no whitespace) but if I changed the "" to " " (i.e. one empty space), it worked. It also worked if I changed the buffer to a character array, like you did.

You said I was creating a string literal. if I initialise the buffer with an actual value, am I still creating a string literal? or does it work differently?

Unfortunately I still don't get it, but I want to so any explanation you can offer would be appreciated.. as is all the help you've given me so far, Prelude.


thx again,