Thread: fstream is just not working for me..

I've been having a lot of problems with fstream lately..
for my class theres this source code that won't compile... i was hoping someone could help me shed some light...

Code:

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

void adding(fstream & f, int & counter)
{
	char name[51];
	long l = f.tellp() / 50; // find out how many records are in the file (Each I record consumes 50 bytes.)
	cout << "There are " << l << " records in the file." << endl;
	cout << "Enter a name ";
	cin.getline(name, 50);
	f.seekp(counter * 50); // move the pointer to the last position.
	f.write(name, 50); // write 50 bytes to the file.
	counter++; // increase the counter.
}

void displaying(fstream & f)
{
	int r=0;
	char name[51];
	cout << "Which record do you want to read ?";
	cin >> r;
	cin.ignore(200, '\n');
	f.seekg(--r * 50); // locate the pointer for the read() function.
	f.read(name, 50); // read 50 bytes from the file and store it into 'name'
	cout << "The name is " << name << endl;
}


void changing(fstream & f)
{
	int r=0;
	char name[51];
	cout << "Which record do you want to change ?";
	cin >> r;
	cin.ignore(200, '\n');
	cout << "Enter a name ";
	cin.getline(name, 50);
	f.seekp(--r * 50);
	f.write(name, 50);
}


int main()
{ // open a binary file for both input and output.
	fstream f("Test.bin", ios::in | ios::out | ios::binary);
	int i=0;
	int counter=0;
	if(f) { // the file can open.
		while(i != 4) {
		cout << "1 for adding records " << endl;
		cout << "2 for displaying records " << endl;
		cout << "3 for changing records " << endl;
		cout << "4 for quitting the program" << endl;
		cin >> i;
		cin.ignore(200, '\n');
		if(i==1)
  	  	  	adding(f, counter);
		else if(i==2)
			displaying(f);
		else if(i==3)
			changing(f);
		}
	}
	else
		cout << "failed" << endl;
	f.close(); // close the file associated with "f"
	return 0;
}

all i get in the console window is

Code:

failed
press any key to continue .....

please let me know why this is happening..
also as a side note. while learning fstream i have had many failed attempts in opening an fstream with both ' ios::in and ios:ut '.. perhaps there's a relationship between them. or i just dont know my fstream well enough..

thanks

The way you're opening the file, Test.bin must exist. You don't use a full path, so it must exist in the current directory. But the current directory is often different than the directory that your program is in, and can change depending on how you run the program.

Put a full path in the code (e.g. "C:\\test.bin") and see if it works better. If it does, then you just need to make sure the current directory is what you expect.

If you're running from Visual C++, edit the Project Properties under Debugging and change the Working Directory to the directory that has that file. If you're running from the command line, then set the current directory of the console to the directory with your file before running the program. If you're doing something else, let us know so we can help.

If the full path doesn't work, make sure the file exists there and is readable and writable, and make sure you used the proper escape sequence for directory separators.

>> But will "if (f)" work?
Yes. The conversion to void* is done automatically, and that value is evaluated in the boolean context to be false if the file did not open.

>> I suspect that you probably want to open the file with the append flag set as well, that way, if the file doesn't already exist, it gets created empty.
I'm not sure about this, but I don't think the file is emptied when the ios::in flag is set. Otherwise how would you be able to read from the file?

Originally Posted by Daved

Yes. The conversion to void* is done automatically, and that value is evaluated in the boolean context to be false if the file did not open.

Thanks! I never knew that. To me that kind of code looks scary, because if you don't understand how the language works in that situation, it can be one of those, "Oh god #!#$#, what the heck does this do?" moments. And even after digging through the documentation for a couple minutes, you're still not sure how or why it works.

Originally Posted by Daved

I'm not sure about this, but I don't think the file is emptied when the ios::in flag is set. Otherwise how would you be able to read from the file?

Well I didn't really mean it gets emptied. If the file already exists then it gets opened for appending. If the file doesn't exist, it gets created empty.

I just compiled his code with the append flag included just to make sure I'm not making things up here, and it worked for me (using MinGW). Of course this could just be a fluke.

Originally Posted by laserlight

operator bool() has an obvious problem: it means that the object is convertible to int, which is often undesirable.

Could you explain this a little more or provide a link or something? Why is converting to an int undesirable?

Aha. Thanks. Thought it was just on that idiom alone.