Thread: ofstream resetting get pointer position

First off topic name is wrong, it's meant to be: ifstream resetting get pointer position

I'm having trouble resetting the get pointer position. I want to read a file, then set the get pointer back to the beginning of the file, so then if my function is called again it will re-read the entire file. The problem is it will only read it once, and for some reason it won't reset properly. Another thing, I'm getting odd output. It will paste everything in the file, then it will repeat the very last line once, this is an example output:

Say the file contains:

Code:

www.hi.com
www.hello.com
www.sup.com

The output will be like this:

Code:

www.hi.com
www.hello.com
www.sup.com
www.sup.com

It always repeats the last line for some odd reason that I cannot see. Ok here are the important bits of code to help diagnose the problem, thanks!:

Code:

class Crawl {

public:
	Crawl();
	void PrintMenu();

private:
	ofstream writeStream;
	ifstream readStream;

};

Crawl::Crawl() {
	writeStream.open("crawler.txt", ios_base::app);
	readStream.open("crawler.txt");
}

void Crawl::PrintCrawl() {

	string x;

	while (!readStream.eof()) {
		readStream >> x;
		cout << x << "\n";
	}

	readStream.seekg(0, ios_base::beg);
	readStream.clear();

}

Yes, because the last time you try to read something, it gets an EOF, then you still print the content of x, which is the old value from the last read.

--
Mats

Oh.. Makes sense, ok well that's an easy fix, but that doesn't fix the reason why it won't read more than once

Code:

	while (!readStream.eof()) {
		readStream >> x;
		if (!readStream.eof())
			cout << x << "\n";
	}

Or, a "better" fix:

Code:

while(readstream >> x)
{
...
}

I don't see anything directly wrong, but perhaps you should try doing clear() first, then seekg(), as it may be that seekg() doesn't "do it's thing" if the stream is not in a good state.


--
Mats

Yep that worked, thanks matsp you're a genius

Incidentally, you might want to use the constructor initialisation list, e.g.,

Code:

Crawl::Crawl() : writeStream("crawler.txt", ios_base::app), readStream("crawler.txt") {}

Originally Posted by laserlight

Incidentally, you might want to use the constructor initialisation list, e.g.,

Code:

Crawl::Crawl() : writeStream("crawler.txt", ios_base::app), readStream("crawler.txt") {}

What would be the advantage of that?

Originally Posted by pobri19

What would be the advantage of that?

It would be slightly more efficient since you create the member streams with what you want instead of default constructing them and only then opening them.

So if I use the initialisation list (which I've never known about until now lol) it will open the files before a Crawl object is actually declared (which doesn't make sense to me, so I'm assuming this is wrong - because the objects etc don't get created until an instance of that object is created so therefore the constructor has nothing to change until it has an object to work with), and before the constructor of that class is actually called? Is that the difference?

Originally Posted by pobri19

So if I use the initialisation list (which I've never known about until now lol) it will open the files before a Crawl object is actually declared, and before the constructor of that class is actually called? Is that the difference?

When the default constructor for Crawl is invoked to create a Crawl object, the writeStream and readStream member variables of that new Crawl object would be initialised by opening the corresponding file. With your original code, there would be an unnecessary gap between the time that they are created and the time that they represent an open input/output stream. During that gap, the stream objects would exist with default states that would immediately be replaced by the desired states.

Also, since these stream objects are non-copyable, it might be good to declare the Crawl copy constructor and copy assignment operator as private.

"Also, since these stream objects are non-copyable, it might be good to declare the Crawl copy constructor and copy assignment operator as private."

Yeah... I have no idea how to do that I'll try and figure that out with Google. Never touched copy constructors or copy assignment operators or whatever before. Thanks for the help.

laserlight: What will declaring them private do?

Originally Posted by pobri19

laserlight: What will declaring them private do?

Make sure that no code outside of the class itself can call them - which essentially means you can't copy or assign objects of this class.

--
Mats