std::istreambuf_iterator "peek" with std::ifstream

Question

When dealing with streams of data, I prefer to write the code in terms of templates and iterators. Often I need to "peek" at the next character. In order to make the code be able to deal no-bidirectional iterators, I have a snippet which looks like this:

template <class I>
I next(I it) {
    return ++it;
}

Obviously, this makes a copy of the iterator, increments the copy and returns it. This tends to work very nicely... except when the std::istreambuf_iterator is operating on a std::ifstream. For example, given a file "test.txt" with the contents "ABCD", and the following code:

#include <fstream>
#include <iostream>

template <class I>
I next(I it) {
    return ++it;
}

int main() {
    std::ifstream file("test.txt", std::ios::binary);
    std::istreambuf_iterator<char> it(file);
    std::cout << *next(it) << std::endl;
    std::cout << *it << std::endl;
}

The output is:

$ ./test 
B
B

Instead of what I hoped for:

$ ./test 
B
A

In other words, incrementing one copy of the iterator, has the net effect of incrementing all of them!

I realize that file stream iterators have limitations in that they can only operate on what is currently in the read buffer associated with the file. So there may not be a solution which matches exactly what I want. Is there a way to do what I want?

score 2 · Accepted Answer · answered Jun 13 '11 at 01:40

2

Well, it actually doesn't increment all iterators but rather consumes the stream, which in the end has the same effect. If you want to peek ahead, you need to do so on the stream itself, AFAIK.

answered Jun 13 '11 at 01:40

Xeo

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yea, I figured. That's why I said "net effect". I figured that the character is "consumed" by incrementing, I was just hoping that there is something clever to do what I want. – Evan Teran Jun 13 '11 at 01:43
You need to `seek` it explicitly, iternator don't work that way. – J-16 SDiZ Jun 13 '11 at 02:37

score 0 · Answer 2 · answered Jan 21 '14 at 22:19

Stream buffer iterators are one-pass iterators. You cannot move backwards and it doesn't have bidirectional functionality like vector::iterator and string::iterator to name a few. There is no way to create an isolated copy of a stream iterator which operates on a file stream independent of any other iterator instances. It is impossible because stream iterators, whether copied or not, work on the same stream object all the time. Copying an iterator does not copy the file stream.

score 0 · Answer 3 · answered Jun 13 '11 at 01:37

0

You mentioned 'peek' in the title of your post. std::istream has a peek() method. Use it.

answered Jun 13 '11 at 01:37

David Hammen

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1

but I want to work with iterators, ideally the code will be able to work with pointers or a `std::istreambuf_iterator` or anything that works like an iterator. – Evan Teran Jun 13 '11 at 01:40

std::istreambuf_iterator "peek" with std::ifstream

3 Answers3