Re: [Boost-users] [Boost.CircularBuffer] begin()/end() arithmetic not working out quite right

Subject: Re: [Boost-users] [Boost.CircularBuffer] begin()/end() arithmetic not working out quite right
From: Jan Gaspar (jano_gaspar_at_[hidden])
Date: 2009-06-11 04:53:09

Hi David,


you just cannot rely on this. I don't think standard says anything about begin() moving forward after push_back(). Correct me if I'm wrong.

The way how it is implemented is that if the circular_buffer is empty begin() returns the same iterator as end().

There is also a note about iterator invalidation for push_back():
"Does not invalidate any iterators with the exception of iterators pointing to the overwritten element."

It means iterator 'a' in your first example will point to the same element as it was before calling push_back() - which is end(). This explains the behaviour you are describing.

Regards,

Jan




----- Original Message ----
From: David Baird <dhbaird_at_[hidden]>
To: Jan Gaspar <jano_gaspar_at_[hidden]>; boost-users_at_[hidden]
Sent: Thursday, 11 June, 2009 1:01:54
Subject: [Boost.CircularBuffer] begin()/end() arithmetic not working out quite  right

Hi,

Firstly, thanks for the work on a circular buffer.  This is very
useful since many of my applications require it.  I am having a
problem though...

I am using Boost 1.38.0.  When I call push_back(), the iterator math
makes it appear that begin() moves backwards and end() remains
constant.  (Based on other STL libraries, I expect that begin() will
remain constant and end() will keep advancing as I call push_back()).

In other words, this assertion fails (but I expect it to succeed):

    circular_buffer<int> buf(8);
    circular_buffer<int>::iterator a;
    circular_buffer<int>::iterator b;
    a = buf.begin();
    buf.push_back(1);
    b = buf.begin();
    assert(a == b); // Fails!!

Also, this code fails too (but I expect it to succeed):

    circular_buffer<int> buf(8);
    circular_buffer<int>::iterator a;
    circular_buffer<int>::iterator b;
    a = buf.end();
    buf.push_back(1);
    b = buf.end();
    assert(a != b); // Also fails!

Below is a full example that you can compile and try out:

#include <boost/circular_buffer.hpp>
#include <stdio.h>

int
main ()
{
    typedef boost::circular_buffer<int> buf_type;
    // Instead of "end" moving ahead, "begin" is moving backwards when //
    // using "push_back".
    {
        buf_type buf1(1024);
        buf_type::iterator a;
        buf_type::iterator b;
        a = buf1.end();
        buf1.push_back(1);
        buf1.push_back(2);
        b = buf1.end();
        printf ("%d\n", a == b);
        // >>>
        // got: 1
        // expected: 0
        printf ("%d, %d\n", a-buf1.begin(), b-buf1.begin());
        // >>>
        // got: 2, 2
        // expected: 0, 2
    }
    {
        buf_type buf2(1024);
        buf_type::iterator c;
        buf_type::iterator d;
        c = buf2.begin();
        buf2.push_back(1);
        buf2.push_back(2);
        d = buf2.begin();
        printf ("%d\n", c == d);
        // >>>
        // got: 0
        // expected: 1
        // (i.e. "begin()" is not still pointing to first item!!
        // This is incorrect, isn't it?)
        printf ("%d, %d\n", buf2.end()-c, buf2.end()-d);
        // >>>
        // got: 0, 2
        // expected: 2, 2
    }
    return 0;
}


Thanks,
David