C++ iterators with linked list

Question

I have a linked list of objects where each object has a linked list of char.

I declared three iterators:

top = someList.begin();

middle = top++;

bottom = middle++;

When I print the list of each of those iterators, it doesn't match up with what the full list looks like.

Basically by the end of those declaration statements top becomes the middle row, middle is what its meant to be and bottom becomes what the top row is meant to be.

I am assuming it's the way I declare the iterators with the increments. Any help is much appreciated.

You're gonna need to post more code before people can properly help you. Something compilable would be nice. — Rose Kunkel
– Rose Kunkel, Commented May 3, 2014 at 5:47
Do you really mean top++ and not top+1? Similarly middle++ and not middle+1? — theharshest
– theharshest, Commented May 3, 2014 at 5:52
The postfix++ operator increments the iterator and returns the original value. — Lachlan Easton
– Lachlan Easton, Commented May 3, 2014 at 5:54

Jarod42 · Accepted Answer · 2014-05-03 05:58:12Z

3

Let's see what happens after each statement. assume position marks 0,1,2,3... for the list
The problem is in the post ++

top = someList.begin();
//top=0

middle = top++;
//middle=0
//top=1

bottom = middle++;
//bottom=0
//middle=1
//top=1

edited May 3, 2014 at 5:58

Jarod42

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answered May 3, 2014 at 5:54

digit plumber

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1 Comment

Vince Over a year ago

I see what is happening. thank you for breaking that down for me.

merlin2011 · Accepted Answer · 2014-05-03 05:57:09Z

1

This is a shot in the dark, based on your description here:

basically by the end of those declaration statements top becomes the middle row, middle is what its meant to be and bottom becomes what the top row is meant to be.

When you assign middle = top++, you are assigning middle to the old value of top, and simultaneously incrementing top. It is the same idea in the second assignment.

answered May 3, 2014 at 5:57

merlin2011

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Comments

Vlad from Moscow · Accepted Answer · 2014-05-03 06:08:24Z

1

Try the following

#include <iterator>

//...
top = someList.begin();

middle = std::next( top );

bottom = std::next( middle );

Or you could write the same without using function std::next

top = someList.begin();

++( middle = top );

++( bottom = middle );

Or more readable

top = someList.begin();

middle = top;
++middle;

bottom = middle;
++bottom;

Or you could use function std::advance

#include <iterator>

//...
top = someList.begin();

middle = someList.begin();
std::advance( middle, 1 );

bottom = someList.begin();
std::advance( bottom, 2 );

edited May 3, 2014 at 6:08

answered May 3, 2014 at 6:00

Vlad from Moscow

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