Channels ▼


Transactional Programming


atomic::list is implemented as a doubly linked list of nodes, just like std::list would be implemented. When an item is inserted into the list, the node containing the item is logged on the transaction stack. When an item is deleted, it is removed from the data structure and stored on the transaction stack. To roll back a delete, the left and right pointers point to live nodes, giving the correct place in the list to insert the node.

There is no possibility of an exception being thrown because roll back only requires pointer manipulation, not any memory allocation. To roll back an insertion, the list node is removed and deleted (which will not throw). Figure 1 shows how list operations can be rolled back.

Figure 1: Operations on atomic::list<int>: (a) Initial list; (b) list after push_front(4); (c) list after pop_back(); (d) list after rolling back step (c); (e) the list after rolling back step (b).


The tree-based containers (atomic::map and the like) are based upon an implementation of red-black trees (a standard technique for balancing trees). When an item is inserted into the data structure, it is logged on the transaction stack. When an item is deleted, it is removed from the data structure and stored on the transaction stack.

Rolling back a deletion is tricky because the object must be inserted in the correct position in the tree. You cannot simply call insert() to insert the node back into the data structure because the comparators might throw, and the nodes might end up in a different order than the original. So the deleted node stores the next node in the data structure. When the deletion is rolled back, the node is reinserted immediately before the next node in the data structure. There are a few cases to consider, but the procedure can be performed reliably. Figure 2 shows how tree operations are rolled back (ignoring tree balancing).

Figure 2: Operations on atomic::set<int>: (a) Initial tree; (b) tree after insert(6); (c) Tree after erase(5); (d) tree after rolling back step (c); this is equivalent to tree (b); (e) tree after rolling back step (b).

With red-back trees, the tree-balancing algorithms must be called after an insertion or a deletion, but these only perform pointer manipulation and will not throw exceptions.

Related Reading

More Insights

Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.