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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 22. Policy-Based Data Structures</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.78.1" /><meta name="keywords" content="ISO C++, policy, container, data, structure, associated, tree, trie, hash, metaprogramming" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="extensions.html" title="Part III.  Extensions" /><link rel="prev" href="bitmap_allocator_impl.html" title="Implementation" /><link rel="next" href="policy_data_structures_using.html" title="Using" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 22. Policy-Based Data Structures</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bitmap_allocator_impl.html">Prev</a> </td><th width="60%" align="center">Part III. 
Extensions
</th><td width="20%" align="right"> <a accesskey="n" href="policy_data_structures_using.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.containers.pbds"></a>Chapter 22. Policy-Based Data Structures</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="section"><a href="policy_data_structures.html#pbds.intro">Intro</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures.html#pbds.intro.issues">Performance Issues</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures.html#pbds.intro.issues.associative">Associative</a></span></dt><dt><span class="section"><a href="policy_data_structures.html#pbds.intro.issues.priority_queue">Priority Que</a></span></dt></dl></dd><dt><span class="section"><a href="policy_data_structures.html#pbds.intro.motivation">Goals</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures.html#pbds.intro.motivation.associative">Associative</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures.html#motivation.associative.policy">Policy Choices</a></span></dt><dt><span class="section"><a href="policy_data_structures.html#motivation.associative.underlying">Underlying Data Structures</a></span></dt><dt><span class="section"><a href="policy_data_structures.html#motivation.associative.iterators">Iterators</a></span></dt><dt><span class="section"><a href="policy_data_structures.html#motivation.associative.functions">Functional</a></span></dt></dl></dd><dt><span class="section"><a href="policy_data_structures.html#pbds.intro.motivation.priority_queue">Priority Queues</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures.html#motivation.priority_queue.policy">Policy Choices</a></span></dt><dt><span class="section"><a href="policy_data_structures.html#motivation.priority_queue.underlying">Underlying Data Structures</a></span></dt><dt><span class="section"><a href="policy_data_structures.html#motivation.priority_queue.binary_heap">Binary Heaps</a></span></dt></dl></dd></dl></dd></dl></dd><dt><span class="section"><a href="policy_data_structures_using.html">Using</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.prereq">Prerequisites</a></span></dt><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.organization">Organization</a></span></dt><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.tutorial">Tutorial</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.tutorial.basic">Basic Use</a></span></dt><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.tutorial.configuring">
Configuring via Template Parameters
</a></span></dt><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.tutorial.traits">
Querying Container Attributes
</a></span></dt><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.tutorial.point_range_iteration">
Point and Range Iteration
</a></span></dt></dl></dd><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.examples">Examples</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.examples.basic">Intermediate Use</a></span></dt><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.examples.query">Querying with <code class="classname">container_traits</code> </a></span></dt><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.examples.container">By Container Method</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.examples.container.hash">Hash-Based</a></span></dt><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.examples.container.branch">Branch-Based</a></span></dt><dt><span class="section"><a href="policy_data_structures_using.html#pbds.using.examples.container.priority_queue">Priority Queues</a></span></dt></dl></dd></dl></dd></dl></dd><dt><span class="section"><a href="policy_data_structures_design.html">Design</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.concepts">Concepts</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.concepts.null_type">Null Policy Classes</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.concepts.associative_semantics">Map and Set Semantics</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#concepts.associative_semantics.set_vs_map">
Distinguishing Between Maps and Sets
</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#concepts.associative_semantics.multi">Alternatives to <code class="classname">std::multiset</code> and <code class="classname">std::multimap</code></a></span></dt></dl></dd><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.concepts.iterator_semantics">Iterator Semantics</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#concepts.iterator_semantics.point_and_range">Point and Range Iterators</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#concepts.iterator_semantics.both">Distinguishing Point and Range Iterators</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.concepts.invalidation">Invalidation Guarantees</a></span></dt></dl></dd><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.concepts.genericity">Genericity</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#concepts.genericity.tag">Tag</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#concepts.genericity.traits">Traits</a></span></dt></dl></dd></dl></dd><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.container">By Container</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.container.hash">hash</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#container.hash.interface">Interface</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#container.hash.details">Details</a></span></dt></dl></dd><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.container.tree">tree</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#container.tree.interface">Interface</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#container.tree.details">Details</a></span></dt></dl></dd><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.container.trie">Trie</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#container.trie.interface">Interface</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#container.trie.details">Details</a></span></dt></dl></dd><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.container.list">List</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#container.list.interface">Interface</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#container.list.details">Details</a></span></dt></dl></dd><dt><span class="section"><a href="policy_data_structures_design.html#pbds.design.container.priority_queue">Priority Queue</a></span></dt><dd><dl><dt><span class="section"><a href="policy_data_structures_design.html#container.priority_queue.interface">Interface</a></span></dt><dt><span class="section"><a href="policy_data_structures_design.html#container.priority_queue.details">Details</a></span></dt></dl></dd></dl></dd></dl></dd><dt><span class="section"><a href="policy_based_data_structures_test.html">Testing</a></span></dt><dd><dl><dt><span class="section"><a href="policy_based_data_structures_test.html#pbds.test.regression">Regression</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#pbds.test.performance">Performance</a></span></dt><dd><dl><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.hash">Hash-Based</a></span></dt><dd><dl><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.hash.text_find">
Text <code class="function">find</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.hash.int_find">
Integer <code class="function">find</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.hash.int_subscript_find">
Integer Subscript <code class="function">find</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.hash.int_subscript_insert">
Integer Subscript <code class="function">insert</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.hash.zlob_int_find">
Integer <code class="function">find</code> with Skewed-Distribution
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.hash.erase_mem">
Erase Memory Use
</a></span></dt></dl></dd><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.branch">Branch-Based</a></span></dt><dd><dl><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.branch.text_insert">
Text <code class="function">insert</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.branch.text_find">
Text <code class="function">find</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.branch.text_lor_find">
Text <code class="function">find</code> with Locality-of-Reference
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.branch.split_join">
<code class="function">split</code> and <code class="function">join</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.branch.order_statistics">
Order-Statistics
</a></span></dt></dl></dd><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.multimap">Multimap</a></span></dt><dd><dl><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.multimap.text_find_small">
Text <code class="function">find</code> with Small Secondary-to-Primary Key Ratios
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.multimap.text_find_large">
Text <code class="function">find</code> with Large Secondary-to-Primary Key Ratios
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.multimap.text_insert_small">
Text <code class="function">insert</code> with Small
Secondary-to-Primary Key Ratios
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.multimap.text_insert_large">
Text <code class="function">insert</code> with Small
Secondary-to-Primary Key Ratios
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.multimap.text_insert_mem_small">
Text <code class="function">insert</code> with Small
Secondary-to-Primary Key Ratios Memory Use
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.multimap.text_insert_mem_large">
Text <code class="function">insert</code> with Small
Secondary-to-Primary Key Ratios Memory Use
</a></span></dt></dl></dd><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.priority_queue">Priority Queue</a></span></dt><dd><dl><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.priority_queue.text_push">
Text <code class="function">push</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.priority_queue.text_push_pop">
Text <code class="function">push</code> and <code class="function">pop</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.priority_queue.int_push">
Integer <code class="function">push</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.priority_queue.int_push_pop">
Integer <code class="function">push</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.priority_queue.text_pop">
Text <code class="function">pop</code> Memory Use
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.priority_queue.text_join">
Text <code class="function">join</code>
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.priority_queue.text_modify_up">
Text <code class="function">modify</code> Up
</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#performance.priority_queue.text_modify_down">
Text <code class="function">modify</code> Down
</a></span></dt></dl></dd><dt><span class="section"><a href="policy_based_data_structures_test.html#pbds.test.performance.observations">Observations</a></span></dt><dd><dl><dt><span class="section"><a href="policy_based_data_structures_test.html#observations.associative">Associative</a></span></dt><dt><span class="section"><a href="policy_based_data_structures_test.html#observations.priority_queue">Priority_Queue</a></span></dt></dl></dd></dl></dd></dl></dd><dt><span class="section"><a href="policy_data_structures_ack.html">Acknowledgments</a></span></dt><dt><span class="bibliography"><a href="policy_data_structures.html#pbds.biblio">Bibliography</a></span></dt></dl></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="pbds.intro"></a>Intro</h2></div></div></div><p>
This is a library of policy-based elementary data structures:
associative containers and priority queues. It is designed for
high-performance, flexibility, semantic safety, and conformance to
the corresponding containers in <code class="literal">std</code> and
<code class="literal">std::tr1</code> (except for some points where it differs
by design).
</p><p>
</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="pbds.intro.issues"></a>Performance Issues</h3></div></div></div><p>
</p><p>
An attempt is made to categorize the wide variety of possible
container designs in terms of performance-impacting factors. These
performance factors are translated into design policies and
incorporated into container design.
</p><p>
There is tension between unravelling factors into a coherent set of
policies. Every attempt is made to make a minimal set of
factors. However, in many cases multiple factors make for long
template names. Every attempt is made to alias and use typedefs in
the source files, but the generated names for external symbols can
be large for binary files or debuggers.
</p><p>
In many cases, the longer names allow capabilities and behaviours
controlled by macros to also be unamibiguously emitted as distinct
generated names.
</p><p>
Specific issues found while unraveling performance factors in the
design of associative containers and priority queues follow.
</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.intro.issues.associative"></a>Associative</h4></div></div></div><p>
Associative containers depend on their composite policies to a very
large extent. Implicitly hard-wiring policies can hamper their
performance and limit their functionality. An efficient hash-based
container, for example, requires policies for testing key
equivalence, hashing keys, translating hash values into positions
within the hash table, and determining when and how to resize the
table internally. A tree-based container can efficiently support
order statistics, i.e. the ability to query what is the order of
each key within the sequence of keys in the container, but only if
the container is supplied with a policy to internally update
meta-data. There are many other such examples.
</p><p>
Ideally, all associative containers would share the same
interface. Unfortunately, underlying data structures and mapping
semantics differentiate between different containers. For example,
suppose one writes a generic function manipulating an associative
container.
</p><pre class="programlisting">
template&lt;typename Cntnr&gt;
void
some_op_sequence(Cntnr&amp; r_cnt)
{
...
}
</pre><p>
Given this, then what can one assume about the instantiating
container? The answer varies according to its underlying data
structure. If the underlying data structure of
<code class="literal">Cntnr</code> is based on a tree or trie, then the order
of elements is well defined; otherwise, it is not, in general. If
the underlying data structure of <code class="literal">Cntnr</code> is based
on a collision-chaining hash table, then modifying
r_<code class="literal">Cntnr</code> will not invalidate its iterators' order;
if the underlying data structure is a probing hash table, then this
is not the case. If the underlying data structure is based on a tree
or trie, then a reference to the container can efficiently be split;
otherwise, it cannot, in general. If the underlying data structure
is a red-black tree, then splitting a reference to the container is
exception-free; if it is an ordered-vector tree, exceptions can be
thrown.
</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.intro.issues.priority_queue"></a>Priority Que</h4></div></div></div><p>
Priority queues are useful when one needs to efficiently access a
minimum (or maximum) value as the set of values changes.
</p><p>
Most useful data structures for priority queues have a relatively
simple structure, as they are geared toward relatively simple
requirements. Unfortunately, these structures do not support access
to an arbitrary value, which turns out to be necessary in many
algorithms. Say, decreasing an arbitrary value in a graph
algorithm. Therefore, some extra mechanism is necessary and must be
invented for accessing arbitrary values. There are at least two
alternatives: embedding an associative container in a priority
queue, or allowing cross-referencing through iterators. The first
solution adds significant overhead; the second solution requires a
precise definition of iterator invalidation. Which is the next
point...
</p><p>
Priority queues, like hash-based containers, store values in an
order that is meaningless and undefined externally. For example, a
<code class="code">push</code> operation can internally reorganize the
values. Because of this characteristic, describing a priority
queues' iterator is difficult: on one hand, the values to which
iterators point can remain valid, but on the other, the logical
order of iterators can change unpredictably.
</p><p>
Roughly speaking, any element that is both inserted to a priority
queue (e.g. through <code class="code">push</code>) and removed
from it (e.g., through <code class="code">pop</code>), incurs a
logarithmic overhead (in the amortized sense). Different underlying
data structures place the actual cost differently: some are
optimized for amortized complexity, whereas others guarantee that
specific operations only have a constant cost. One underlying data
structure might be chosen if modifying a value is frequent
(Dijkstra's shortest-path algorithm), whereas a different one might
be chosen otherwise. Unfortunately, an array-based binary heap - an
underlying data structure that optimizes (in the amortized sense)
<code class="code">push</code> and <code class="code">pop</code> operations, differs from the
others in terms of its invalidation guarantees. Other design
decisions also impact the cost and placement of the overhead, at the
expense of more difference in the the kinds of operations that the
underlying data structure can support. These differences pose a
challenge when creating a uniform interface for priority queues.
</p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="pbds.intro.motivation"></a>Goals</h3></div></div></div><p>
Many fine associative-container libraries were already written,
most notably, the C++ standard's associative containers. Why
then write another library? This section shows some possible
advantages of this library, when considering the challenges in
the introduction. Many of these points stem from the fact that
the ISO C++ process introduced associative-containers in a
two-step process (first standardizing tree-based containers,
only then adding hash-based containers, which are fundamentally
different), did not standardize priority queues as containers,
and (in our opinion) overloads the iterator concept.
</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.intro.motivation.associative"></a>Associative</h4></div></div></div><p>
</p><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="motivation.associative.policy"></a>Policy Choices</h5></div></div></div><p>
Associative containers require a relatively large number of
policies to function efficiently in various settings. In some
cases this is needed for making their common operations more
efficient, and in other cases this allows them to support a
larger set of operations
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
Hash-based containers, for example, support look-up and
insertion methods (<code class="function">find</code> and
<code class="function">insert</code>). In order to locate elements
quickly, they are supplied a hash functor, which instruct
how to transform a key object into some size type; a hash
functor might transform <code class="constant">"hello"</code>
into <code class="constant">1123002298</code>. A hash table, though,
requires transforming each key object into some size-type
type in some specific domain; a hash table with a 128-long
table might transform <code class="constant">"hello"</code> into
position <code class="constant">63</code>. The policy by which the
hash value is transformed into a position within the table
can dramatically affect performance. Hash-based containers
also do not resize naturally (as opposed to tree-based
containers, for example). The appropriate resize policy is
unfortunately intertwined with the policy that transforms
hash value into a position within the table.
</p></li><li class="listitem"><p>
Tree-based containers, for example, also support look-up and
insertion methods, and are primarily useful when maintaining
order between elements is important. In some cases, though,
one can utilize their balancing algorithms for completely
different purposes.
</p><p>
Figure A shows a tree whose each node contains two entries:
a floating-point key, and some size-type
<span class="emphasis"><em>metadata</em></span> (in bold beneath it) that is
the number of nodes in the sub-tree. (The root has key 0.99,
and has 5 nodes (including itself) in its sub-tree.) A
container based on this data structure can obviously answer
efficiently whether 0.3 is in the container object, but it
can also answer what is the order of 0.3 among all those in
the container object: see <a class="xref" href="policy_data_structures.html#biblio.clrs2001" title="Introduction to Algorithms, 2nd edition">[biblio.clrs2001]</a>.
</p><p>
As another example, Figure B shows a tree whose each node
contains two entries: a half-open geometric line interval,
and a number <span class="emphasis"><em>metadata</em></span> (in bold beneath
it) that is the largest endpoint of all intervals in its
sub-tree. (The root describes the interval <code class="constant">[20,
36)</code>, and the largest endpoint in its sub-tree is
99.) A container based on this data structure can obviously
answer efficiently whether <code class="constant">[3, 41)</code> is
in the container object, but it can also answer efficiently
whether the container object has intervals that intersect
<code class="constant">[3, 41)</code>. These types of queries are
very useful in geometric algorithms and lease-management
algorithms.
</p><p>
It is important to note, however, that as the trees are
modified, their internal structure changes. To maintain
these invariants, one must supply some policy that is aware
of these changes. Without this, it would be better to use a
linked list (in itself very efficient for these purposes).
</p></li></ol></div><div class="figure"><a id="id-1.3.5.9.2.5.3.3.4"></a><p class="title"><strong>Figure 22.1. Node Invariants</strong></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="../images/pbds_node_invariants.png" align="middle" alt="Node Invariants" /></div></div></div><br class="figure-break" /></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="motivation.associative.underlying"></a>Underlying Data Structures</h5></div></div></div><p>
The standard C++ library contains associative containers based on
red-black trees and collision-chaining hash tables. These are
very useful, but they are not ideal for all types of
settings.
</p><p>
The figure below shows the different underlying data structures
currently supported in this library.
</p><div class="figure"><a id="id-1.3.5.9.2.5.3.4.4"></a><p class="title"><strong>Figure 22.2. Underlying Associative Data Structures</strong></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="../images/pbds_different_underlying_dss_1.png" align="middle" alt="Underlying Associative Data Structures" /></div></div></div><br class="figure-break" /><p>
A shows a collision-chaining hash-table, B shows a probing
hash-table, C shows a red-black tree, D shows a splay tree, E shows
a tree based on an ordered vector(implicit in the order of the
elements), F shows a PATRICIA trie, and G shows a list-based
container with update policies.
</p><p>
Each of these data structures has some performance benefits, in
terms of speed, size or both. For now, note that vector-based trees
and probing hash tables manipulate memory more efficiently than
red-black trees and collision-chaining hash tables, and that
list-based associative containers are very useful for constructing
"multimaps".
</p><p>
Now consider a function manipulating a generic associative
container,
</p><pre class="programlisting">
template&lt;class Cntnr&gt;
int
some_op_sequence(Cntnr &amp;r_cnt)
{
...
}
</pre><p>
Ideally, the underlying data structure
of <code class="classname">Cntnr</code> would not affect what can be
done with <code class="varname">r_cnt</code>. Unfortunately, this is not
the case.
</p><p>
For example, if <code class="classname">Cntnr</code>
is <code class="classname">std::map</code>, then the function can
use
</p><pre class="programlisting">
std::for_each(r_cnt.find(foo), r_cnt.find(bar), foobar)
</pre><p>
in order to apply <code class="classname">foobar</code> to all
elements between <code class="classname">foo</code> and
<code class="classname">bar</code>. If
<code class="classname">Cntnr</code> is a hash-based container,
then this call's results are undefined.
</p><p>
Also, if <code class="classname">Cntnr</code> is tree-based, the type
and object of the comparison functor can be
accessed. If <code class="classname">Cntnr</code> is hash based, these
queries are nonsensical.
</p><p>
There are various other differences based on the container's
underlying data structure. For one, they can be constructed by,
and queried for, different policies. Furthermore:
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
Containers based on C, D, E and F store elements in a
meaningful order; the others store elements in a meaningless
(and probably time-varying) order. By implication, only
containers based on C, D, E and F can
support <code class="function">erase</code> operations taking an
iterator and returning an iterator to the following element
without performance loss.
</p></li><li class="listitem"><p>
Containers based on C, D, E, and F can be split and joined
efficiently, while the others cannot. Containers based on C
and D, furthermore, can guarantee that this is exception-free;
containers based on E cannot guarantee this.
</p></li><li class="listitem"><p>
Containers based on all but E can guarantee that
erasing an element is exception free; containers based on E
cannot guarantee this. Containers based on all but B and E
can guarantee that modifying an object of their type does
not invalidate iterators or references to their elements,
while containers based on B and E cannot. Containers based
on C, D, and E can furthermore make a stronger guarantee,
namely that modifying an object of their type does not
affect the order of iterators.
</p></li></ol></div><p>
A unified tag and traits system (as used for the C++ standard
library iterators, for example) can ease generic manipulation of
associative containers based on different underlying data
structures.
</p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="motivation.associative.iterators"></a>Iterators</h5></div></div></div><p>
Iterators are centric to the design of the standard library
containers, because of the container/algorithm/iterator
decomposition that allows an algorithm to operate on a range
through iterators of some sequence. Iterators, then, are useful
because they allow going over a
specific <span class="emphasis"><em>sequence</em></span>. The standard library
also uses iterators for accessing a
specific <span class="emphasis"><em>element</em></span>: when an associative
container returns one through <code class="function">find</code>. The
standard library consistently uses the same types of iterators
for both purposes: going over a range, and accessing a specific
found element. Before the introduction of hash-based containers
to the standard library, this made sense (with the exception of
priority queues, which are discussed later).
</p><p>
Using the standard associative containers together with
non-order-preserving associative containers (and also because of
priority-queues container), there is a possible need for
different types of iterators for self-organizing containers:
the iterator concept seems overloaded to mean two different
things (in some cases). <em><span class="remark"> XXX
"ds_gen.html#find_range"&gt;Design::Associative
Containers::Data-Structure Genericity::Point-Type and Range-Type
Methods</span></em>.
</p><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="associative.iterators.using"></a>Using Point Iterators for Range Operations</h6></div></div></div><p>
Suppose <code class="classname">cntnr</code> is some associative
container, and say <code class="varname">c</code> is an object of
type <code class="classname">cntnr</code>. Then what will be the outcome
of
</p><pre class="programlisting">
std::for_each(c.find(1), c.find(5), foo);
</pre><p>
If <code class="classname">cntnr</code> is a tree-based container
object, then an in-order walk will
apply <code class="classname">foo</code> to the relevant elements,
as in the graphic below, label A. If <code class="varname">c</code> is
a hash-based container, then the order of elements between any
two elements is undefined (and probably time-varying); there is
no guarantee that the elements traversed will coincide with the
<span class="emphasis"><em>logical</em></span> elements between 1 and 5, as in
label B.
</p><div class="figure"><a id="id-1.3.5.9.2.5.3.5.4.5"></a><p class="title"><strong>Figure 22.3. Range Iteration in Different Data Structures</strong></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="../images/pbds_point_iterators_range_ops_1.png" align="middle" alt="Node Invariants" /></div></div></div><br class="figure-break" /><p>
In our opinion, this problem is not caused just because
red-black trees are order preserving while
collision-chaining hash tables are (generally) not - it
is more fundamental. Most of the standard's containers
order sequences in a well-defined manner that is
determined by their <span class="emphasis"><em>interface</em></span>:
calling <code class="function">insert</code> on a tree-based
container modifies its sequence in a predictable way, as
does calling <code class="function">push_back</code> on a list or
a vector. Conversely, collision-chaining hash tables,
probing hash tables, priority queues, and list-based
containers (which are very useful for "multimaps") are
self-organizing data structures; the effect of each
operation modifies their sequences in a manner that is
(practically) determined by their
<span class="emphasis"><em>implementation</em></span>.
</p><p>
Consequently, applying an algorithm to a sequence obtained from most
containers may or may not make sense, but applying it to a
sub-sequence of a self-organizing container does not.
</p></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="associative.iterators.cost"></a>Cost to Point Iterators to Enable Range Operations</h6></div></div></div><p>
Suppose <code class="varname">c</code> is some collision-chaining
hash-based container object, and one calls
</p><pre class="programlisting">c.find(3)</pre><p>
Then what composes the returned iterator?
</p><p>
In the graphic below, label A shows the simplest (and
most efficient) implementation of a collision-chaining
hash table. The little box marked
<code class="classname">point_iterator</code> shows an object
that contains a pointer to the element's node. Note that
this "iterator" has no way to move to the next element (
it cannot support
<code class="function">operator++</code>). Conversely, the little
box marked <code class="classname">iterator</code> stores both a
pointer to the element, as well as some other
information (the bucket number of the element). the
second iterator, then, is "heavier" than the first one-
it requires more time and space. If we were to use a
different container to cross-reference into this
hash-table using these iterators - it would take much
more space. As noted above, nothing much can be done by
incrementing these iterators, so why is this extra
information needed?
</p><p>
Alternatively, one might create a collision-chaining hash-table
where the lists might be linked, forming a monolithic total-element
list, as in the graphic below, label B. Here the iterators are as
light as can be, but the hash-table's operations are more
complicated.
</p><div class="figure"><a id="id-1.3.5.9.2.5.3.5.5.7"></a><p class="title"><strong>Figure 22.4. Point Iteration in Hash Data Structures</strong></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="../images/pbds_point_iterators_range_ops_2.png" align="middle" alt="Point Iteration in Hash Data Structures" /></div></div></div><br class="figure-break" /><p>
It should be noted that containers based on collision-chaining
hash-tables are not the only ones with this type of behavior;
many other self-organizing data structures display it as well.
</p></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="associative.iterators.invalidation"></a>Invalidation Guarantees</h6></div></div></div><p>Consider the following snippet:</p><pre class="programlisting">
it = c.find(3);
c.erase(5);
</pre><p>
Following the call to <code class="classname">erase</code>, what is the
validity of <code class="classname">it</code>: can it be de-referenced?
can it be incremented?
</p><p>
The answer depends on the underlying data structure of the
container. The graphic below shows three cases: A1 and A2 show
a red-black tree; B1 and B2 show a probing hash-table; C1 and C2
show a collision-chaining hash table.
</p><div class="figure"><a id="id-1.3.5.9.2.5.3.5.6.6"></a><p class="title"><strong>Figure 22.5. Effect of erase in different underlying data structures</strong></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="../images/pbds_invalidation_guarantee_erase.png" align="middle" alt="Effect of erase in different underlying data structures" /></div></div></div><br class="figure-break" /><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
Erasing 5 from A1 yields A2. Clearly, an iterator to 3 can
be de-referenced and incremented. The sequence of iterators
changed, but in a way that is well-defined by the interface.
</p></li><li class="listitem"><p>
Erasing 5 from B1 yields B2. Clearly, an iterator to 3 is
not valid at all - it cannot be de-referenced or
incremented; the order of iterators changed in a way that is
(practically) determined by the implementation and not by
the interface.
</p></li><li class="listitem"><p>
Erasing 5 from C1 yields C2. Here the situation is more
complicated. On the one hand, there is no problem in
de-referencing <code class="classname">it</code>. On the other hand,
the order of iterators changed in a way that is
(practically) determined by the implementation and not by
the interface.
</p></li></ol></div><p>
So in the standard library containers, it is not always possible
to express whether <code class="varname">it</code> is valid or not. This
is true also for <code class="function">insert</code>. Again, the
iterator concept seems overloaded.
</p></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="motivation.associative.functions"></a>Functional</h5></div></div></div><p>
</p><p>
The design of the functional overlay to the underlying data
structures differs slightly from some of the conventions used in
the C++ standard. A strict public interface of methods that
comprise only operations which depend on the class's internal
structure; other operations are best designed as external
functions. (See <a class="xref" href="policy_data_structures.html#biblio.meyers02both" title="Class Template, Member Template - or Both?">[biblio.meyers02both]</a>).With this
rubric, the standard associative containers lack some useful
methods, and provide other methods which would be better
removed.
</p><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="motivation.associative.functions.erase"></a><code class="function">erase</code></h6></div></div></div><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
Order-preserving standard associative containers provide the
method
</p><pre class="programlisting">
iterator
erase(iterator it)
</pre><p>
which takes an iterator, erases the corresponding
element, and returns an iterator to the following
element. Also standardd hash-based associative
containers provide this method. This seemingly
increasesgenericity between associative containers,
since it is possible to use
</p><pre class="programlisting">
typename C::iterator it = c.begin();
typename C::iterator e_it = c.end();
while(it != e_it)
it = pred(*it)? c.erase(it) : ++it;
</pre><p>
in order to erase from a container object <code class="varname">
c</code> all element which match a
predicate <code class="classname">pred</code>. However, in a
different sense this actually decreases genericity: an
integral implication of this method is that tree-based
associative containers' memory use is linear in the total
number of elements they store, while hash-based
containers' memory use is unbounded in the total number of
elements they store. Assume a hash-based container is
allowed to decrease its size when an element is
erased. Then the elements might be rehashed, which means
that there is no "next" element - it is simply
undefined. Consequently, it is possible to infer from the
fact that the standard library's hash-based containers
provide this method that they cannot downsize when
elements are erased. As a consequence, different code is
needed to manipulate different containers, assuming that
memory should be conserved. Therefor, this library's
non-order preserving associative containers omit this
method.
</p></li><li class="listitem"><p>
All associative containers include a conditional-erase method
</p><pre class="programlisting">
template&lt;
class Pred&gt;
size_type
erase_if
(Pred pred)
</pre><p>
which erases all elements matching a predicate. This is probably the
only way to ensure linear-time multiple-item erase which can
actually downsize a container.
</p></li><li class="listitem"><p>
The standard associative containers provide methods for
multiple-item erase of the form
</p><pre class="programlisting">
size_type
erase(It b, It e)
</pre><p>
erasing a range of elements given by a pair of
iterators. For tree-based or trie-based containers, this can
implemented more efficiently as a (small) sequence of split
and join operations. For other, unordered, containers, this
method isn't much better than an external loop. Moreover,
if <code class="varname">c</code> is a hash-based container,
then
</p><pre class="programlisting">
c.erase(c.find(2), c.find(5))
</pre><p>
is almost certain to do something
different than erasing all elements whose keys are between 2
and 5, and is likely to produce other undefined behavior.
</p></li></ol></div></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="motivation.associative.functions.split"></a>
<code class="function">split</code> and <code class="function">join</code>
</h6></div></div></div><p>
It is well-known that tree-based and trie-based container
objects can be efficiently split or joined (See
<a class="xref" href="policy_data_structures.html#biblio.clrs2001" title="Introduction to Algorithms, 2nd edition">[biblio.clrs2001]</a>). Externally splitting or
joining trees is super-linear, and, furthermore, can throw
exceptions. Split and join methods, consequently, seem good
choices for tree-based container methods, especially, since as
noted just before, they are efficient replacements for erasing
sub-sequences.
</p></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="motivation.associative.functions.insert"></a>
<code class="function">insert</code>
</h6></div></div></div><p>
The standard associative containers provide methods of the form
</p><pre class="programlisting">
template&lt;class It&gt;
size_type
insert(It b, It e);
</pre><p>
for inserting a range of elements given by a pair of
iterators. At best, this can be implemented as an external loop,
or, even more efficiently, as a join operation (for the case of
tree-based or trie-based containers). Moreover, these methods seem
similar to constructors taking a range given by a pair of
iterators; the constructors, however, are transactional, whereas
the insert methods are not; this is possibly confusing.
</p></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="motivation.associative.functions.compare"></a>
<code class="function">operator==</code> and <code class="function">operator&lt;=</code>
</h6></div></div></div><p>
Associative containers are parametrized by policies allowing to
test key equivalence: a hash-based container can do this through
its equivalence functor, and a tree-based container can do this
through its comparison functor. In addition, some standard
associative containers have global function operators, like
<code class="function">operator==</code> and <code class="function">operator&lt;=</code>,
that allow comparing entire associative containers.
</p><p>
In our opinion, these functions are better left out. To begin
with, they do not significantly improve over an external
loop. More importantly, however, they are possibly misleading -
<code class="function">operator==</code>, for example, usually checks for
equivalence, or interchangeability, but the associative
container cannot check for values' equivalence, only keys'
equivalence; also, are two containers considered equivalent if
they store the same values in different order? this is an
arbitrary decision.
</p></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.intro.motivation.priority_queue"></a>Priority Queues</h4></div></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="motivation.priority_queue.policy"></a>Policy Choices</h5></div></div></div><p>
Priority queues are containers that allow efficiently inserting
values and accessing the maximal value (in the sense of the
container's comparison functor). Their interface
supports <code class="function">push</code>
and <code class="function">pop</code>. The standard
container <code class="classname">std::priorityqueue</code> indeed support
these methods, but little else. For algorithmic and
software-engineering purposes, other methods are needed:
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
Many graph algorithms (see
<a class="xref" href="policy_data_structures.html#biblio.clrs2001" title="Introduction to Algorithms, 2nd edition">[biblio.clrs2001]</a>) require increasing a
value in a priority queue (again, in the sense of the
container's comparison functor), or joining two
priority-queue objects.
</p></li><li class="listitem"><p>The return type of <code class="classname">priority_queue</code>'s
<code class="function">push</code> method is a point-type iterator, which can
be used for modifying or erasing arbitrary values. For
example:</p><pre class="programlisting">
priority_queue&lt;int&gt; p;
priority_queue&lt;int&gt;::point_iterator it = p.push(3);
p.modify(it, 4);
</pre><p>These types of cross-referencing operations are necessary
for making priority queues useful for different applications,
especially graph applications.</p></li><li class="listitem"><p>
It is sometimes necessary to erase an arbitrary value in a
priority queue. For example, consider
the <code class="function">select</code> function for monitoring
file descriptors:
</p><pre class="programlisting">
int
select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *errorfds,
struct timeval *timeout);
</pre><p>
then, as the select documentation states:
</p><p>
<span class="quote">“<span class="quote">
The nfds argument specifies the range of file
descriptors to be tested. The select() function tests file
descriptors in the range of 0 to nfds-1.</span>”</span>
</p><p>
It stands to reason, therefore, that we might wish to
maintain a minimal value for <code class="varname">nfds</code>, and
priority queues immediately come to mind. Note, though, that
when a socket is closed, the minimal file description might
change; in the absence of an efficient means to erase an
arbitrary value from a priority queue, we might as well
avoid its use altogether.
</p><p>
The standard containers typically support iterators. It is
somewhat unusual
for <code class="classname">std::priority_queue</code> to omit them
(See <a class="xref" href="policy_data_structures.html#biblio.meyers01stl" title="Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library">[biblio.meyers01stl]</a>). One might
ask why do priority queues need to support iterators, since
they are self-organizing containers with a different purpose
than abstracting sequences. There are several reasons:
</p><div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem"><p>
Iterators (even in self-organizing containers) are
useful for many purposes: cross-referencing
containers, serialization, and debugging code that uses
these containers.
</p></li><li class="listitem"><p>
The standard library's hash-based containers support
iterators, even though they too are self-organizing
containers with a different purpose than abstracting
sequences.
</p></li><li class="listitem"><p>
In standard-library-like containers, it is natural to specify the
interface of operations for modifying a value or erasing
a value (discussed previously) in terms of a iterators.
It should be noted that the standard
containers also use iterators for accessing and
manipulating a specific value. In hash-based
containers, one checks the existence of a key by
comparing the iterator returned by <code class="function">find</code> to the
iterator returned by <code class="function">end</code>, and not by comparing a
pointer returned by <code class="function">find</code> to <span class="type">NULL</span>.
</p></li></ol></div></li></ol></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="motivation.priority_queue.underlying"></a>Underlying Data Structures</h5></div></div></div><p>
There are three main implementations of priority queues: the
first employs a binary heap, typically one which uses a
sequence; the second uses a tree (or forest of trees), which is
typically less structured than an associative container's tree;
the third simply uses an associative container. These are
shown in the figure below with labels A1 and A2, B, and C.
</p><div class="figure"><a id="id-1.3.5.9.2.5.4.3.3"></a><p class="title"><strong>Figure 22.6. Underlying Priority Queue Data Structures</strong></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="../images/pbds_different_underlying_dss_2.png" align="middle" alt="Underlying Priority Queue Data Structures" /></div></div></div><br class="figure-break" /><p>
No single implementation can completely replace any of the
others. Some have better <code class="function">push</code>
and <code class="function">pop</code> amortized performance, some have
better bounded (worst case) response time than others, some
optimize a single method at the expense of others, etc. In
general the "best" implementation is dictated by the specific
problem.
</p><p>
As with associative containers, the more implementations
co-exist, the more necessary a traits mechanism is for handling
generic containers safely and efficiently. This is especially
important for priority queues, since the invalidation guarantees
of one of the most useful data structures - binary heaps - is
markedly different than those of most of the others.
</p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="motivation.priority_queue.binary_heap"></a>Binary Heaps</h5></div></div></div><p>
Binary heaps are one of the most useful underlying
data structures for priority queues. They are very efficient in
terms of memory (since they don't require per-value structure
metadata), and have the best amortized <code class="function">push</code> and
<code class="function">pop</code> performance for primitive types like
<span class="type">int</span>.
</p><p>
The standard library's <code class="classname">priority_queue</code>
implements this data structure as an adapter over a sequence,
typically
<code class="classname">std::vector</code>
or <code class="classname">std::deque</code>, which correspond to labels
A1 and A2 respectively in the graphic above.
</p><p>
This is indeed an elegant example of the adapter concept and
the algorithm/container/iterator decomposition. (See <a class="xref" href="policy_data_structures.html#biblio.nelson96stlpq" title="Priority Queues and the STL">[biblio.nelson96stlpq]</a>). There are
several reasons why a binary-heap priority queue
may be better implemented as a container instead of a
sequence adapter:
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
<code class="classname">std::priority_queue</code> cannot erase values
from its adapted sequence (irrespective of the sequence
type). This means that the memory use of
an <code class="classname">std::priority_queue</code> object is always
proportional to the maximal number of values it ever contained,
and not to the number of values that it currently
contains. (See <code class="filename">performance/priority_queue_text_pop_mem_usage.cc</code>.)
This implementation of binary heaps acts very differently than
other underlying data structures (See also pairing heaps).
</p></li><li class="listitem"><p>
Some combinations of adapted sequences and value types
are very inefficient or just don't make sense. If one uses
<code class="classname">std::priority_queue&lt;std::vector&lt;std::string&gt;
&gt; &gt;</code>, for example, then not only will each
operation perform a logarithmic number of
<code class="classname">std::string</code> assignments, but, furthermore, any
operation (including <code class="function">pop</code>) can render the container
useless due to exceptions. Conversely, if one uses
<code class="classname">std::priority_queue&lt;std::deque&lt;int&gt; &gt;
&gt;</code>, then each operation uses incurs a logarithmic
number of indirect accesses (through pointers) unnecessarily.
It might be better to let the container make a conservative
deduction whether to use the structure in the graphic above, labels A1 or A2.
</p></li><li class="listitem"><p>
There does not seem to be a systematic way to determine
what exactly can be done with the priority queue.
</p><div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem"><p>
If <code class="classname">p</code> is a priority queue adapting an
<code class="classname">std::vector</code>, then it is possible to iterate over
all values by using <code class="function">&amp;p.top()</code> and
<code class="function">&amp;p.top() + p.size()</code>, but this will not work
if <code class="varname">p</code> is adapting an <code class="classname">std::deque</code>; in any
case, one cannot use <code class="classname">p.begin()</code> and
<code class="classname">p.end()</code>. If a different sequence is adapted, it
is even more difficult to determine what can be
done.
</p></li><li class="listitem"><p>
If <code class="varname">p</code> is a priority queue adapting an
<code class="classname">std::deque</code>, then the reference return by
</p><pre class="programlisting">
p.top()
</pre><p>
will remain valid until it is popped,
but if <code class="varname">p</code> adapts an <code class="classname">std::vector</code>, the
next <code class="function">push</code> will invalidate it. If a different
sequence is adapted, it is even more difficult to
determine what can be done.
</p></li></ol></div></li><li class="listitem"><p>
Sequence-based binary heaps can still implement
linear-time <code class="function">erase</code> and <code class="function">modify</code> operations.
This means that if one needs to erase a small
(say logarithmic) number of values, then one might still
choose this underlying data structure. Using
<code class="classname">std::priority_queue</code>, however, this will generally
change the order of growth of the entire sequence of
operations.
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