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<h1>Generic Programming Techniques</h1>
<p>This is an incomplete survey of some of the generic programming
techniques used in the <a href="../index.htm">boost</a> libraries.</p>
<h2>Table of Contents</h2>
<ul>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#concept">The Anatomy of a Concept</a></li>
<li><a href="#traits">Traits</a></li>
<li><a href="#tag_dispatching">Tag Dispatching</a></li>
<li><a href="#adaptors">Adaptors</a></li>
<li><a href="#type_generator">Type Generators</a></li>
<li><a href="#object_generator">Object Generators</a></li>
<li><a href="#policy">Policy Classes</a></li>
</ul>
<h2><a name="introduction">Introduction</a></h2>
<p>Generic programming is about generalizing software components so that
they can be easily reused in a wide variety of situations. In C++, class
and function templates are particularly effective mechanisms for generic
programming because they make the generalization possible without
sacrificing efficiency.</p>
<p>As a simple example of generic programming, we will look at how one
might generalize the <tt>memcpy()</tt> function of the C standard
library. An implementation of <tt>memcpy()</tt> might look like the
following:<br>
<br>
</p>
<blockquote>
<pre>
void* memcpy(void* region1, const void* region2, size_t n)
{
const char* first = (const char*)region2;
const char* last = ((const char*)region2) + n;
char* result = (char*)region1;
while (first != last)
*result++ = *first++;
return result;
}
</pre>
</blockquote>
The <tt>memcpy()</tt> function is already generalized to some extent by
the use of <tt>void*</tt> so that the function can be used to copy arrays
of different kinds of data. But what if the data we would like to copy is
not in an array? Perhaps it is in a linked list. Can we generalize the
notion of copy to any sequence of elements? Looking at the body of
<tt>memcpy()</tt>, the function's <b><i>minimal requirements</i></b> are
that it needs to to <i>traverse</i> through the sequence using some sort
of pointer, <i>access</i> elements pointed to, <i>write</i> the elements
to the destination, and <i>compare</i> pointers to know when to stop. The
C++ standard library groups requirements such as these into
<b><i>concepts</i></b>, in this case the <a href=
"http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>
concept (for <tt>region2</tt>) and the <a href=
"http://www.sgi.com/tech/stl/OutputIterator.html">Output Iterator</a>
concept (for <tt>region1</tt>).
<p>If we rewrite the <tt>memcpy()</tt> as a function template, and use
the <a href="http://www.sgi.com/tech/stl/InputIterator.html">Input
Iterator</a> and <a href=
"http://www.sgi.com/tech/stl/OutputIterator.html">Output Iterator</a>
concepts to describe the requirements on the template parameters, we can
implement a highly reusable <tt>copy()</tt> function in the following
way:<br>
<br>
</p>
<blockquote>
<pre>
template <typename InputIterator, typename OutputIterator>
OutputIterator
copy(InputIterator first, InputIterator last, OutputIterator result)
{
while (first != last)
*result++ = *first++;
return result;
}
</pre>
</blockquote>
<p>Using the generic <tt>copy()</tt> function, we can now copy elements
from any kind of sequence, including a linked list that exports iterators
such as <tt>std::<a href=
"http://www.sgi.com/tech/stl/List.html">list</a></tt>.<br>
<br>
</p>
<blockquote>
<pre>
#include <list>
#include <vector>
#include <iostream>
int main()
{
const int N = 3;
std::vector<int> region1(N);
std::list<int> region2;
region2.push_back(1);
region2.push_back(0);
region2.push_back(3);
std::copy(region2.begin(), region2.end(), region1.begin());
for (int i = 0; i < N; ++i)
std::cout << region1[i] << " ";
std::cout << std::endl;
}
</pre>
</blockquote>
<h2><a name="concept">Anatomy of a Concept</a></h2>
A <b><i>concept</i></b> is a set requirements, where the requirements
consist of valid expressions, associated types, invariants, and
complexity guarantees. A type that satisfies the set of requirements is
said to <b><i>model</i></b> the concept. A concept can extend the
requirements of another concept, which is called
<b><i>refinement</i></b>.
<ul>
<li><a name="valid_expression"><b>Valid Expressions</b></a> are C++
expressions which must compile successfully for the objects involved in
the expression to be considered <i>models</i> of the concept.</li>
<li><a name="associated_type"><b>Associated Types</b></a> are types
that are related to the modeling type in that they participate in one
or more of the valid expressions. Typically associated types can be
accessed either through typedefs nested within a class definition for
the modeling type, or they are accessed through a <a href=
"#traits">traits class</a>.</li>
<li><b>Invariants</b> are run-time characteristics of the objects that
must always be true, that is, the functions involving the objects must
preserve these characteristics. The invariants often take the form of
pre-conditions and post-conditions.</li>
<li><b>Complexity Guarantees</b> are maximum limits on how long the
execution of one of the valid expressions will take, or how much of
various resources its computation will use.</li>
</ul>
<p>The concepts used in the C++ Standard Library are documented at the <a
href="http://www.sgi.com/tech/stl/table_of_contents.html">SGI STL
site</a>.</p>
<h2><a name="traits">Traits</a></h2>
<p>A traits class provides a way of associating information with a
compile-time entity (a type, integral constant, or address). For example,
the class template <tt><a href=
"http://www.sgi.com/tech/stl/iterator_traits.html">std::iterator_traits<T></a></tt>
looks something like this:</p>
<blockquote>
<pre>
template <class Iterator>
struct iterator_traits {
typedef ... iterator_category;
typedef ... value_type;
typedef ... difference_type;
typedef ... pointer;
typedef ... reference;
};
</pre>
</blockquote>
The traits' <tt>value_type</tt> gives generic code the type which the
iterator is "pointing at", while the <tt>iterator_category</tt> can be
used to select more efficient algorithms depending on the iterator's
capabilities.
<p>A key feature of traits templates is that they're
<i>non-intrusive</i>: they allow us to associate information with
arbitrary types, including built-in types and types defined in
third-party libraries, Normally, traits are specified for a particular
type by (partially) specializing the traits template.</p>
<p>For an in-depth description of <tt>std::iterator_traits</tt>, see <a
href="http://www.sgi.com/tech/stl/iterator_traits.html">this page</a>
provided by SGI. Another very different expression of the traits idiom in
the standard is <tt>std::numeric_limits<T></tt> which provides
constants describing the range and capabilities of numeric types.</p>
<h2><a name="tag_dispatching">Tag Dispatching</a></h2>
<p>Tag dispatching is a way of using function overloading to
dispatch based on properties of a type, and is often used hand in
hand with traits classes. A good example of this synergy is the
implementation of the <a href=
"http://www.sgi.com/tech/stl/advance.html"><tt>std::advance()</tt></a>
function in the C++ Standard Library, which increments an iterator
<tt>n</tt> times. Depending on the kind of iterator, there are different
optimizations that can be applied in the implementation. If the iterator
is <a href="http://www.sgi.com/tech/stl/RandomAccessIterator.html">random
access</a> (can jump forward and backward arbitrary distances), then the
<tt>advance()</tt> function can simply be implemented with <tt>i +=
n</tt>, and is very efficient: constant time. Other iterators must be
<tt>advance</tt>d in steps, making the operation linear in n. If the
iterator is <a href=
"http://www.sgi.com/tech/stl/BidirectionalIterator.html">bidirectional</a>,
then it makes sense for <tt>n</tt> to be negative, so we must decide
whether to increment or decrement the iterator.</p>
<p>The relation between tag dispatching and traits classes is that the
property used for dispatching (in this case the
<tt>iterator_category</tt>) is often accessed through a traits class. The
main <tt>advance()</tt> function uses the <a href=
"http://www.sgi.com/tech/stl/iterator_traits.html"><tt>iterator_traits</tt></a>
class to get the <tt>iterator_category</tt>. It then makes a call the the
overloaded <tt>advance_dispatch()</tt> function. The appropriate
<tt>advance_dispatch()</tt> is selected by the compiler based on whatever
type the <tt>iterator_category</tt> resolves to, either <a href=
"http://www.sgi.com/tech/stl/input_iterator_tag.html"><tt>input_iterator_tag</tt></a>,
<a href=
"http://www.sgi.com/tech/stl/bidirectional_iterator_tag.html"><tt>bidirectional_iterator_tag</tt></a>,
or <a href=
"http://www.sgi.com/tech/stl/random_access_iterator_tag.html"><tt>random_access_iterator_tag</tt></a>.
A <b><i>tag</i></b> is simply a class whose only purpose is to convey
some property for use in tag dispatching and similar techniques. Refer to
<a href="http://www.sgi.com/tech/stl/iterator_tags.html">this page</a>
for a more detailed description of iterator tags.</p>
<blockquote>
<pre>
namespace std {
struct input_iterator_tag { };
struct bidirectional_iterator_tag { };
struct random_access_iterator_tag { };
namespace detail {
template <class InputIterator, class Distance>
void advance_dispatch(InputIterator& i, Distance n, <b>input_iterator_tag</b>) {
while (n--) ++i;
}
template <class BidirectionalIterator, class Distance>
void advance_dispatch(BidirectionalIterator& i, Distance n,
<b>bidirectional_iterator_tag</b>) {
if (n >= 0)
while (n--) ++i;
else
while (n++) --i;
}
template <class RandomAccessIterator, class Distance>
void advance_dispatch(RandomAccessIterator& i, Distance n,
<b>random_access_iterator_tag</b>) {
i += n;
}
}
template <class InputIterator, class Distance>
void advance(InputIterator& i, Distance n) {
typename <b>iterator_traits<InputIterator>::iterator_category</b> category;
detail::advance_dispatch(i, n, <b>category</b>);
}
}
</pre>
</blockquote>
<h2><a name="adaptors">Adaptors</a></h2>
<p>An <i>adaptor</i> is a class template which builds on another type or
types to provide a new interface or behavioral variant. Examples of
standard adaptors are <a href=
"http://www.sgi.com/tech/stl/ReverseIterator.html">std::reverse_iterator</a>,
which adapts an iterator type by reversing its motion upon
increment/decrement, and <a href=
"http://www.sgi.com/tech/stl/stack.html">std::stack</a>, which adapts a
container to provide a simple stack interface.</p>
<p>A more comprehensive review of the adaptors in the standard can be
found <a href=
"http://www.cs.rpi.edu/~wiseb/xrds/ovp2-3b.html#SECTION00015000000000000000">
here</a>.</p>
<h2><a name="type_generator">Type Generators</a></h2>
<p><b>Note:</b> The <i>type generator</i> concept has largely been
superseded by the more refined notion of a <a href=
"../libs/mpl/doc/ref/Metafunction.html"><i>metafunction</i></a>. See
<i><a href="http://www.boost-consulting.com/mplbook">C++ Template
Metaprogramming</a></i> for an in-depth discussion of metafunctions.</p>
<p>A <i>type generator</i> is a template whose only purpose is to
synthesize a new type or types based on its template argument(s)<a href=
"#1">[1]</a>. The generated type is usually expressed as a nested typedef
named, appropriately <tt>type</tt>. A type generator is usually used to
consolidate a complicated type expression into a simple one. This example
uses an old version of <tt><a href=
"../libs/iterator/doc/iterator_adaptor.html">iterator_adaptor</a></tt>
whose design didn't allow derived iterator types. As a result, every
adapted iterator had to be a specialization of <tt>iterator_adaptor</tt>
itself and generators were a convenient way to produce those types.</p>
<blockquote>
<pre>
template <class Predicate, class Iterator,
class Value = <i>complicated default</i>,
class Reference = <i>complicated default</i>,
class Pointer = <i>complicated default</i>,
class Category = <i>complicated default</i>,
class Distance = <i>complicated default</i>
>
struct filter_iterator_generator {
typedef iterator_adaptor<
Iterator,filter_iterator_policies<Predicate,Iterator>,
Value,Reference,Pointer,Category,Distance> <b>type</b>;
};
</pre>
</blockquote>
<p>Now, that's complicated, but producing an adapted filter iterator
using the generator is much easier. You can usually just write:</p>
<blockquote>
<pre>
boost::filter_iterator_generator<my_predicate,my_base_iterator>::type
</pre>
</blockquote>
<h2><a name="object_generator">Object Generators</a></h2>
<p>An <i>object generator</i> is a function template whose only purpose
is to construct a new object out of its arguments. Think of it as a kind
of generic constructor. An object generator may be more useful than a
plain constructor when the exact type to be generated is difficult or
impossible to express and the result of the generator can be passed
directly to a function rather than stored in a variable. Most Boost
object generators are named with the prefix "<tt>make_</tt>", after
<tt>std::<a href=
"http://www.sgi.com/tech/stl/pair.html">make_pair</a>(const T&, const U&)</tt>.</p>
<p>For example, given:</p>
<blockquote>
<pre>
struct widget {
void tweak(int);
};
std::vector<widget *> widget_ptrs;
</pre>
</blockquote>
By chaining two standard object generators, <tt>std::<a href=
"http://www.dinkumware.com/htm_cpl/functio2.html#bind2nd">bind2nd</a>()</tt>
and <tt>std::<a href=
"http://www.dinkumware.com/htm_cpl/functio2.html#mem_fun">mem_fun</a>()</tt>,
we can easily tweak all widgets:
<blockquote>
<pre>
void tweak_all_widgets1(int arg)
{
for_each(widget_ptrs.begin(), widget_ptrs.end(),
<b>bind2nd</b>(std::<b>mem_fun</b>(&widget::tweak), arg));
}
</pre>
</blockquote>
<p>Without using object generators the example above would look like
this:</p>
<blockquote>
<pre>
void tweak_all_widgets2(int arg)
{
for_each(struct_ptrs.begin(), struct_ptrs.end(),
<b>std::binder2nd<std::mem_fun1_t<void, widget, int> ></b>(
std::<b>mem_fun1_t<void, widget, int></b>(&widget::tweak), arg));
}
</pre>
</blockquote>
<p>As expressions get more complicated the need to reduce the verbosity
of type specification gets more compelling.</p>
<h2><a name="policy">Policy Classes</a></h2>
<p>A policy class is a template parameter used to transmit behavior. An
example from the standard library is <tt>std::<a href=
"http://www.dinkumware.com/htm_cpl/memory.html#allocator">allocator</a></tt>,
which supplies memory management behaviors to standard <a href=
"http://www.sgi.com/tech/stl/Container.html">containers</a>.</p>
<p>Policy classes have been explored in detail by <a href=
"http://www.moderncppdesign.com/">Andrei Alexandrescu</a> in <a href=
"http://www.informit.com/articles/article.asp?p=167842">this chapter</a>
of his book, <i>Modern C++ Design</i>. He writes:</p>
<blockquote>
<p>In brief, policy-based class design fosters assembling a class with
complex behavior out of many little classes (called policies), each of
which takes care of only one behavioral or structural aspect. As the
name suggests, a policy establishes an interface pertaining to a
specific issue. You can implement policies in various ways as long as
you respect the policy interface.</p>
<p>Because you can mix and match policies, you can achieve a
combinatorial set of behaviors by using a small core of elementary
components.</p>
</blockquote>
<p>Andrei's description of policy classes suggests that their power is
derived from granularity and orthogonality. Less-granular policy
interfaces have been shown to work well in practice, though. <a href=
"http://cvs.sourceforge.net/viewcvs.py/*checkout*/boost/boost/libs/utility/Attic/iterator_adaptors.pdf">
This paper</a> describes an old version of <tt><a href=
"../libs/iterator/doc/iterator_adaptor.html">iterator_adaptor</a></tt>
that used non-orthogonal policies. There is also precedent in the
standard library: <tt><a href=
"http://www.dinkumware.com/htm_cpl/string2.html#char_traits">std::char_traits</a></tt>,
despite its name, acts as a policies class that determines the behaviors
of <a href=
"http://www.dinkumware.com/htm_cpl/string2.html#basic_string">std::basic_string</a>.</p>
<h2>Notes</h2>
<a name="1">[1]</a> Type generators are sometimes viewed as a workaround
for the lack of ``templated typedefs'' in C++.
<hr>
<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan -->18
August 2004<!--webbot bot="Timestamp" endspan i-checksum="14885" -->
</p>
<p>© Copyright David Abrahams 2001. Permission to copy, use, modify,
sell and distribute this document is granted provided this copyright
notice appears in all copies. This document is provided "as is" without
express or implied warranty, and with no claim as to its suitability for
any purpose.
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