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From: daniel_james_at_[hidden]
Date: 2007-11-18 15:18:05
Author: danieljames
Date: 2007-11-18 15:18:04 EST (Sun, 18 Nov 2007)
New Revision: 41210
URL: http://svn.boost.org/trac/boost/changeset/41210
Log:
Move the 'implementation variations' page to the new site. Fixes #1355.
Removed:
trunk/more/imp_vars.htm
Deleted: trunk/more/imp_vars.htm
Boost-Commit list run by bdawes at acm.org, david.abrahams at rcn.com, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk
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-<title>Boost Implementation Variations</title>
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-<table summary="Navigational header"
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- <tr>
- <td bgcolor="#FFFFFF"><img src="../boost.png" alt="boost.png (6897 bytes)" width="277" height="86"></td>
- <td>Home</td>
- <td>Libraries</td>
- <td>People</td>
- <td>FAQ</td>
- <td>More</td>
- </tr>
-</table>
-<h1>Boost Implementation Variations</h1>
-<h2>Separation of interface and implementation</h2>
-<p>The interface specifications for boost.org library components (as well as for
-quality software in general) are conceptually separate from implementations of
-those interfaces. This may not be obvious, particularly when a component is
-implemented entirely within a header, but this separation of interface and
-implementation is always assumed. From the perspective of those concerned with
-software design, portability, and standardization, the interface is what is
-important, while the implementation is just a detail.</p>
-<p>Dietmar Kühl, one of the original boost.org contributors, comments "The
-main contribution is the interface, which is augmented with an implementation,
-proving that it is possible to implement the corresponding class and providing a
-free implementation."</p>
-
-<h2>Implementation variations</h2>
-
-<p>There may be a need for multiple implementations of an interface, to
-accommodate either platform dependencies or performance tradeoffs. Examples of
-platform dependencies include compiler shortcomings, file systems, thread
-mechanisms, and graphical user interfaces. The classic example of a performance
-tradeoff is a fast implementation which uses a lot of memory versus a slower
-implementation which uses less memory.</p>
-<p>Boost libraries generally use a <a href="../libs/config/config.htm">configuration
-header</a>, boost/config.hpp, to capture compiler and platform
-dependencies. Although the use of boost/config.hpp is not required, it is
-the preferred approach for simple configuration problems. </p>
-<h2>Boost policy</h2>
-<p>The Boost policy is to avoid platform dependent variations in interface
-specifications, but supply implementations which are usable over a wide range of
-platforms and applications. That means boost libraries will use the
-techniques below described as appropriate for dealing with platform
-dependencies.</p>
-<p>The Boost policy toward implementation variations designed to enhance
-performance is to avoid them unless the benefits greatly exceed the full
-costs. The term "full costs" is intended to include both
-tangible costs like extra maintenance, and intangible cost like increased
-difficulty in user understanding.</p>
-
-<h2>Techniques for providing implementation variations</h2>
-
-<p>Several techniques may be used to provide implementation variations. Each is
-appropriate in some situations, and not appropriate in other situations.</p>
-<h3>Single general purpose implementation</h3>
-<p>The first technique is to simply not provide implementation variation at
-all. Instead, provide a single general purpose implementation, and forgo
-the increased complexity implied by all other techniques.</p>
-<p><b>Appropriate:</b> When it is possible to write a single portable
-implementation which has reasonable performance across a wide range of
-platforms. Particularly appropriate when alternative implementations differ only
-in esoteric ways.</p>
-<p><b>Not appropriate:</b> When implementation requires platform specific
-features, or when there are multiple implementation possible with widely
-differing performance characteristics.</p>
-<p>Beman Dawes comments "In design discussions some implementation is often
-alleged to be much faster than another, yet a timing test discovers no
-significant difference. The lesson is that while algorithmic differences may
-affect speed dramatically, coding differences such as changing a class from
-virtual to non-virtual members or removing a level of indirection are unlikely
-to make any measurable difference unless deep in an inner loop. And even in an
-inner loop, modern CPUs often execute such competing code sequences in the
-same number of clock cycles! A single general purpose implementation is
-often just fine."</p>
-<p>Or as Donald Knuth said, "Premature optimization is the root of all
-evil." (Computing Surveys, vol 6, #4, p 268).</p>
-<h3>Macros</h3>
-<p>While the evils of macros are well known, there remain a few cases where
-macros are the preferred solution:</p>
-<blockquote>
- <ul>
- <li> Preventing multiple inclusion of headers via #include guards.</li>
- <li> Passing minor configuration information from a configuration
- header to other files.</li>
- </ul>
-</blockquote>
-<p><b>Appropriate:</b> For small compile-time variations which would
-otherwise be costly or confusing to install, use, or maintain. More appropriate
-to communicate within and between library components than to communicate with
-library users.</p>
-<p><b>Not appropriate: </b> If other techniques will do.</p>
-<p>To minimize the negative aspects of macros:</p>
-<blockquote>
- <ul>
- <li>Only use macros when they are clearly superior to other
- techniques. They should be viewed as a last resort.</li>
- <li>Names should be all uppercase, and begin with the namespace name. This
- will minimize the chance of name collisions. For example, the #include
- guard for a boost header called foobar.h might be named BOOST_FOOBAR_H.</li>
- </ul>
-</blockquote>
-<h3>Separate files</h3>
-<p>A library component can have multiple variations, each contained in its own
-separate file or files. The files for the most appropriate variation are
-copied to the appropriate include or implementation directories at installation
-time.</p>
-<p>The way to provide this approach in boost libraries is to include specialized
-implementations as separate files in separate sub-directories in the .ZIP
-distribution file. For example, the structure within the .ZIP distribution file
-for a library named foobar which has both default and specialized variations
-might look something like:</p>
-<blockquote>
- <pre>foobar.h // The default header file
-foobar.cpp // The default implementation file
-readme.txt // Readme explains when to use which files
-self_contained/foobar.h // A variation with everything in the header
-linux/foobar.cpp // Implementation file to replace the default
-win32/foobar.h // Header file to replace the default
-win32/foobar.cpp // Implementation file to replace the default</pre>
-</blockquote>
-<p><b>Appropriate:</b> When different platforms require different
-implementations, or when there are major performance differences between
-possible implementations. </p>
-<p><b>Not appropriate:</b> When it makes sense to use more that one of the
-variations in the same installation.</p>
-<h3>Separate components</h3>
-<p>Rather than have several implementation variations of a single component,
-supply several separate components. For example, the Boost library currently
-supplies <code>scoped_ptr</code> and <code>shared_ptr</code> classes rather than
-a single <code>smart_ptr</code> class parameterized to distinguish between the
-two cases. There are several ways to make the component choice:</p>
-<blockquote>
- <ul>
- <li>Hardwired by the programmer during coding.</li>
- <li>Chosen by programmer written runtime logic (trading off some extra
- space, time, and program complexity for the ability to select the
- implementation at run-time.)</li>
- </ul>
-</blockquote>
-<p><b>Appropriate: </b>When the interfaces for the variations diverge, and when
-it is reasonably to use more than one of the variations. When run-time selection
-of implementation is called for.</p>
-<p><b>Not appropriate:</b> When the variations are data type, traits, or
-specialization variations which can be better handled by making the component a
-template. Also not appropriate when choice of variation is best done by some
-setup or installation mechanism outside of the program itself. Thus
-usually not appropriate to cope with platform differences.</p>
-<p><b>Note:</b> There is a related technique where the interface is specified as
-an abstract (pure virtual) base class (or an interface definition language), and
-the implementation choice is passed off to some third-party, such as a
-dynamic-link library or object-request broker. While that is a powerful
-technique, it is way beyond the scope of this discussion.</p>
-<h3>Template-based approaches</h3>
-<p>Turning a class or function into a template is often an elegant way to cope
-with variations. Template-based approaches provide optimal space and time
-efficiency in return for constraining the implementation selection to compile
-time. </p>
-<p>Important template techniques include:</p>
-<blockquote>
- <ul>
- <li>Data type parameterization. This allows a single component to
- operate on a variety of data types, and is why templates were originally
- invented.</li>
- <li>Traits parameterization. If parameterization is complex, bundling
- up aspects into a single traits helper class can allow great variation
- while hiding messy details. The C++ Standard Library provides
- several examples of this idiom, such as <code>iterator_traits<></code>
- (24.3.1 lib.iterator.traits) and <tt>char_traits<></tt> (21.2
- lib.char.traits).</li>
- <li>Specialization. A template parameter can be used purely for the
- purpose of selecting a specialization. For example:</li>
- </ul>
- <blockquote>
- <blockquote>
- <pre>SomeClass<fast> my_fast_object; // fast and small are empty classes
-SomeClass<small> my_small_object; // used just to select specialization</pre>
- </blockquote>
- </blockquote>
-</blockquote>
-<p><b>Appropriate: </b>When the need for variation is due to data type or
-traits, or is performance related like selecting among several algorithms, and
-when a program might reasonably use more than one of the variations.</p>
-<p><b>Not appropriate:</b> When the interfaces for variations are
-different, or when choice of variation is best done by some mechanism outside of
-the program itself. Thus usually not appropriate to cope with platform
-differences.</p>
-<hr>
-<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 October, 2003<!--webbot bot="Timestamp" endspan i-checksum="38549" --></p>
-
-<p>© Copyright Beman Dawes 2001</p>
-
-<p>Distributed under the Boost Software License, Version 1.0. (See
- accompanying file LICENSE_1_0.txt or copy
- at <a href=
- "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt>)
-</p>
-
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