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