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<h1>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>
<b>
<h2>Implementation variations</h2>
</b>
<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/index.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>
<b>
<h2>Techniques for providing implementation variations</h2>
</b>
<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 CPU’s 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 -->23 June, 2000<!--webbot bot="Timestamp" endspan i-checksum="19836" --></p>
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