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< div class = "section" title = "Algorithm" >
< div class = "titlepage" >< div >< div >< h2 class = "title" style = "clear: both" >
< a name = "fusion.algorithm" ></ a >< a class = "link" href = "algorithm.html" title = "Algorithm" > Algorithm</ a >
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< div class = "toc" >< dl >
< dt >< span class = "section" >< a href = "algorithm/iteration.html" > Iteration</ a ></ span ></ dt >
< dd >< dl >
< dt >< span class = "section" >< a href = "algorithm/iteration/functions.html" > Functions</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "algorithm/iteration/metafunctions.html" > Metafunctions</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "algorithm/query.html" > Query</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "algorithm/query/functions.html" > Functions</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "algorithm/transformation.html" > Transformation</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "algorithm/transformation/functions.html" > Functions</ a ></ span ></ dt >
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< a name = "fusion.algorithm.lazy_evaluation" ></ a >< h4 >
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< a name = "id752896" ></ a >
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< a class = "link" href = "algorithm.html#fusion.algorithm.lazy_evaluation" > Lazy Evaluation</ a >
</ h4 >
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< p >
Unlike < a href = "http://www.boost.org/libs/mpl/index.html" target = "_top" > MPL</ a > , Fusion
algorithms are lazy and non sequence-type preserving. What does that mean?
It means that when you operate on a sequence through a Fusion algorithm that
returns a sequence, the sequence returned may not be of the same class as the
original. This is by design. Runtime efficiency is given a high priority. Like
< a href = "http://www.boost.org/libs/mpl/index.html" target = "_top" > MPL</ a > , and unlike
< a href = "http://en.wikipedia.org/wiki/Standard_Template_Library" target = "_top" > STL</ a > ,
fusion algorithms are functional in nature such that algorithms are non mutating
(no side effects). However, due to the high cost of returning full sequences
such as vectors and lists, < span class = "emphasis" >< em > Views</ em ></ span > are returned from Fusion
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algorithms instead. For example, the < a class = "link" href = "algorithm/transformation/functions/transform.html" title = "transform" >< code class = "computeroutput" >< span class = "identifier" > transform</ span ></ code ></ a > algorithm does not actually
return a transformed version of the original sequence. < a class = "link" href = "algorithm/transformation/functions/transform.html" title = "transform" >< code class = "computeroutput" >< span class = "identifier" > transform</ span ></ code ></ a > returns a < a class = "link" href = "view/transform_view.html" title = "transform_view" >< code class = "computeroutput" >< span class = "identifier" > transform_view</ span ></ code ></ a > . This view holds a
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reference to the original sequence plus the transform function. Iteration over
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the < a class = "link" href = "view/transform_view.html" title = "transform_view" >< code class = "computeroutput" >< span class = "identifier" > transform_view</ span ></ code ></ a >
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will apply the transform function over the sequence elements on demand. This
< span class = "emphasis" >< em > lazy</ em ></ span > evaluation scheme allows us to chain as many algorithms
as we want without incurring a high runtime penalty.
</ p >
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< a name = "fusion.algorithm.sequence_extension" ></ a >< h4 >
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< a name = "id752973" ></ a >
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< a class = "link" href = "algorithm.html#fusion.algorithm.sequence_extension" > Sequence Extension</ a >
</ h4 >
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< p >
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The < span class = "emphasis" >< em > lazy</ em ></ span > evaluation scheme where < a class = "link" href = "algorithm.html" title = "Algorithm" > Algorithms</ a >
return < a class = "link" href = "view.html" title = "View" > Views</ a > also allows operations such
as < a class = "link" href = "algorithm/transformation/functions/push_back.html" title = "push_back" >< code class = "computeroutput" >< span class = "identifier" > push_back</ span ></ code ></ a > to be totally generic. In
Fusion, < a class = "link" href = "algorithm/transformation/functions/push_back.html" title = "push_back" >< code class = "computeroutput" >< span class = "identifier" > push_back</ span ></ code ></ a > is actually a generic algorithm
that works on all sequences. Given an input sequence < code class = "computeroutput" >< span class = "identifier" > s</ span ></ code >
and a value < code class = "computeroutput" >< span class = "identifier" > x</ span ></ code > , Fusion's < a class = "link" href = "algorithm/transformation/functions/push_back.html" title = "push_back" >< code class = "computeroutput" >< span class = "identifier" > push_back</ span ></ code ></ a > algorithm simply returns
a < a class = "link" href = "view/joint_view.html" title = "joint_view" >< code class = "computeroutput" >< span class = "identifier" > joint_view</ span ></ code ></ a > :
a view that holds a reference to the original sequence < code class = "computeroutput" >< span class = "identifier" > s</ span ></ code >
and the value < code class = "computeroutput" >< span class = "identifier" > x</ span ></ code > . Functions
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that were once sequence specific and need to be implemented N times over N
different sequences are now implemented only once. That is to say that Fusion
sequences are cheaply extensible. However, an important caveat is that the
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result of a sequence extending operation like < a class = "link" href = "algorithm/transformation/functions/push_back.html" title = "push_back" >< code class = "computeroutput" >< span class = "identifier" > push_back</ span ></ code ></ a > does not retain the properties
of the original sequence such as associativity of < a class = "link" href = "container/set.html" title = "set" >< code class = "computeroutput" >< span class = "identifier" > set</ span ></ code ></ a > (s). To regain the original sequence,
< a class = "link" href = "container/conversion/functions.html" title = "Functions" > Conversion</ a > functions
are provided. You may use one of the < a class = "link" href = "container/conversion/functions.html" title = "Functions" > Conversion</ a >
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functions to convert back to the original sequence type.
</ p >
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< a name = "fusion.algorithm.header" ></ a >< h4 >
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< a name = "id753100" ></ a >
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< a class = "link" href = "algorithm.html#fusion.algorithm.header" > Header</ a >
</ h4 >
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< pre class = "programlisting" >< span class = "preprocessor" > #include</ span > < span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > /</ span >< span class = "identifier" > fusion</ span >< span class = "special" > /</ span >< span class = "identifier" > algorithm</ span >< span class = "special" > .</ span >< span class = "identifier" > hpp</ span >< span class = "special" > > </ span >
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< span class = "preprocessor" > #include</ span > < span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > /</ span >< span class = "identifier" > fusion</ span >< span class = "special" > /</ span >< span class = "identifier" > include</ span >< span class = "special" > /</ span >< span class = "identifier" > algorithm</ span >< span class = "special" > .</ span >< span class = "identifier" > hpp</ span >< span class = "special" > > </ span >
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Schwinger< 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" target = "_top" > http://www.boost.org/LICENSE_1_0.txt</ a > )
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