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< a name = "fusion.algorithm" > < / a > < a class = "link" href = "algorithm.html" title = "Algorithm" > Algorithm< / a >
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< dt > < span class = "section" > < a href = "algorithm/iteration.html" > Iteration< / a > < / span > < / dt >
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< dt > < span class = "section" > < a href = "algorithm/iteration/functions.html" > Functions< / a > < / span > < / dt >
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< a name = "fusion.algorithm.lazy_evaluation" > < / a > < h4 >
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< a name = "id677456" > < / 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 = "id677533" > < / 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 = "id677660" > < / 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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< td align = "right" > < div class = "copyright-footer" > Copyright <20> 2001-2007 Joel de Guzman, Dan Marsden, Tobias
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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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