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POWhat is this pattern's name?
 

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  1. POWhat is this pattern's name?
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    <p>I have written a few template classes as part of a larger template class. One allows me to include or exclude member variables from a class. Another allows me to produce either the passed template class parameter or a placeholder that NOPs instead.</p> <p>These allow classes to use their own template parameters to include or exclude data members through a single very generic class definition, and the operations provide an interface that prevents type mismatch issues.</p> <p>This code sample shows my implementation in use as I assume it will be easier to recognise in action (the classes relating to the pattern have been called "ghost").</p> <pre><code>typedef int field1type, field2type, field3type; template &lt;bool field1exists, bool field2exists, bool field3exists&gt; struct GhostShowcase { typename Ghost&lt;int, field1exists&gt;::type intField; typename Ghost&lt;float, field2exists&gt;::type floatField; typename Ghost&lt;char, field3exists&gt;::type charField; }; GhostShowcase&lt;true,false,true&gt; foo; // has int and char fields // but an unusable float field GhostShowcase&lt;true,true,true&gt; bar; // has all fields usable </code></pre> <p>The samples below show how to use operators with the types derived from ghost classes. The testing function <code>printAll()</code> uses <code>ghostStaticOperator()</code> to call static functions in a class derived from <code>GhostStaticOperation</code>. This class is either the class that was passed as a template parameter to <code>GhostStaticOperation</code> or an automatically generated class which has a static function with the same signature but does nothing. The class chosen is based on the boolean template parameter <code>exists</code>. in <code>main()</code> two calls are made; the first passes the template parameter true and the second passes false. </p> <p>These samples show compilable source (in g++ 4.5.2, with -std=c++0x set). This file can be named anything.</p> <pre><code>#include "ghost.h" #include &lt;iostream&gt; #include &lt;functional&gt; using namespace Aryana; using namespace std; struct PrintingOperator : public std::unary_function&lt;int, void&gt; { static inline void operation(int toPrint) {cout &lt;&lt; toPrint;} }; struct IncrementOperator : public std::unary_function&lt;int&amp;, void&gt; { static inline void operation(int&amp; toIncrement) {++toIncrement;} }; template &lt;bool exists&gt; void printAll() { typedef GhostStaticOperator&lt;PrintingOperator, exists&gt; printClass; typedef GhostStaticOperator&lt;IncrementOperator, exists&gt; incrClass; typename Ghost&lt;int, exists&gt;::type ghostObject; cout &lt;&lt; "using ghost printer: "; ghostStaticOperation&lt;printClass&gt;(ghostObject); cout &lt;&lt; "\nusing ghost incrementor...\n"; ghostStaticOperation&lt;incrClass&gt;(ghostObject); cout &lt;&lt; "using ghost printer: "; ghostStaticOperation&lt;printClass&gt;(ghostObject); cout &lt;&lt; "\nfinished\n"; } int main(int, char**) { cout &lt;&lt; "ghost operation with exists=true:\n"; printAll&lt;true&gt;(); cout &lt;&lt; "ghost operation with exists=false:\n"; printAll&lt;false&gt;(); } </code></pre> <p>This sample should be named "ghost.h" and put in the same directory as the previous file.</p> <pre><code>typedef char ghost_null_argument[0]; typedef void ghost_null_return; template &lt;class S, bool exists&gt; class Ghost; template &lt;class S, bool exists&gt; class GhostOperator; template &lt;class S, bool exists&gt; class GhostStaticOperator; template &lt;class S&gt; class Ghost&lt;S, false&gt; { Ghost(); // private constructor to prevent instantiation public: typedef ghost_null_argument type; typedef S original_type; }; template &lt;class S&gt; class Ghost&lt;S, true&gt; { Ghost(); // private constructor to prevent instantiation public: typedef S type; typedef S original_type; }; template &lt;class S, bool exists&gt; class GhostOperator; template &lt;class S&gt; class GhostStaticOperator&lt;S, false&gt; { GhostStaticOperator(); // private constructor to prevent instantiation public: typedef ghost_null_return result_type; typedef ghost_null_argument argument_type; struct operator_type { inline static result_type operation(argument_type){}; }; }; template &lt;class S&gt; class GhostStaticOperator&lt;S, true&gt; { GhostStaticOperator(); // private constructor to prevent instantiation public: typedef S operator_type; typedef typename operator_type::result_type result_type; typedef typename operator_type::argument_type argument_type; }; // must define argument_type and result_type in the operator class // result_type will be ghost_null_argument if the class is ghosting template &lt;class S&gt; inline typename S::result_type ghostStaticOperation(typename S::argument_type argument) { return S::operator_type::operation(argument); } </code></pre>
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    1. USfuseinabowl
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    1. COThis is not at all a short or self-contained example. I have no idea what this code does.
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    2. CO@DeadMG I do not feel I can shorten the large source files, but I will try to add to the question. Please explain in what context you would like the example.
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      1. POWhat is this pattern's name?
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    3. COTemplate metaprogramming similar to traits class? Seriously though I'm with DeadMG this is information overload. Try to make a concise statement of the design and you may answer your own question by doing so.
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      1. POWhat is this pattern's name?
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