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POtemplate rebinding vs. late template binding or how to call piggy backing a template in a type
 

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  1. POtemplate rebinding vs. late template binding or how to call piggy backing a template in a type
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    <p>Often conceptually a <em>template</em> must be passed as a <em>type argument</em> and the compiler complains because this is not legal c++ - at least until and including c++11 (<strong>Update II:</strong> except see last example of non specialized template definition)</p> <p>These concepts are often used and should have a name. See the code examples below if it is not clear what I mean.</p> <p>My first thought was that this may also be called <em>passing an incomplete type</em> but this is not correct. Another user also states that he has no word for it and freely names it <a href="https://stackoverflow.com/q/15029611/2712726">late binding of template arguments</a>. I think his terminology visualizes the concept well.</p> <p>My question is how do you call the idiom behind this or the involved templates properly?</p> <p><strong>UPDATE</strong> Kerrek suggests naming the idiom <strong>template rebinding</strong>. This name renders only a few google results. However I think it is a very good name too because the standard allocators call their related wrapped internal classes <strong>rebind</strong>.</p> <p>In the following example you could configure if your database uses <code>map</code> or <code>hashmap</code> internally:</p> <pre><code>#include &lt;map&gt; struct map_wrapper { template&lt;typename K, typename V&gt; using type = std::map&lt;K,V&gt;; }; template&lt;typename storage&gt; struct my_database { typename storage::template type&lt;int, int&gt; x; typename storage::template type&lt;int, double&gt; y; }; main () { my_database&lt;map_wrapper&gt; db; db.x[0] = 1; db.y[0] = 2.0; } </code></pre> <p>Or similarly</p> <pre><code>#include &lt;map&gt; #include &lt;boost/mpl/apply.hpp&gt; template &lt;typename storage&gt; struct my_database { typename boost::mpl::apply&lt;storage, int&gt;::type x; typename boost::mpl::apply&lt;storage, double&gt;::type y; }; int main () { my_database&lt; std::map&lt;int, boost::mpl::_1&gt; &gt; db; db.x[0] = 1; db.y[0] = 2.0; } </code></pre> <p><strong>Update II:</strong> To my embarrassment I did not know the following solution which is just passing a template as argument to a template instantiation. In this special case it is legal to pass a non-type.</p> <pre><code>#include &lt;map&gt; #include &lt;unordered_map&gt; template&lt;template&lt;typename...&gt; class Storage&gt; struct my_database { Storage &lt;long,char&gt; x; Storage &lt;long,double&gt; y; }; int main () { my_database&lt; std::map &gt; db1; db1.x[0] = '1'; db1.y[0] = 2.2; my_database&lt; std::unordered_map &gt; db2; db2.x[0] = '1'; db2.y[0] = 2.2; } </code></pre> <p>Examples of other ways to <strong>rebind</strong> or <strong>late bind</strong> template arguments are of course welcomed.</p>
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    1. USPatrick Fromberg
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    1. CO@Kerrek, I am not certain about the netiquette but do you welcome if I improve my question according to your suggestions and thus invalidate your comments? In any case I invite anyone to edit and improve my question.
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      1. USPatrick Fromberg
    2. COThat's perfectly fine. Comments are ephemeral, and you can flag them as "obsolete" if the original comment author doesn't delete them. If you make a comment like "@user: I've updated my post", then more often than not the commenter will come back and delete their obsolete comments...
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      1. POtemplate rebinding vs. late template binding or how to call piggy backing a template in a type
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    3. COI'd call your wrapper maneuver "template rebinding". I guess that's a moderately conventional term, but don't bank on it.
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      1. USKerrek SB
 

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