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POFill container with template parameters
 

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  1. POFill container with template parameters
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    <p>I want to fill the template parameters passed to a variadic template into an array with fixed length. For that purpose I wrote the following helper function templates</p> <pre><code>template&lt;typename ForwardIterator, typename T&gt; void fill(ForwardIterator i) { } template&lt;typename ForwardIterator, typename T, T head, T... tail&gt; void fill(ForwardIterator i) { *i = head; fill&lt;ForwardIterator, T, tail...&gt;(++i); } </code></pre> <p>the following class template</p> <pre><code>template&lt;typename T, T... args&gt; struct params_to_array; template&lt;typename T, T last&gt; struct params_to_array&lt;T, last&gt; { static const std::size_t SIZE = 1; typedef std::array&lt;T, SIZE&gt; array_type; static const array_type params; private: void init_params() { array_type result; fill&lt;typename array_type::iterator, T, head, tail...&gt;(result.begin()); return result; } }; template&lt;typename T, T head, T... tail&gt; struct params_to_array&lt;T, head, tail...&gt; { static const std::size_t SIZE = params_to_array&lt;T, tail...&gt;::SIZE + 1; typedef std::array&lt;T, SIZE&gt; array_type; static const array_type params; private: void init_params() { array_type result; fill&lt;typename array_type::iterator, T, last&gt;(result.begin()); return result; } }; </code></pre> <p>and initialized the static constants via</p> <pre><code>template&lt;typename T, T last&gt; const typename param_to_array&lt;T, last&gt;::array_type param_to_array&lt;T, last&gt;::params = param_to_array&lt;T, last&gt;::init_params(); </code></pre> <p>and</p> <pre><code>template&lt;typename T, T head, T... tail&gt; const typename param_to_array&lt;T, head, tail...&gt;::array_type param_to_array&lt;T, head, tail...&gt;::params = param_to_array&lt;T, head, tail...&gt;::init_params(); </code></pre> <p>Now the array</p> <pre><code>param_to_array&lt;int, 1, 3, 4&gt;::params </code></pre> <p>is a <code>std::array&lt;int, 3&gt;</code> and contains the values <code>1</code>, <code>3</code> and <code>4</code>. I think there must be a simpler way to achieve this behavior. Any suggestions?</p> <p><b>Edit:</b> As Noah Roberts suggested in his answer I modified my program like the following: I wrote a new struct counting the elements in a parameter list:</p> <pre><code>template&lt;typename T, T... args&gt; struct count; template&lt;typename T, T head, T... tail&gt; struct count&lt;T, head, tail...&gt; { static const std::size_t value = count&lt;T, tail...&gt;::value + 1; }; template&lt;typename T, T last&gt; stuct count&lt;T, last&gt; { static const std::size_t value = 1; }; </code></pre> <p>and wrote the following function</p> <pre><code>template&lt;typename T, T... args&gt; std::array&lt;T, count&lt;T, args...&gt;::value&gt; params_to_array() { std::array&lt;T, count&lt;T, args...&gt;::value&gt; result; fill&lt;typename std::array&lt;T, count&lt;T, args...&gt;::value&gt;::iterator, T, args...&gt;(result.begin()); return result; } </code></pre> <p>Now I get with</p> <pre><code>params_to_array&lt;int, 10, 20, 30&gt;() </code></pre> <p>a <code>std::array&lt;int, 3&gt;</code> with the content <code>10</code>, <code>20</code> and <code>30</code>. Any further suggestions?</p>
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