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POusing nested std::array to create an multidimensional array without knowing dimensions or extents until runtime
 

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  1. POusing nested std::array to create an multidimensional array without knowing dimensions or extents until runtime
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    <p>Is it possible for a class to have a member which is a multidimensional array whose dimensions and extents are not known until runtime?</p> <p>I have found (via <a href="http://cpptruths.blogspot.com/2011/10/multi-dimensional-arrays-in-c11.html" rel="nofollow noreferrer" title="guide">this guide</a>) a way to create a struct to easily nest std::arrays at compile time using template metaprogramming:</p> <pre><code>#include &lt;array&gt; /* this struct allows for the creation of an n-dimensional array type */ template &lt;typename T,size_t CurrentDimExtent,size_t... NextDimExtent&gt; struct MultiDimArray{ public: //define the type name nestedType to be a recursive template definition. using nestedType=typename MultiDimArray&lt;T,NextDimExtent...&gt;::type; using type=std::array&lt;nestedType,CurrentDimExtent&gt;; }; /* This struct is the template specialization which handles the base case of the final dimensional extent */ template &lt;typename T,size_t DimExtent&gt; struct MultiDimArray&lt;T,DimExtent&gt;{ using type=std::array&lt;T,DimExtent&gt;; }; </code></pre> <p>this still falls short of satisfying my requirement in two ways (that I know of):</p> <ol> <li>In order to declare a variable (or a pointer to a variable) of this type you must state the dimensions.</li> <li>This only works when the DimExtents are constant expressions (set at compile time). </li> </ol> <p>To demonstrate why number 2 is a distinct problem, here is a class with a set number of dimensions (2) using a void* to reference the multidimensional array:</p> <pre><code>template &lt;typename T&gt; class TwoDimGrid{ public: TwoDimGrid(const size_t extent1,const size_t extent2): _twoDimArray(new MultiDimArray&lt;T,extent1,extent2&gt;); private: void* _twoDimArray; }; </code></pre> <p>This will not compile as extent1 and extent2 are not constant expressions.</p> <p>other notes:</p> <ul> <li>I would like to see if it's possible to accomplish using std:array, rather than native arrays or a dynamically resizing container like std::vector.</li> <li>Please use smart pointers where appropriate (I didn't as I'm not really sure how to handle a smart void pointer).</li> </ul> <h1>Edit</h1> <p>I have fallen into the trap of <a href="https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem">The XY Problem</a> with X being the first sentence of this question and Y being how to accomplish it with std::array. I therefore created a <a href="https://stackoverflow.com/questions/20226270/is-it-possible-in-c-for-a-class-to-have-a-member-which-is-a-multidimensional-a">new question</a> and am leaving this one here in case it's ever possible to solve Y problem.</p>
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    1. COstd::array needs to know its size in compile time, you can't have it deduce it in runtime, use std::vector
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    2. CO@user814628 even using std::vector, I would still run into the problem of having to know the number of dimensions at compile time.
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    3. COCan you use boost::multi_array?
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