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POHow can one make a variadic-based chained-indexing function?
 

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  1. POHow can one make a variadic-based chained-indexing function?
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    <p>If I have an object <code>a</code> that either a built-in array or a class-type with a suitable <code>operator []</code>, and its returned type can be indexed itself, how should I write a generic function that can index all of them with a variadic call instead of separated bracket block? In other words, I can make expressions like:</p> <pre><code>a[i0][i1]...[iK] </code></pre> <p>and I want to be able to write it as a single function:</p> <pre><code>slice( a, i0, i1, ..., iK ) </code></pre> <p>since the rules of C++ require <code>operator []</code> to work on single arguments, making it less-compatible with variadic stuff. (This question is based on a Usenet thread where I tried asking something similar; ending up solving only possibly-nested built-in arrays myself.)</p> <p>A first stab:</p> <pre><code>template &lt; typename T, typename U &gt; constexpr auto slice( T &amp;&amp;t, U &amp;&amp;u ) noexcept(???) -&gt; ??? { return ce_forward&lt;T&gt;(t)[ ce_forward&lt;U&gt;(u) ]; } template &lt; typename T, typename U, typename V, typename ...W &gt; constexpr auto slice( T &amp;&amp;t, U &amp;&amp;u, V &amp;&amp;v, W... &amp;&amp;w ) noexcept(???) -&gt; ??? { return slice( ce_forward&lt;T&gt;(t)[ce_forward&lt;U&gt;(u)], ce_forward&lt;V&gt;(v), ce_forward&lt;W&gt;(w)... ); } </code></pre> <p>The <code>ce_forward</code> function templates are a <code>constexpr</code>-marked <code>std::forward</code>. (Some things in the standard that could be marked <code>constexpr</code> aren't.) I'm trying to figure out the right stuff to put in the return type and exception specification spots. Some cases and caveats I know of are:</p> <ul> <li>The built-in <code>operator []</code> requires one operand to be a data-pointer (or decayed array reference) and the other to be an enumeration or integer type. The operands can be in either order. I don't know if an operand could be replaced with a class-type with an unambiguous (non-explicit?) conversion to an appropriate type.</li> <li>A class type can define <code>operator []</code> as non-static member function(s). Any such function can only have one parameter (besides <code>this</code>).</li> <li>The built-in operator can't throw. (Unless operands could be based on an UDT conversion; said conversion is the only possible throw point.) The undefined behavior caused by boundary errors is beyond our purview here.</li> <li>Whether the last indexing call returns a l-value reference, r-value reference, or value should be reflected in the return type of <code>slice</code>.</li> <li>If the (final?) call is by-value, then <code>std::is_nothrow_move_constructible&lt;ReturnType&gt;::value</code> should be OR'd to the exception specification. (By-reference returns are <code>noexcept</code>.)</li> <li>If the built-in operator involves an array, the returned reference for that step should be a r-value reference if the array is one too. (This is kind-of a defect because pointers, unlike arrays, lose their target's l- vs. r-value status, so the traditional return would always be a l-value reference.)</li> <li>For class-type indexing operators, make sure the exception-spec and return-type sections refer to the same overload (if more than one) that the function body uses. Be wary for overloads that differ only in the qualification of <code>this</code> (<code>const</code>/<code>volatile</code>/both/neither and/or <code>&amp;</code>/<code>&amp;&amp;</code>/neither).</li> </ul>
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      1. POHow can one make a variadic-based chained-indexing function?
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    1. COIt must be `constexpr`?
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    2. COFor what I'm using it for, it should be `constexpr` whenever possible.
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    3. COHave you considered simply putting the same expression as in the `return` statement for both return type and `noexcept` specification? E.g. `noexcept(noexcept(std::declval<T>()[std::declval<U>()])) -> decltype(std::declval<T>()[std::declval<U>()])`.
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      1. POHow can one make a variadic-based chained-indexing function?
      1. USLuc Danton
 

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