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POHow can an array work with the conditional operator?
 

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  1. POHow can an array work with the conditional operator?
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    <p>This is a retelling of <a href="https://stackoverflow.com/q/16163456/1010226" title="Can you throw within a conditional expression? (was: How can bounds-checking be extended to multiple dimensions?)">my previous post</a>, since I changed the question (so it probably didn't get flagged as a new question and was missed). I'll hopefully trim it down too.</p> <p>I had functions like:</p> <pre><code>#include &lt;cstddef&gt; #include &lt;type_traits&gt; template &lt; typename E, typename T &gt; inline constexpr auto checked_slice( E &amp;&amp;, T &amp;&amp;t ) noexcept -&gt; T &amp;&amp; { return static_cast&lt;T &amp;&amp;&gt;(t); } template &lt; typename E, typename T, std::size_t N, typename U, typename ...V &gt; inline constexpr auto checked_slice( E &amp;&amp;e, T (&amp;t)[N], U &amp;&amp;u, V &amp;&amp;...v ) -&gt; typename remove_some_extents&lt;T, sizeof...(V)&gt;::type &amp; { typedef typename std::remove_reference&lt;U&gt;::type u_type; typedef typename std::common_type&lt;u_type, std::size_t&gt;::type cmp_type; return ( u &lt; u_type{} ) || ( static_cast&lt;cmp_type&gt;(u) &gt;= static_cast&lt;cmp_type&gt;(N) ) ? throw e : checked_slice( static_cast&lt;E &amp;&amp;&gt;(e), t[static_cast&lt;U &amp;&amp;&gt;( u )], static_cast&lt;V &amp;&amp;&gt;(v)... ); } </code></pre> <p>where <code>remove_some_extents</code> is a custom class template that's like calling the <code>std::remove_extent</code> meta-function a given number of times.</p> <p>When I tried running the program, I got a bunch of errors like: "invalid initialization of reference of type <code>Whatever(&amp;)[X][Y]</code> from expression of type <code>Whatever(*)[Y]</code>" (or <code>Whatever(&amp;)[Z]</code> from <code>Whatever*</code>). My workaround was to convert the conditional expression to an <code>if</code>-<code>else</code> pair (and removing the <code>constexpr</code>).</p> <p>I'm trying to figure out what's wrong, so I'm poking around the section about the conditional operator in the C++ (2011) standard. That's section 5.16. When one of the two possible actions is a throw command (or is otherwise a <code>void</code> expression), then the conditional has the type of the other expression, but the standard conversions, including array-to-pointer, is applied to that other expression. (This is in paragraph 2.) I think that's what's messing me up. Is there any way around it? I thought returning an array reference suppresses the a-to-p conversion. Why does it work when made into an <code>if/else</code>?</p>
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