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POHow can I use decltype to get the type of a reference?
 

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  1. POHow can I use decltype to get the type of a reference?
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    <p>I'm working on some code using decltype in CodeGear RAD Studio. I've tried the naive solution, which looks not unlike this:</p> <pre><code>int main(int, char**) { int i; int &amp;ir = i; decltype((ir)) ir_clone = ir; } </code></pre> <p>Of course, this fails to compile: Internal Compiler Error. I rather suspect that there is nothing particularly wrong with that code and there is a compiler bug regarding reference expressions. (Incidentally, g++ has no problem with the code and compiles it fine.) That doesn't help solve the problem, however, as the platform is non-negotiable.</p> <p>If, above, I had written</p> <pre><code> decltype(ir) ir_clone = ir; /* No extra parens */ </code></pre> <p>it compiles and works as expected. However, the problem doesn't end there, since that doesn't correctly compute constness from the environment. In particular:</p> <pre><code>struct S { int i; } s; const S* p = &amp;s; decltype(p-&gt;i) i0 = s.i; /* i0 is an int */ decltype((p-&gt;i)) i1 = s.i; /* i1 is a const int&amp; */ </code></pre> <p>If I don't use the parens to make the argument an expression, I lose the constness of the argument, which I need.</p> <p>Another tool I can use is simple templates, like so:</p> <pre><code>template&lt;class T&gt; struct unref { typedef T type; } template&lt;class T&gt; struct unref&lt;T&amp;&gt; { typedef T type; } </code></pre> <p>That lets me strip away the reference portion of a type, by using <code>unref&lt;int&amp;&gt;::type</code>.</p> <p>What I can't seem to figure out is how to put all these tools together to get a successful expression for the type I need. For one of things I need, I'm working on a generalized macro that does 'foreach'. (Yes, I know Boost does it better.) It needs to handle the following scenarios:</p> <pre><code>(vector&lt;int&gt;) vi =&gt; vector&lt;int&gt; (vector&lt;int&gt;&amp;)vir =&gt; vector&lt;int&gt; (const vector&lt;int&gt;) cvi =&gt; const vector&lt;int&gt; (const vector&lt;int&gt;&amp;)cvir =&gt; const vector&lt;int&gt; (const P*) cp-&gt;vi =&gt; const vector&lt;int&gt; (P*) p-&gt;vi =&gt; vector&lt;int&gt; </code></pre> <p>So far, my simple attempts fail:</p> <pre><code>unref&lt;decltype(cp-&gt;vi)&gt; /* is vector&lt;int&gt;, not what I need. */ unref&lt;decltype((cp-&gt;vi))&gt; /* is const vector&lt;int&gt;, which is right. */ unref&lt;decltype(vir)&gt; /* is vector&lt;int&gt;, which is right. */ unref&lt;decltype((vir))&gt; /* Internal Compiler Error, which is a headache. */ </code></pre> <p>Any ideas to get me on the right track? Hopefully there's just something simple I'm missing. Maybe I'm attacking the problem from the wrong angle.</p>
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    1. USJohan
    1. USalficles
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    1. COfrankly, the description is too long, and i guess with typos (declspec), and not clear why decltype is needed at all if there are only simple known types used. i understand that it's a contrived example, but it doesn't help to understand the problem.
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      1. POHow can I use decltype to get the type of a reference?
      1. USGene Bushuyev
    2. COTypos corrected. declspec was wrong. As for the length of the question, I was trying to 'show my work', so as to get more precise answers. If it helps, consider that a macro that uses this might look like `foreach(item, container)`. (Yes, I know Boost does it better.) I need to be able to intuit things about the type of the container in order to determine what kind of iterator object to use.
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      1. POHow can I use decltype to get the type of a reference?
      1. USalficles
    3. COCan you use `auto` instead?
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      1. USGManNickG
 

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