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POCRTP-related compiler error on typedef
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  1. POCRTP-related compiler error on typedef
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    copied!<p>I have rather simple, I imagine, question about <a href="http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern" rel="nofollow">CRTP</a>, but I cannot seem to find an answer to it. Probably, because it is so simple, no one thought of asking it. I am new to the concept, so, please don't laugh too hard ;).</p> <p> Here is the code (it's sort of an attempt to do something similar to STL containers):</p> <pre class="lang-c++ prettyprint-override"><code>template&lt; typename tT &gt; struct TBase { typedef tT T; }; template&lt; typename tTBase &gt; struct TTraitsBase { typedef typename tTBase::T T; }; template&lt; typename tTHelpee, typename tTTraits &gt; struct THelper { typedef typename tTTraits::T T; typedef typename tTHelpee::T Th; /* This generates a compiler error: 'T' not being a member of TDerived&lt;tT&gt; */ T Foo( void ) { return static_cast&lt; tTHelpee* &gt; ( this )-&gt;mVal; } }; template&lt; typename tT &gt; struct TDerived : TBase&lt; tT &gt; , THelper&lt; TDerived&lt; tT &gt; , TTraitsBase&lt; TBase&lt; tT &gt; &gt; &gt; { using TBase&lt; tT &gt;::T; T mVal; }; int main() { TDerived&lt; int &gt;::T lTmp = -1; TDerived&lt; int &gt; lObj; lObj.mVal = -1; std::cout &lt;&lt; lObj.Foo() &lt;&lt; std::endl; return 0; } </code></pre> <p>Everything compiles if I comment the offending <code>typedef typename _THelpee::T Th;</code>. And that what confuses me: if compiler does not like <code>typedef typename _THelpee::T Th;</code>, why does it let through <code>static_cast&lt; _THelpee* &gt; ( this )-&gt;mVal</code>? I assume, it has something to do with not being able to instantiate <code>THelper</code> when instantiating <code>TDerived</code>, but no clear understanding. Can someone, please, give a brief explanation and/or some references on what is happening here? Thank you.</p> <p>EDIT: removed '_T' prefix.</p>
 

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