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POisAbstract template and visual studio
 

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    <p>The following template will decide if T is abstract with g++.</p> <pre><code>/** isAbstract&lt;T&gt;::result is 1 if T is abstract, 0 if otherwise. */ template&lt;typename T&gt; class isAbstract { class No { }; class Yes { No no[3]; }; template&lt;class U&gt; static No test( U (*)[1] ); // not defined template&lt;class U&gt; static Yes test( ... ); // not defined public: enum { result = sizeof( isAbstract&lt;T&gt;::template test&lt;T&gt;( 0 ) ) == sizeof(Yes) }; }; </code></pre> <p>For example: struct myClass2 { virtual void f() {} }; struct myClass1 { virtual void f() = 0; };</p> <pre><code>bool twoAbstract = isAbstract&lt;myClass2&gt;::result; bool oneAbstract = isAbstract&lt;myClass1&gt;::result; </code></pre> <p>However, it fails in visual studio 9.0 with the error:</p> <pre><code>error C2784: 'AiLive::isAbstract&lt;T&gt;::No AiLive::isAbstract&lt;T&gt;::test(U (*)[1])' : could not deduce template argument for 'U (*)[1]' from 'myClass2' </code></pre> <p>Does anyone have an idea of what the problem is and how to fix this?</p> <p><a href="http://msdn.microsoft.com/en-us/library/bb982572.aspx" rel="nofollow noreferrer">MSDN</a> Reports that they now have an <code>is_abstract</code> class since VS2008 as part of TR1 (inside the header <code>type_traits</code>). However, it seems to be missing from my installation.</p> <p>PS. For reasons that are long and boring, I can't reimplement this via Boost.</p> <p><em>Update</em></p> <p>Also, tried replacing,</p> <pre><code>template&lt;class U&gt; static No test( U (*)[1] ); </code></pre> <p>with each of,</p> <pre><code>template&lt;class U&gt; static No test( U (*x)[1] ); template&lt;class U&gt; static No test( U (*)() ); template&lt;class U&gt; static No test( U (*x)() ); </code></pre> <p>and</p> <pre><code>template &lt;typename U&gt; struct U2 : public U { U2( U* ) {} }; // Match if I can make a U2 template &lt;typename U&gt; static No test( U2&lt;U&gt; x ); </code></pre> <p>and</p> <pre><code>// Match if I can make a U2 template &lt;typename U&gt; static No test( U2&lt;T&gt; x ); </code></pre> <p>No luck - all say that template argument cannot be deduced for U.</p>
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    1. COI think the code is fine, suspecting a VC++ bug. Maybe try with this: template<class U> static No test( U (*)() ); or with a name for the parameter (maybe it chokes because the parameter has no name?).
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      1. USJohannes Schaub - litb
    2. COThanks. Tried those, but works the same. (I've added those cases and an extra one above).
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    3. COHmm, just looked deeper. my recommendation U(*)() wouldn't work it seems, 'cause a function-type with an abstract return type is not invalid. It's just not possible to have such a function. leading to a "real" compile time error with comeau :)
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      1. USJohannes Schaub - litb
 

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