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POmsvc 11 only respects C++ extern templates in certain situations
 

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  1. POmsvc 11 only respects C++ extern templates in certain situations
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    <p>I'm messing around with C++ extern templates to speed up compilation. I've noticed that Visual Studio 2012 has made some improvements in its handling of <code>extern template</code>, but I'm still finding issues. Here's an example:</p> <p><strong>main.cpp:</strong></p> <pre><code>#include &lt;iostream&gt; #include "Calc.h" using namespace std; int main(int argc, char** argv) { Calc&lt;int&gt; c1; cout &lt;&lt; c1.add(1, 2) &lt;&lt; endl; } </code></pre> <p><strong>Calc.h:</strong></p> <pre><code>#pragma once template &lt;class A_Type&gt; class Calc { public: A_Type add(A_Type x, A_Type y) { return x + y; } }; extern template class Calc&lt;int&gt;; </code></pre> <p>Normally I'd include <strong>Calc.cpp</strong> with <code>template class Calc&lt;int&gt;;</code>, ensuring the template is only instantiated once. For this example, I am only compiling <strong>Main.cpp</strong> and expecting it to fail.</p> <p>The problem is, Visual Studio 2012 will happily compile and link the code above. It's not respecting the <code>extern template</code>. The only solution I've found is to do this in the header:</p> <p><strong>Calc.h:</strong></p> <pre><code>#pragma once template &lt;class A_Type&gt; class Calc { public: A_Type add(A_Type x, A_Type y); }; template &lt;class A_Type&gt; A_Type Calc&lt;A_Type&gt;::add(A_Type x, A_Type y) { return x + y; } extern template class Calc&lt;int&gt;; </code></pre> <p>Visual Studio fails to link if the header is like this (complaining about a missing definition for <code>Calc&lt;int&gt;::add</code>, as expected). <strong>However</strong>, <em>g++ 4.6.3</em> Fails to link either of these examples.</p> <p>Who is right? If both the prototypes and definitions of functions in a class are specified in the same header file, is there really a difference? Is there any way to specify the "shorter-form" <em>Calc.h</em> and have it fail as expected in Visual Studio 2012 (as well as gcc)?</p>
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    1. USjmacdonagh
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      1. VTUpMod
    1. COYour first example doesn't include `extern`, is something missing?
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      1. USMark Ransom
    2. COOops, bad copy/paste ;) Thanks!
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