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POC++ Static member initialization (template fun inside)
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  1. POC++ Static member initialization (template fun inside)
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    copied!<p>For static member initialization I use a nested helper struct, which works fine for non templated classes. However, if the enclosing class is parameterized by a template, the nested initialization class is not instantiated, if the helper object is not accessed in the main code. For illustration, a simplified example (In my case, I need to initialize a vector). </p> <pre><code>#include &lt;string&gt; #include &lt;iostream&gt; struct A { struct InitHelper { InitHelper() { A::mA = "Hello, I'm A."; } }; static std::string mA; static InitHelper mInit; static const std::string&amp; getA(){ return mA; } }; std::string A::mA; A::InitHelper A::mInit; template&lt;class T&gt; struct B { struct InitHelper { InitHelper() { B&lt;T&gt;::mB = "Hello, I'm B."; // [3] } }; static std::string mB; static InitHelper mInit; static const std::string&amp; getB() { return mB; } static InitHelper&amp; getHelper(){ return mInit; } }; template&lt;class T&gt; std::string B&lt;T&gt;::mB; //[4] template&lt;class T&gt; typename B&lt;T&gt;::InitHelper B&lt;T&gt;::mInit; int main(int argc, char* argv[]) { std::cout &lt;&lt; "A = " &lt;&lt; A::getA() &lt;&lt; std::endl; // std::cout &lt;&lt; "B = " &lt;&lt; B&lt;int&gt;::getB() &lt;&lt; std::endl; // [1] // B&lt;int&gt;::getHelper(); // [2] } </code></pre> <p>With g++ 4.4.1: </p> <ul> <li><p>[1] and [2] commented: </p> <pre>A = Hello, I'm A.</pre> <p>Works as intended</p></li> <li><p>[1] uncommented: </p> <pre>A = Hello, I'm A. B = </pre> <p>I would expect, that the InitHelper initializes mB</p></li> <li>[1] and [2] uncommented: <pre>A = Hello, I'm A. B = Hello, I'm B.</pre> Works as intended</li> <li>[1] commented, [2] uncommented:<br> Segfault in the static initialization stage at [3] </li> </ul> <p>Thus my question: Is this a compiler bug or is the bug sitting between the monitor and the chair? And if the latter is the case: Is there an elegant solution (i.e. without explicitly calling a static initialization method)?</p> <p><strong>Update I:</strong><br> This seems to be a desired behavior (as defined in the ISO/IEC C++ 2003 standard, 14.7.1):</p> <blockquote> <p>Unless a member of a class template or a member template has been explicitly instantiated or explicitly specialized, the specialization of the member is implicitly instantiated when the specialization is referenced in a context that requires the member definition to exist; in particular, the initialization (and any associated side-effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist.</p> </blockquote>
 

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