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POIssues with C++ template arguments for inheritance
 

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  1. POIssues with C++ template arguments for inheritance
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    <p>I have a questions about C++ templates. More specifally, by using template arguments for inheritance. I am facing strange behaviour in a closed-source 3rd party library. There is a C method</p> <pre><code>factoryReg(const char*, ICallback*) </code></pre> <p>which allows to register a subclass of ICallback and overwrite the (simplified) methods:</p> <pre><code>class ICallback { public: virtual void ENTRY(void* data) = 0; virtual void EXIT(void* data) = 0; const char* getName() { return _name; } const ICallback(const char* name) : _name(name) {} virtual ~ICallback() {} private: const char* _name; }; </code></pre> <p>I have </p> <pre><code> class BaseCallback : public ICallback { public: BaseCallback(const char* name) : ICallback(name) {} virtual void ENTRY(void* data) { std::cout &lt;&lt; "in ENTRY base" &lt;&lt; std::endl; } virtual void EXIT(void* data) { std::cout &lt;&lt; "in EXIT base" &lt;&lt; std::endl; }; class SpecialCallback : public BaseCallback { public: SpecialCallback(const char* name) : BaseCallback(name) {} virtual void ENTRY(void* data) { // actually, it's 3rd party code too - assumed to do something like ... BaseCallback::ENTRY(); } // no redecl. of EXIT(void* data) }; template &lt;typename Base&gt; TemplCallback : public Base { public: TemplCallback(Base&amp; myT) : Base(myT.getName()), _myT(myT) virtual void ENTRY(void* data) { std::cout &lt;&lt; "in ENTRY templ." &lt;&lt; std::endl; _myT.ENTRY(); } virtual void EXIT(void* data) { std::cout &lt;&lt; "in EXIT templ." &lt;&lt; std::endl; _myT.EXIT(); } private: Base&amp; _myT; } </code></pre> <p>Upon registering</p> <pre><code>SpecialCallback spc("validName"); TemplCallback&lt;SpecialCallback&gt; myCallback(spc); factoryReg(spc.getName(), &amp;myCallback); ... // output: "in ENTRY base" // "in EXIT base" </code></pre> <p>the callback somehow does not work (debug output not being put out // breakpoints do not apply).</p> <p>If I omit implementation of the EXIT(void* data) method in my template class TemplCallback - everything works fine! </p> <pre><code>// output: "in ENTRY templ." // "in EXIT base" </code></pre> <p>Is this expected behaviour? I have been told it might be an issue of the MSVC compiler 13.10.6030 I use. Not sure about that.</p> <p>BTW: The template idea presented here might not be the best choice for whatever I am trying to do ;) But I am still interested in the matter itself, regardless about design questions.</p>
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    1. COWhat is the type of `_myT`?
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      1. USMike Seymour
    2. COCan you flesh this pidgin C++ out to something that at least has a hope of compiling? It's hard to be a language lawyer with pseudocode.
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      1. USWarren Young
    3. COSorry, I realize the code is a bit thin: fleshed it out now.
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