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POTemplate Friend for Superclass
 

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  1. POTemplate Friend for Superclass
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    <p>I have a non-templatized class (Par_list_elem), and I would like to give access to its internals to the class Par_list (to build an intrusive list).</p> <p>The catch: I need Par_list_elem and <strong>all of its subclasses</strong> to be accessible to Par_list. In detail, the only fields that need to be accessible are _next and _prev; explicitly limiting to those fields would be nice, but isn't required.</p> <p>I've made a few preliminary attempts at doing this, the latest of which is below:</p> <pre><code> template &lt;class T&gt; class Par_list { public: Par_list() : _head(0) {} ~Par_list(); //Insert element into list bool insert(T elem); //Remove element identified by iterator void erase(iterator itr); private: T* _head; }; class Par_list_elem { public: Par_list_elem() : _next(0), _prev(0) {} //Get next element in list Par_list_elem* next() { return _next; } private: Par_list_elem* _next; Par_list_elem* _prev; template &lt;typename&gt; friend class Par_list; }; template &lt;class T&gt; void Par_list&lt;T&gt;::erase(Par_list&lt;T&gt;::iterator itr) { T* e = *itr; T* p; if ((p = e-&gt;_prev) != 0) p-&gt;_next = e-&gt;_next; else _head = e-&gt;_next; if ((e-&gt;_next) != 0) (e-&gt;_next)-&gt;_prev = p; delete e; } template &lt;class T&gt; bool Par_list&lt;T&gt;::insert(T* nelem) { T* curr = _head; if (curr != 0) { while (curr-&gt;_next != 0) curr = curr-&gt;next(); curr-&gt;_next = nelem; } else _head = nelem; nelem-&gt;_prev = curr; nelem-&gt;_next = 0; return true; } </code></pre> <p>test.cpp</p> <pre><code>#include "parsnip_list_back.h" class elem : parsnip::Par_list_elem { int _elem; }; int main (int argc, char** argv) { parsnip::Par_list&lt;elem&gt; plist; return 0; } </code></pre> <p>Some information seems to be available here: <a href="https://stackoverflow.com/questions/3292795/template-friend">Template friend</a> But the goal is different in enough detail that I'm stuck.</p> <p>Thanks!</p> <p>--------UPDATE---------</p> <p>The following sort of error occurs for each instance of an access of a private member of Par_list_elem. I'm compiling on gcc 4.4. </p> <pre><code>parsnip_list_back.h:66: error: ‘parsnip::Par_list_elem* parsnip::Par_list_elem::_prev’ is private parsnip_list_back.h:124: error: within this context </code></pre> <p>So, with the current implementation, even the superclass isn't giving up its privates.</p>
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    1. COThat doesn't really make sense; you say you need access to "all of its subclasses", but then that you only need access to the fields defined in the base class - which is exactly what the `friend` declaration in your code gives you. What exactly is the problem?
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      1. USMike Seymour
    2. COPar_class cannot access the _next and _prev fields of subclasses of Par_list_elem. Does that make sense?
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      1. USbcr
    3. COIt should be able to, since they are members of the class with the `friend` declaration, not of the subclass, and it did when I tried it. What compiler are you using? What error are you getting? Could you post an example of a subclass of `Par_list_elem`, and some code in `Par_list` that tries to access one of these members?
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      1. POTemplate Friend for Superclass
      1. USMike Seymour
 

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