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POAny way to resolve forward declaration in a template class?
 

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  1. POAny way to resolve forward declaration in a template class?
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    <p>I have a template class defined as such:</p> <pre><code>template &lt;typename T&gt; void ProxyNoOp(T *) {} template &lt;typename T&gt; void ProxyDelete(T * ptrT) {delete ptrT;} template &lt;typename T, typename C, void (* Release)(T *) = ProxyNoOp&lt;T&gt; &gt; class Proxy { public: class Container : public std::list&lt;T *&gt; { public: Container() {} ~Container() {clear();} void clear() { iterator clsEnd = end(); for (iterator clsCurrent = begin(); clsCurrent != clsEnd; ++clsCurrent) { T * ptrT = *clsCurrent; static_cast&lt;Proxy *&gt;(ptrT)-&gt;m_ptrContainer = NULL; Release(ptrT); } } }; private: iterator m_clsPosition; C * m_ptrContainer; public: Proxy() : m_ptrContainer(NULL) {} ~Proxy() { if ( m_ptrContainer != NULL ) { Container * ptrContainer = static_cast&lt;Container *&gt;(m_ptrContainer); static_cast&lt;std::list&lt;T *&gt; &gt;(ptrContainer)-&gt;erase(m_clsPosition); } } C * GetContainer() const {return m_ptrContainer;} }; </code></pre> <p>A lot of the code has been removed or changed for the sake of brevity. The idea of the data structure is that not only does a container maintain references to contained elements, but the contained elements maintain a reference to the container PLUS their position in the container, making removal quick (constant time) and automatic (called in the destructor)</p> <p>A problem with this approach can be demonstrated by this code:</p> <pre><code>#include "proxy.h" // the above template class class Child; class Parent : public Proxy&lt;Child, Parent&gt;::Container {}; class Child : public Proxy&lt;Child, Parent&gt; {}; </code></pre> <p>When compiling <code>Parent</code>, the definition of <code>Proxy&lt;Child, Parent&gt;::Container</code> causes an error at this line: <code>static_cast&lt;Proxy *&gt;(ptrT)-&gt;m_ptrContainer = NULL;</code> - at the point in the compilation, <code>Child</code> has not been defined, only declared. If I change the declaration order of <code>Parent</code> and <code>Child</code> to be this:</p> <pre><code>#include "proxy.h" class Parent; class Child : public Proxy&lt;Child, Parent&gt; {}; class Parent : public Proxy&lt;Child, Parent&gt;::Container {}; </code></pre> <p>the definition of <code>Proxy&lt;Child, Parent&gt;</code> causes an error at this line: <code>Container * ptrContainer = static_cast&lt;Container *&gt;(m_ptrContainer);</code>, the reason being the same as before - at that point in the compilation, <code>Parent</code> is not fully defined.</p> <p>Is there any way I can get around this and still use templates? Is there a standard data structure in STL, BOOST, etc. that accomplishes this?</p> <p><strong>EDIT:</strong></p> <p>The code will not compile as-is, I should have been clearer about that (to quote myself from above: <em>A lot of the code has been removed or changed for the sake of brevity.</em>). It doesn't actually use <code>std::list&lt;T *&gt;</code>, but a custom doubly linked that is effectively the same. I didn't want to post a ton of ancillary code when only asking about this one issue.</p> <p>I wasn't asking for critique on the data structure itself (although I am fine to hear it), but for those wondering why it was created here are a couple examples:</p> <p>A game engine has models and instances of those models, such as a war game where there is one tank model and any number of tanks created from it. The classes could be written as such:</p> <pre><code>class Instance; class Model : public Proxy&lt;Model, Instance, ProxyDelete&lt;Instance&gt; &gt;::Container {...}; class Instance : public Proxy&lt;Model, Instance, ProxyDelete&lt;Instance&gt; &gt; {...}; </code></pre> <p>When a model is deleted, all instances made from that model should also be deleted (hence the <code>ProxyDelete&lt;Instance&gt;</code> template parameter) since it doesn't make sense for an instance to exist without the model it was made from.</p> <p>Suppose your graphical user interface (GUI) renderer keeps track of all visible elements (buttons, frames, etc.) so it knows what to render, but text is rendered differently since it is a different call to the graphics API:</p> <pre><code>class GuiTextElement; class GuiRenderer : public Proxy&lt;GuiRenderer, GuiTextElement, ProxyNoOp&lt;GuiTextElement&gt; &gt;::Container {...}; class GuiTextElement : public Proxy&lt;GuiRenderer, GuiTextElement, ProxyNoOp&lt;GuiTextElement&gt; &gt; {...}; </code></pre> <hr> <p>The less astute will say "why don't you simply add <code>std::list&lt;Instance *&gt;</code> as a member of the <code>Model</code> and <code>std::list&lt;GuiTextElement *&gt;</code> as a member of <code>GuiRenderer</code>?". Here are a few reasons:</p> <ol> <li>Elements that are "owned" by their containers are not automatically deleted when the container is destroyed (yes, I am aware of <code>boost::ptr_list</code>).</li> <li>There is no way, without adding another class member, to refer to the container using a contained element. For example, to get a <code>Model</code> from an <code>Instance</code>.</li> <li>If you did add a class member to accomplish #2, you would also need to update it when elements are removed from their containers.</li> <li>For the speed fanatics - if contained elements don't maintain an iterator to their position in the container, before that element can be removed the container must first be searched for it.</li> </ol>
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      1. POAny way to resolve forward declaration in a template class?
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    1. COif elements maintain thier position, I think that would make removal _slower_ not faster.
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      1. POAny way to resolve forward declaration in a template class?
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    2. CONow that I've read the question, nothing here makes sense. And everything I see here terrifies me.
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      1. POAny way to resolve forward declaration in a template class?
      1. USMooing Duck
    3. COHowever, the general solution to this sort of problem is to design `Proxy` so it _doesn't_ require `Parent` or `Child` (pick one) to be defined.
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      1. POAny way to resolve forward declaration in a template class?
      1. USMooing Duck
 

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