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PORecursive Template idiom how to avoid that the base class is friend of the child classes
 

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  1. PORecursive Template idiom how to avoid that the base class is friend of the child classes
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    <p>I use the <a href="https://stackoverflow.com/questions/10332725/how-to-automatically-register-a-class-on-creation">recursive template idiom</a> to automatically register all children of a base class in a factory. However in my design the child class must have as a friend class the base class. As the Constructor of my Base class should be private to avoid instantiation of this class other than via the factory.</p> <p>The overal aim would be that the registration of the factory is done in the BaseSolver and the ChildClasses cannot be instantiated other than via the factory.</p> <p>Here is the code of my base class which automatically registers all children in the SolverFactory.</p> <pre><code>template&lt;class T&gt; struct BaseSolver: AbstractSolver { protected: BaseSolver() { reg=reg;//force specialization } virtual ~BaseSolver(){} /** * create Static constructor. */ static AbstractSolver* create() { return new T; } static bool reg; /** * init Registers the class in the Solver Factory */ static bool init() { SolverFactory::instance().registerType(T::name, BaseSolver::create); return true; } }; template&lt;class T&gt; bool BaseSolver&lt;T&gt;::reg = BaseSolver&lt;T&gt;::init(); </code></pre> <p>And here the header file of my child class:</p> <pre><code>class SolverD2Q5 : public BaseSolver&lt;SolverD2Q5&gt;{ private: //how can I avoid this? friend class BaseSolver; SolverD2Q5(); static const std::string name; } </code></pre> <p>This works fine. However I really do not like to have to add the BaseSolver as a friend class, however I <strong><em>do not</em></strong> want the constructor and the static member name to be public. </p> <p>Is there a more elegant solution or a better layout to avoid this?</p>
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    1. COWhy does BaseSolver need to be a friend class? Could you add `friend class T` to BaseSolver instead?
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    2. COHow should that help I do not want to access private methods in the BaseSolver but the BaseSolver should access private members of the Solver.
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    3. COI like this design the way it is. Why is the `friend` declaration an issue?
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