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POTemplated Singleton Policy using CRTP in C++
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copied!<p>There are many folks out there that claim their singleton implementation to be robust and general because it uses metaprogramming constructs.</p> <p>My goal is to enforce a singleton policy onto a derived class so that I do not have to explicitly (manually) declare the derived class' constructors as private. I think there's a way to naively add the instance static variable and the getter as a policy by making the templated singleton a friend of the class you derive. But that's not at all elegant.</p> <p>I started with this code, that, among other things, is given as being a correct (i.e. complete) design of a singleton, while it is clearly allowing for multiple instances:</p> <pre><code>template <class CWrappedClass> class CSingleton { protected: static CWrappedClass* ms_instance; private: CSingleton(){} CSingleton(const CSingleton& ) {} CSingleton& operator = (const CSingleton&) {} public: static CWrappedClass& GetInstance() { if (ms_instance == NULL) ms_instance = new CWrappedClass; return *ms_instance; } }; template <class CWrappedClass> CWrappedClass* CSingleton<CWrappedClass>::ms_instance = NULL; </code></pre> <p>And a singleton client of this "policy", using CRTP:</p> <pre><code>class CThing : public CSingleton<CThing> { // friend class CSingleton<CThing>; // only if ctor is private! public: void DoNothing() { std::cout<<" Nothing \n"; } CThing() { std::cout<<" single "; } }; </code></pre> <p><strong>NOTE</strong> this is not a correct implementation of a CRTP Singleton policy, it's merely part of the question!</p> <p>The code <em>won't</em> compile as is. The base singleton policy class has its constructor declared private, so it can't support derived instances unless the child is a friend of this class. People usually make the constructors protected, which means there's nothing to keep a user from making derived class non-singletonian.</p> <p><strong>Problem/Question</strong>: Is there any mechanism to enforce a singleton policy without having to make the derived class' constructors private manually?</p>

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