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POHow do I re-use an interface implementation in many classes?
 

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  1. POHow do I re-use an interface implementation in many classes?
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    <p>This question could also have been entitled "How to do reference-counting without ATL". Some similar questions have been asked <a href="https://stackoverflow.com/q/6507372/98903" title="Must the IUnknown interface be re-implemented by every new COM class?">here</a> and <a href="https://stackoverflow.com/q/10779834/98903" title="ATL COM - How To Reuse Code for Interface Method">here</a>, but in the former a different question was answered and in both cases, ATL is involved. My question is more general to C++ and not so much about COM.</p> <p>Suppose we have an <code>IUnknown</code> "interface", like so:</p> <pre><code>class IUnknown { public: virtual ULONG AddRef() = 0; virtual ULONG Release() = 0; virtual ULONG QueryInterface(void * iid, void **ppv) = 0; }; </code></pre> <p>...and let's throw in a few other interfaces that are part of a fictional SDK:</p> <pre><code>class IAnimal : public IUnknown { public: virtual IAnimal** GetParents() = 0; }; class IMammal : public IAnimal { public: virtual ULONG Reproduce() = 0; }; </code></pre> <p>Since I am going to be implementing several animals and mammals, I would rather not copy-paste the <code>AddRef()</code> and <code>Release()</code> implementations in every class, so I wrote <code>UnknownBase</code>:</p> <pre><code>class UnknownBase : public IUnknown { public: UnknownBase() { _referenceCount = 0; } ULONG AddRef() { return ++_referenceCount; } ULONG Release() { ULONG result = --_referenceCount; if (result == 0) { delete this; } return result; } private: ULONG _referenceCount; }; </code></pre> <p>...so that I may use it to, say, implement a <code>Cat</code>:</p> <pre><code>class Cat : public IMammal, UnknownBase { public: ULONG QueryInterface(void *, void**); IAnimal** GetParents(); ULONG Reproduce(); }; ULONG Cat::QueryInterface(void * iid, void **ppv) { // TODO: implement return E_NOTIMPL; } IAnimal** Cat::GetParents() { // TODO: implement return NULL; } ULONG Cat::Reproduce() { // TODO: implement return 0; } </code></pre> <p>...however, the compiler disagrees:</p> <pre><code>c:\path\to\farm.cpp(42): error C2259: 'Cat' : cannot instantiate abstract class due to following members: 'ULONG IUnknown::AddRef(void)' : is abstract c:\path\to\iunknown.h(8) : see declaration of 'IUnknown::AddRef' 'ULONG IUnknown::Release(void)' : is abstract c:\path\to\iunknown.h(9) : see declaration of 'IUnknown::Release' </code></pre> <p>What am I missing?</p>
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    1. USOlivier Dagenais
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    1. COSide note - If your `unknownbase` is only present to provide an impl to those two functions and your original interface is only used through it, you can consider moving the impl directly into the interface itself.
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      1. POHow do I re-use an interface implementation in many classes?
      1. USKarthik T
    2. COAh, that would be easier, but in this case I don't control `IUnknown` or any of the other interfaces. I'm merely implementing them.
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      1. POHow do I re-use an interface implementation in many classes?
      1. USOlivier Dagenais
 

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