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    <p>To simplify answer let's think about virtual/non-virtual as duplicated or non-duplicated content.</p> <pre><code>class LibDerived : LibBase </code></pre> <p>declares: I allow LibBase be twice (or more ) entered into descending of LibDerived</p> <pre><code>class MyBase : virtual LibBase {}; </code></pre> <p>declares: I allow compiler to optimize two entries of LibBase in MyBase descendings into single one. </p> <p>When these two declarations meet each one, the first is more priority so MyDerived gets 2 implmentation of LibBase. But power of c++ is possibility to resolve it! Just make overriding on MyDerived virtual functions to select which you want to use. Ore another way - create universal wrapper of MyDerived derived from interface LibBase that aggregate any instance: LibDerived, MyBase, ... and call expected method from aggregate.</p>
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    1. COCan you explain further the resolution you mentioned? When you say overriding on MyDerived virtual functions to select the ones I want, do you mean just "accept the fact that you have two LibBase objects, and override the methods to expose only one of them"? Unfortunately I don't think the universal wrapper would do what I would like because it would then not be compatible with the rest of the library. I'd like the each of the derived class objects to "appear" as if it's a native object of the library, but simply have a different implementation for some of the methods.
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    2. CO`virtual` in heritance is not for allowing a compiler to optimize out base classes. A `virtual` base class is for guaranteeing that there is only one base class sub-object no matter how many times that type appears as a virtual base class in the inheritance tree of a derived class object. A non-virtual base class is guaranteed to add a new base class sub-object. There's no priority and there's no option to add a second virtual base for the same type.
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    3. COCheshirekow, let me answer in back order. Aggregate exactly allows to choose between multiple implementation, you just make wrapper around impl. And MyConcrete, for example at constructor time, accepts pointer to expected implementation and exposes it by delagating each call to aggreate. "accept the fact that you have two LibBase" - unfortunately "yes" nonvirtual declaration grants you manually need override each virtual function to make decision which member of two copies must be invoked.
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