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    <p>This is multiple inheritance, and there are two inherited functions with the same signature, <strong>both of which have implementation</strong>. That's where C++ is different from Java.</p> <p>Calling <code>write</code> on an expression whose static type is <code>MyBigClass</code> would therefore be ambiguous as to which of the inherited functions was desired.</p> <p><strike>If <code>write</code> is only called through base class pointers, then defining <code>write</code> in the derived class is NOT necessary, contrary to the claim in the question.</strike> Now that the question changed to include a pure specifier, implementing that function in the derived class is necessary to make the class concrete and instantiable.</p> <p><code>MyWriter::write</code> cannot be used for the virtual call mechanism of <code>MyBigClass</code>, because the virtual call mechanism requires a function that accepts an implicit <code>IWriter* const this</code>, and <code>MyWriter::write</code> accepts an implicit <code>MyWriter* const this</code>. A new function is required, which must take into account the address difference between the <code>IWriter</code> subobject and the <code>MyWriter</code> subobject.</p> <p>It would be theoretically possible for the compiler to create this new function automatically, but it would be fragile, since a change in a base class could suddenly cause a new function to be chosen for forwarding. It's less fragile in Java, where only single inheritance is possible (there's only one choice for what function to forward to), but in C++, which supports full multiple inheritance, the choice is ambiguous, and we haven't even started on diamond inheritance or virtual inheritance yet.</p> <p>Actually, this problem (difference between subobject addresses) is solved for virtual inheritance. But it requires additional overhead that's not necessary most of the time, and a C++ guiding principle is "you don't pay for what you don't use".</p>
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    1. POWhy does C++ not let baseclasses implement a derived class' inherited interface?
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    1. COBut IWriter::write is a pure virtual function. The compiler can note that and make MyWriter prevent MyBigClass from being abstract.
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      1. USJohannes Schaub - litb
    2. CO@JohannesSchaub-litb: Of course not. `MyWriter::write` isn't virtual and has no connection to `IWriter`, and doesn't have the right adjustment thunk for the `this` pointer. I guess you're asking the compiler to write the forwarding code automatically? Have you tried `using MyWriter::write;`?
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    3. COwhy does it matter whether or not it is virtual? What prevents putting a rule into the C++ Standard making the case well-formed?
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      1. USJohannes Schaub - litb
 

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