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POMultiple polymorphism in C++
 

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  1. POMultiple polymorphism in C++
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    <p>There's the compile error " ' A ' is an ambiguous base of ' C ' " existing on the test <code>diamond multiple inheritance</code> below,</p> <pre><code>struct A { void f();}; struct B1: A {}; struct B2: A {}; struct C: B1, B2 {}; void g() {A *o = new C;} </code></pre> <p>It's solved by <code>virtual inheritance</code> below,</p> <pre><code>struct B1: virtual A {}; struct B2: virtual A {}; </code></pre> <p>Now there's the compile error " no unique final overridder for ' virtual void A::f() ' in ' C ' " existing on another test <code>diamond multiple polymorphism</code> below,</p> <pre><code>struct A {virtual void f();}; struct B1: virtual A {void f();}; struct B2: virtual A {void f();}; struct C: B1, B2 {}; void g() { A *o = new C; o-&gt;f(); } </code></pre> <p>And it can't be solved even by <code>dynamic_cast</code> below,</p> <pre><code>dynamic_cast&lt;B1 *&gt;(o)-&gt;f(); </code></pre> <p><strong>@NOTE</strong></p> <p><code>dynamic_cast&lt;T *&gt;(obj_ptr)</code> is actually used to perform <code>typesafe downcast</code>, i.e. when the run-time type Tobj of the object that the obj_ptr points to is a subtype of T, it returns obj_ptr itself; otherwise the NULL pointer. It is mistaken being thought of upcasting the subtype Tobj to the supertype T, otherwise performing upcast at run-time directly contradicts the principle of polymorphism.</p> <p>IMHO, <code>virtual inheritance</code> is succinct enough to solve the first-half problem at compile-time. In contrast, can you present anything to solve the second-half problem at run-time?</p> <p><strong>@EDIT</strong></p> <p>Thanks for your pointing out <code>dynamic_cast</code> doesn't do the job. Corrected.</p> <p>To sovle the 2nd-half problem, it seems to have no choice but implement the overrider on the final subclass of the diamond hierarchy below,</p> <pre><code>struct C: B1, B2 { void f() {B1::f();} //Hardcode to your choice of overrider }; </code></pre>
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    1. COAre you sure `dynamic_cast` solves the problem? Did you try it?
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    2. COAvoiding diamond inheritance is more succinct than `dynamic_cast`...
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      1. USMark B
 

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