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POHow can a base class satisfy the definition of a parent's pure virtual function using another parent's function
 

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  1. POHow can a base class satisfy the definition of a parent's pure virtual function using another parent's function
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    <p>I am extending an existing C++ project. I have a base class that derives from two parent classes. One of the parents has a pure virtual function. I want that pure virtual function to be defined by a function implemented in the other parent.</p> <p>So, I want another parent to satisfy the base class's obligation to define a parent's pure virtual function. I have tried two approaches, both have lead to compiler errors.<br> <strong>Any ideas?</strong></p> <p>Here is a C++ program demonstrating my first idea, hoping the compiler would just use <code>base2</code>'s definition of <code>vfunc()</code>.</p> <pre><code>// This is my first approach, hoping the parent base2 of derived would satisfy the need to define // base1's pure virtual vfunc. class base1 { public: virtual int vfunc() = 0; }; class base2 { public: int vfunc() { return 0;} //defined }; class derived : public base1, public base2 { public: //empty }; int main() { derived d; base1 &amp; b1 = d; int result = b1.vfunc(); return result; } </code></pre> <p>The compiler reports that <code>derived</code> is still an abstract class:</p> <pre><code>$ gcc a.cc a.cc: In function ‘int main()’: a.cc:26: error: cannot declare variable ‘d’ to be of abstract type ‘derived’ a.cc:18: note: because the following virtual functions are pure within ‘derived’: a.cc:7: note: virtual int base1::vfunc() </code></pre> <p>Here is my second attempt:</p> <pre><code>// This is my second attempt, defining a vfunc in the derived class that calls the other parent. class base1 { public: virtual int vfunc() = 0; }; class base2 { public: int vfunc() { return 0; } // defined }; class derived : public base1, public base2 { public: int vfunc() { return base2::vfunc(); } // call the other parent's vfunc }; int main() { derived d; base1 &amp; b1 = d; int result = b1.vfunc(); return result; } </code></pre> <p>I actually expected this to do it for me, but instead the linker is giving me a bunch of vtable errors that I do not understand: ( Mac OS 10.6, gcc 4.2.1 )</p> <pre><code>$ gcc inheritance_tester.cc Undefined symbols: "vtable for __cxxabiv1::__vmi_class_type_info", referenced from: typeinfo for derivedin ccmeHq8C.o "___cxa_pure_virtual", referenced from: vtable for base1in ccmeHq8C.o "___gxx_personality_v0", referenced from: _main in ccmeHq8C.o base2::vfunc() in ccmeHq8C.o derived::vfunc() in ccmeHq8C.o base1::base1() in ccmeHq8C.o base2::base2() in ccmeHq8C.o derived::derived()in ccmeHq8C.o CIE in ccmeHq8C.o "vtable for __cxxabiv1::__class_type_info", referenced from: typeinfo for base1in ccmeHq8C.o typeinfo for base2in ccmeHq8C.o ld: symbol(s) not found </code></pre>
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    1. COderived's vfunc should `return base::vfunc();`, but thats probably a typo.
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    2. COWhy do you try to give base2 member the same name as the virtual function? Its more reabable if you postfix it with `impl` or somthing like that.
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