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POUsing an abstract class in C++
 

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    <p>I'm trying to use an abstract class when passing an extended object as an parameter to a function, but my attempts so far have led to some compiler errors.</p> <p>I have a few clues as to what the problem is, I'm obviously not allowed to instantiate an abstract class, and I believe some of the code in MyClass is trying to do this, even though this is not my intention. Some researching has suggested that I should reference the object as a pointer to achieve what I want, but my attempts so far have failed and I'm not even sure that this is the answer (hence my asking here).</p> <p>I'll submit now that I'm more familiar with Java than C++, and I'm sure part of my problem is due to this.</p> <p>Here is an example of what I'm trying to do in my program:</p> <pre><code>class A { public: virtual void action() = 0; }; class B : public A { public: B() {} void action() { // Do stuff } }; class MyClass { public: void setInstance(A newInstance) { instance = newInstance; } void doSomething() { instance.action(); } private: A instance; }; int main(int argc, char** argv) { MyClass c; B myInstance; c.setInstance(myInstance); c.doSomething(); return 0; } </code></pre> <p>This example produces the same compiler error that I am getting in my program:</p> <pre><code>sean@SEAN-PC:~/Desktop$ gcc -o test test.cpp test.cpp:20: error: cannot declare parameter ‘newInstance’ to be of abstract type ‘A’ test.cpp:2: note: because the following virtual functions are pure within ‘A’: test.cpp:4: note: virtual void A::action() test.cpp:30: error: cannot declare field ‘MyClass::instance’ to be of abstract type ‘A’ test.cpp:2: note: since type ‘A’ has pure virtual functions test.cpp: In function ‘int main(int, char**)’: test.cpp:36: error: cannot allocate an object of abstract type ‘A’ test.cpp:2: note: since type ‘A’ has pure virtual functions </code></pre> <h2>Update</h2> <p>Thanks for the feedback everyone.</p> <p>I've since changed "MyClass::instance to contain a pointer of type A, but I now get some bizarre errors related to the vtable:</p> <pre><code>sean@SEAN-PC:~/Desktop$ gcc -o test test.cpp /tmp/ccoEdRxq.o:(.rodata._ZTI1B[typeinfo for B]+0x0): undefined reference to `vtable for __cxxabiv1::__si_class_type_info' /tmp/ccoEdRxq.o:(.rodata._ZTI1A[typeinfo for A]+0x0): undefined reference to `vtable for __cxxabiv1::__class_type_info' /tmp/ccoEdRxq.o:(.rodata._ZTV1A[vtable for A]+0x8): undefined reference to `__cxa_pure_virtual' collect2: ld returned 1 exit status </code></pre> <p>My modified code is as follows (A and B have not been modified):</p> <pre><code>class MyClass { public: void setInstance(A* newInstance) { instance = newInstance; } void doSomething() { instance-&gt;action(); } private: A* instance; }; int main(int argc, char** argv) { MyClass c; B myInstance; c.setInstance(&amp;myInstance); c.doSomething(); return 0; } </code></pre>
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