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POLearning C++: returning references AND getting around slicing
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  1. POLearning C++: returning references AND getting around slicing
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    copied!<p>I'm having a devil of a time understanding references. Consider the following code:</p> <pre><code>class Animal { public: virtual void makeSound() {cout &lt;&lt; "rawr" &lt;&lt; endl;} }; class Dog : public Animal { public: virtual void makeSound() {cout &lt;&lt; "bark" &lt;&lt; endl;} }; Animal* pFunc() { return new Dog(); } Animal&amp; rFunc() { return *(new Dog()); } Animal vFunc() { return Dog(); } int main() { Animal* p = pFunc(); p-&gt;makeSound(); Animal&amp; r1 = rFunc(); r1.makeSound(); Animal r2 = rFunc(); r2.makeSound(); Animal v = vFunc(); v.makeSound(); } </code></pre> <p>And the results are: "bark bark rawr rawr". </p> <p>In a Java way of thinking, (which has apparently corrupted my conceptualization of C++), the results would be "bark bark bark bark". I understand from my <a href="https://stackoverflow.com/q/4403726/348056">previous question</a> that this difference is due to slicing and I now have a good understanding of what slicing is. </p> <p>But let's say that I want a function that returns an Animal value that is really a Dog. </p> <ol start="14"> <li>Do I understand correctly that the closest that I can get is a <em>reference</em>? </li> <li>Furthermore, is it incumbent upon the one using the rFunc interface to see that the reference returned is assign an Animal&amp;? (Or otherwise intentionally assign the reference to an Animal which, via slicing, discards polymorphism.) </li> <li>How on earth am I supposed to return a reference to a newly generated object without doing the stupid thing I did above in rFunc? (At least I've heard this is stupid.)</li> </ol> <hr> <p>Update: since everyone seems to agree so far that rFunc it illegitimate, that brings up another related questions: </p> <p>If I pass back a pointer how do I communicate to the programmer that the pointer is not theirs to delete if this is the case? Or alternatively how do I communicate that the pointer is subject to deletion at any time (from the same thread but a different function) so that the calling function should not store it, if this is the case. Is the only way to communicate this through comments? That seems sloppy.</p> <p><em>Note: All this is leading up to an idea for a templated shared_pimpl concept I was working on. Hopefully I'll learn enough to post something about that in a couple of days.</em></p>
 

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