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POWhat is the proper way to overload operators in abstract base classes?
 

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  1. POWhat is the proper way to overload operators in abstract base classes?
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    <p>Suppose I have an abstract base class, that just defines a container on which addition can be performed:</p> <pre><code>class Base { public: virtual ~Base() {} virtual Base operator+(const Base&amp; rhs) =0; }; </code></pre> <p>Then I want subclasses of Base to provide the actual operation:</p> <pre><code>class Derived: public Base { public: Base operator+(const Base&amp; rhs) { // won't compile // actual implementation } }; </code></pre> <p>Here is my problem: operator+() is supposed to return a new Base object, but Base being abstract it won't compile. </p> <p>I tried to get around that by using a factory to return a reference to a Base object, but then in the body of the operator I find myself doing casts, because the addition only makes sense on Derived objects. </p> <p>In any case, it feels like I am biting my own tail, is there a proper solution to this?</p> <p>UPDATE: Based on the answers so far, it seems I am using the wrong pattern. I want to separate the interface from the implementation, so that library code only has to know the interface and client code provides the implementation. I tried to do that by providing the interface as an abstract base class, and the implementation as subclasses. </p> <p>UPDATE2: My question was actually 2 questions, a concrete one (about overloading operators in abstract classes) and another about my intent (how do I allow the client to customize the implementation). The former has been answered: don't. For the latter, it seems that the Interface Class pattern I use is actually a good one to solve that problem (according to <a href="http://www.octopull.demon.co.uk/c++/implementation_hiding.html" rel="noreferrer">Griffiths and Radford</a>), it's just that I should not mess with overloaded operators.</p>
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