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POC++: Adding methods to a polymorphic class hierarchy without violating SRP?
 

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  1. POC++: Adding methods to a polymorphic class hierarchy without violating SRP?
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    <p>I have a design issue that I constantly keep encountering.</p> <p>For the sake of illustration, let's assume that I have a polymorphic class hierarchy</p> <pre><code>class A { public: virtual ~A() {} ... }; class B: public A { ... }; class C: public B { ... }; class D: public A { ... }; ... </code></pre> <p>I want to be able to print instances of these classes in a polymorphic way, i.e. each class has its own way of printing itself. The obvious way of achieving this would be to add</p> <pre><code>virtual void print(OutputStream &amp;os) = 0; </code></pre> <p>into the base class and override this method in every subclass. However, if the original responsibility of the classes is not related to printing, this will add another responsibility to them, thus violating <a href="http://en.wikipedia.org/wiki/Single_responsibility_principle" rel="nofollow noreferrer">SRP</a>.</p> <p>My question is: what is a proper way of achieving the desired behavior without violating SRP?</p> <p>In <a href="https://stackoverflow.com/q/2352533/2580955">this post</a>, a solution based on the <a href="http://en.wikipedia.org/wiki/Visitor_design_pattern" rel="nofollow noreferrer">Visitor design pattern</a> is proposed. However, then I would need to create a class which has to know about every subclass of <code>A</code>. I would like to be able to add and remove subclasses without a need to always modify the visitor.</p> <p>Is there some other, SRP-preserving way than the two ways described above?</p>
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    1. COWhat does your print() method do? Does its output contain any information about internals of the class? In that case, I would put its definition into the class, because the only other option is to break encapsulation and that is even less agile. All other solutions I am aware of would be related to implementing dynamic dispatch for function arguments in C++ and that can be ugly.
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    2. CO@Markus Mayr: Let's assume that the emitted information can be obtained from the public interface of every subclass, but not necessarily by using only the base class interface. Otherwise, as you have already mentioned, ecapsulation would need to be broken.
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