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POShould I change my design to prevent dynamic casts?
 

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  1. POShould I change my design to prevent dynamic casts?
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    <p>I have read several threads about dynamic casts in C++, all full of people claiming it indicates bad design. In other languages I never gave it much thought when checking the type of an object. I never use it as an alternative to polymorphism and only when strong coupling seems perfectly acceptable. One of these situations i encounter quite often: having a list (i use std::vector in C++) of objects, all derived from a common base class. The list is managed by an object that is allowed to know the different subclasses (often it's a small hierarchy of private classes within the managing objects class). By keeping them in a single list (array, vector, ..) I can still benefit from polymorphism, but when an operation is meant to act on objects of a specific subclass I use a dynamic cast or something similar.</p> <p>Is there a different approach to this type of problem without dynamic casts or type checking that I am missing? I am really curious how programmers that avoid these at all costs would handle them.</p> <p>If my description is too abstract I could write a simple example in C++ (Edit: see below).</p> <pre><code>class EntityContacts { private: class EntityContact { private: virtual void someVirtualFunction() { }; // Only there to make dynamic_cast work public: b2Contact* m_contactData; }; class InternalEntityContact : public EntityContact { public: InternalEntityContact(b2Fixture* fixture1, b2Fixture* fixture2){ m_internalFixture1 = fixture1; m_internalFixture2 = fixture2; }; b2Fixture* m_internalFixture1; b2Fixture* m_internalFixture2; }; class ExternalEntityContact : public EntityContact { public: ExternalEntityContact(b2Fixture* internalFixture, b2Fixture* externalFixture){ m_internalFixture = internalFixture; m_externalFixture = externalFixture; }; b2Fixture* m_internalFixture; b2Fixture* m_externalFixture; }; PhysicsEntity* m_entity; std::vector&lt;EntityContact*&gt; m_contacts; public: EntityContacts(PhysicsEntity* entity) { m_entity = entity; } void addContact(b2Contact* contactData) { // Create object for internal or external contact EntityContact* newContact; if (m_entity-&gt;isExternalContact(contactData)) { b2Fixture* iFixture; b2Fixture* eFixture; m_entity-&gt;getContactInExFixtures(contactData, iFixture, eFixture); newContact = new ExternalEntityContact(iFixture, eFixture); } else newContact = new InternalEntityContact(contactData-&gt;GetFixtureA(), contactData-&gt;GetFixtureB()); // Add object to vector m_contacts.push_back(newContact); }; int getExternalEntityContactCount(PhysicsEntity* entity) { // Return number of external contacts with the entity int result = 0; for (int i = 0; i &lt; m_contacts.size(); ++i) { ExternalEntityContact* externalContact = dynamic_cast&lt;ExternalEntityContact*&gt;(m_contacts[i]); if (externalContact != NULL &amp;&amp; getFixtureEntity(externalContact-&gt;m_externalFixture) == entity) result++; } return result; } }; </code></pre> <p>It is a simplified version of a class that i use for collision detection in a game that uses box2d physics. I hope that the box2d details don't distract too much from what i am trying to show. I have a very similar class 'Event' that creates different types of event handlers which is structured in the same way (with subclasses of a base class EventHandler instead of EntityContact). </p>
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    1. COwithout knowing which other languages you use do note that for example `instanceof` in Java is similarly frowned upon. There are legitimate uses but in general it doesn't follow OO concepts. From an OO point of view one could ask why the manager object needs to know about the subclasses, if acting on a specific subclass should be abstracted etc.
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    2. CO`dynamic_cast` feels like a design shortcut to me. Everything can be made to derive from a common base class(`class Base {};`) and stored in a list(`std::list<Base> l`). But when we use `dynamic_cast` later on it leads to a few questions: Should we have used inheritance? Or have we abstracted enough? - we have other tools available(templates and interfaces)
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