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copied!<p>I think this statement reflects the confusion here (emphasis mine):</p> <blockquote> <p>I do not understand what specifically is required in a class to be eligible for being a base clas (<strong>not a polymorphic one</strong>)?</p> </blockquote> <p>In idiomatic C++, there are two uses for deriving from a class:</p> <ul> <li><strong>private</strong> inheritance, used for mixins and aspect oriented programming using templates.</li> <li><strong>public</strong> inheritance, used for <em>polymorphic situations only</em>. <strong>EDIT</strong>: Okay, I guess this could be used in a few mixin scenarios too -- such as <code>boost::iterator_facade</code> -- which show up when the <a href="http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern" rel="nofollow noreferrer">CRTP</a> is in use.</li> </ul> <p>There is absolutely no reason to publicly derive a class in C++ if you're not trying to do something polymorphic. The language comes with free functions as a standard feature of the language, and free functions are what you should be using here.</p> <p>Think of it this way -- do you really want to force clients of your code to convert to using some proprietary string class simply because you want to tack on a few methods? Because unlike in Java or C# (or most similar object oriented languages), when you derive a class in C++ most users of the base class need to know about that kind of a change. In Java/C#, classes are usually accessed through references, which are similar to C++'s pointers. Therefore, there's a level of indirection involved which decouples the clients of your class, allowing you to substitute a derived class without other clients knowing.</p> <p>However, in C++, classes are <strong>value types</strong> -- unlike in most other OO languages. The easiest way to see this is what's known as <a href="https://stackoverflow.com/questions/274626/what-is-the-slicing-problem-in-c">the slicing problem</a>. Basically, consider:</p> <pre><code>int StringToNumber(std::string copyMeByValue) { std::istringstream converter(copyMeByValue); int result; if (converter >> result) { return result; } throw std::logic_error("That is not a number."); } </code></pre> <p>If you pass your own string to this method, the copy constructor for <code>std::string</code> will be called to make a copy, <strong>not the copy constructor for your derived object</strong> -- no matter what child class of <code>std::string</code> is passed. This can lead to inconsistency between your methods and anything attached to the string. The function <code>StringToNumber</code> cannot simply take whatever your derived object is and copy that, simply because your derived object probably has a different size than a <code>std::string</code> -- but this function was compiled to reserve only the space for a <code>std::string</code> in automatic storage. In Java and C# this is not a problem because the only thing like automatic storage involved are reference types, and the references are always the same size. Not so in C++.</p> <p>Long story short -- don't use inheritance to tack on methods in C++. That's not idiomatic and results in problems with the language. Use non-friend, non-member functions where possible, followed by composition. Don't use inheritance unless you're template metaprogramming or want polymorphic behavior. For more information, see Scott Meyers' <a href="http://rads.stackoverflow.com/amzn/click/0321334876" rel="nofollow noreferrer">Effective C++</a> Item 23: Prefer non-member non-friend functions to member functions.</p> <p>EDIT: Here's a more complete example showing the slicing problem. You can see it's output on <a href="http://codepad.org/72tZmoWx" rel="nofollow noreferrer">codepad.org</a></p> <pre><code>#include <ostream> #include <iomanip> struct Base { int aMemberForASize; Base() { std::cout << "Constructing a base." << std::endl; } Base(const Base&) { std::cout << "Copying a base." << std::endl; } ~Base() { std::cout << "Destroying a base." << std::endl; } }; struct Derived : public Base { int aMemberThatMakesMeBiggerThanBase; Derived() { std::cout << "Constructing a derived." << std::endl; } Derived(const Derived&) : Base() { std::cout << "Copying a derived." << std::endl; } ~Derived() { std::cout << "Destroying a derived." << std::endl; } }; int SomeThirdPartyMethod(Base /* SomeBase */) { return 42; } int main() { Derived derivedObject; { //Scope to show the copy behavior of copying a derived. Derived aCopy(derivedObject); } SomeThirdPartyMethod(derivedObject); } </code></pre>

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