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    <p>First of all, the <a href="http://www.parashift.com/c++-faq/priv-inherit-like-compos.html" rel="nofollow">alternative for composition is private inheritance</a> (and not public one) since both model a <em>has-a</em> relationship.</p> <p>The important question is how can we expose <code>Sprite</code> public members (e.g. <code>changeImage</code>) to <code>VisibleGameObject</code> clients? I present the 4 methods that I know:</p> <h2>(Private) inheritance</h2> <p>I understand that you want to avoid (multiple) inheritance, but for the sake of completeness, I present one suggestion based on private inheritance:</p> <pre><code>class VisibleGameObject: private Sprite, public GameObject { ... }; </code></pre> <p>In this case <code>VisibleGameObject</code> privately derives from <code>Sprite</code>. Then users of former cannot access any member of the latter (as if it it were a private member). In particular, <code>Sprite</code>'s public and protected members are hidden to <code>VisibleGameObject</code> clients.</p> <p>Had the inheritance been public, then <strong>all</strong> <code>Sprite</code>'s public and protected members would be exposed by <code>VisibleGameObject</code> to its clients. With private inheritance we have a finer control of which methods should be exposed through using declarations. For instance, this exposes <code>Sprite::changeImage</code>:</p> <pre><code>class VisibleGameObject1: private Sprite, public GameObject { public: using Sprite::changeImage; ... }; </code></pre> <h2>Forwarding methods</h2> <p>We can give to <code>VisibleGameObject</code> public methods that forward the call to <code>m_sprite</code> as show below.</p> <pre><code>class VisibleGameObject2: public GameObject { public: void changeImage() { m_sprite.changeImage(); } private: Sprite m_sprite; ... }; </code></pre> <p>I believe <strong>this is the best design</strong>, especially as far as encapsulation is concerned. However, it might require a lot of typing in respect to other alternatives.</p> <h2>Structure dereference operator</h2> <p>Even plain old C provides types that exposes another type's interface as if it was its own: pointers.</p> <p>Indeed, suppose that <code>p</code> is of type <code>Sprite*</code>. Then by using the structure dereference operator <code>-&gt;</code> we can access members of <code>Sprite</code> (pointed by <code>p</code>) as shown below.</p> <pre><code>p-&gt;changeImage(); </code></pre> <p>C++ allows us to endow classes with customised struct dereference operators (a feature well used by smart pointers). Our example becomes:</p> <pre><code>class VisibleGameObject3 : public GameObject { public: Sprite* operator -&gt;() { return &amp;m_sprite; } private: Sprite m_sprite; ... }; </code></pre> <p>and</p> <pre><code>VisibleGameObject v; v-&gt;changeImage(); </code></pre> <p>Although convenient, this method has many flaws:</p> <ol> <li>As for public inheritance, this approach doesn't give a fine control over which <code>Sprite</code> public members should be exposed.</li> <li>It works only for one member (that is, you cannot use the same trick to expose two members interfaces).</li> <li>It messes up with the interface. Indeed, consider for instance that <code>VisualGameObject</code> has a method <code>doSomething()</code>. Then, to call this method on an object <code>v</code> one should do <code>v.doSomething()</code> whereas to call <code>changeImage()</code> one should uses <code>v-&gt;changeImage()</code>. This is confusing.</li> <li>It makes <code>VisibleGameInterface</code> to look like a smart pointer. This is semantically wrong!</li> </ol> <h2>C++11 Wrapper Pattern</h2> <p>Finally, there's Sutter's C++11 Wrapper Pattern (watch his <a href="http://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2012-Herb-Sutter-Concurrency-and-Parallelism" rel="nofollow">presentation</a>, specifically the second <a href="https://skydrive.live.com/?cid=f1b8ff18a2aec5c5&amp;id=F1B8FF18A2AEC5C5!1176" rel="nofollow">slide</a> of page 9):</p> <pre><code>class VisibleGameObject4 : public GameObject { private: Sprite m_sprite; public: template &lt;typename F&gt; auto operator()(F f) -&gt; decltype(f(m_sprite)) { return f(m_sprite); } }; </code></pre> <p>Clients use it this way:</p> <pre><code>VisibleGameObject4 v4; v4( [](Sprite&amp; s) { return s.changeImage(); } ); </code></pre> <p>As we can see, compared to the forwarding methods approach this transfer the burden of typing from the class writter to the class clients.</p>
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