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copied!<p>First: you're mixing two concepts: polymorphism, and value vs. reference semantics.</p> <h2>Runtime polymorphism</h2> <p>Polymorphism comes in many shapes. Depending on the runtime you use, other options are available. </p> <p>An interpreted language (like Ruby, Python, Javascript, ...) allows for 'duck typing': if an object merely <em>has</em> a method called <code>foo</code>, you can call it. Typically these languages do garbage collection, so the notion of pointers vs. objects isn't too relevant.</p> <p>C++ has a different viewpoint: polymorphism is allowed, but in a more strict way. Enforcing a common base class (which may be abstract) allows the compiler to check the semantics of your code: this way the compiler assures that you really meant the <code>foo</code> method which implements the intended interface, and not some mishmash of <code>foo</code>s.</p> <p>This polymorphism is realized through the use of a <code>virtual</code> function: a pointer to a function, which may vary amongst implementations. The caller of <code>foo</code> will first have to look up the value of the function pointer, and then jump to that function.</p> <p>So far for polymorphism.</p> <h2>Containment</h2> <p>Now for containment: if you create an array of <code>line</code> objects in C++, these objects are right next to each other in memory; they're contained <em>by value</em>. When you pass the array to a function, the called function can only receive an array of the same type. Otherwise, taking a step of <code>sizeof(shape)</code> into the array we would end up in the mid of a <code>line</code>.</p> <p>In order to fix that, you can contain the objects 'by reference' - in C++ we use pointers for that.</p> <h2>Compile-time polymorphism</h2> <p>But there is another way to achieve polymorphic functions: templates. You <em>can</em> write your <code>drawshapes</code> function with a template argument that says which type of object you are using:</p> <pre><code>template< typename tShape, size_t N > void drawshapes( tShape (&aShapes)[N] ) { for( tShape* shape=aShapes; shape != aShapes+N; ++shape ) { shape->draw(); } } </code></pre> <p>(Note: there are stl functions to simplify this, but that's out of the scope of the question.</p> <pre><code>std::for_each( shapes, shapes+10, mem_fun_ref( &shape::draw ) ); </code></pre> <p>)</p>

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