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    <p>It's allowed because of <code>is-a</code> relationship.</p> <p>When you <em>publicly</em><sup>1</sup> derive <code>Derived</code> from <code>Base</code>, you're annoucing to the compiler that <code>Derived</code> <strong>is a</strong> <code>Base</code>. Hence it should be allowed to do this:</p> <pre><code>Base base = derived; </code></pre> <p>and then use <code>base</code> as it is. that is:</p> <pre><code>base.message(); //calls Base::message(); </code></pre> <p>Read this:</p> <ul> <li><a href="http://en.wikipedia.org/wiki/Is-a" rel="nofollow">Is-A Relationship</a></li> </ul> <p><sup>1. If you <em>privately</em> derive <code>Derived</code> from <code>Base</code>, then it is <code>has-a</code> relationship. That is sort of composition. Read <a href="http://www.parashift.com/c++-faq-lite/private-inheritance.html" rel="nofollow">this</a> and <a href="http://en.wikipedia.org/wiki/Has-a" rel="nofollow">this</a>. </sup></p> <p>However, in your case, if you don't want slicing, then you can do this:</p> <pre><code>Base &amp; base = derived; base.message(); //calls Derived::message(); </code></pre> <hr> <p>From your comment : </p> <blockquote> <p>Wouldn't it better for C++ to prevent object slicing while only allow the pointer/reference to work for is-a relationshp ??? </p> </blockquote> <p>No. Pointer and Reference doesn't maintain <code>is-a</code> relationship if the base has virtual function(s). </p> <pre><code> Base *pBase = &amp;derived; pBase-&gt;message(); //doesn't call Base::message(). //that indicates, pBase is not a pointer to object of Base type. </code></pre> <p>When you want one object of one type to behave like an object of it's base type, then that is called <code>is-a</code> relationship. If you use pointer or reference of base type, then it will not call the <code>Base::message()</code>, which indicates, pointer or reference doesn't have like a pointer or reference to an object of <em>base</em> type.</p>
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    1. POWhy is 'object slice' needed in C++ ? Why it is allowed ? For more bugs?
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    1. POWhy is 'object slice' needed in C++ ? Why it is allowed ? For more bugs?
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    1. CO+1: Better than mine.
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      1. USLightness Races in Orbit
    2. CO"is-a" relationship only means "public inheritance", then could be used for c++ polymorhism. But polymorphism only work with pointer or reference , not the object !
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    3. COWouldn't it better for C++ to prevent object slicing while only allow the pointer/reference to work for is-a relationshp ???
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