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    <p>Note after answer acceptance: <em>This is NOT a complete answer. Read other answers like <a href="https://stackoverflow.com/questions/656224/when-should-i-use-c-private-inheritance/656235#656235">here</a> (conceptually) and <strong><a href="https://stackoverflow.com/questions/656224/when-should-i-use-c-private-inheritance/675451#675451">here</a></strong> (both theoretic and practic) if you are interested in the question. This is just a fancy trick that can be achieved with private inheritance. While it is</em> fancy <em>it is not the answer to the question.</em></p> <p>Besides the basic usage of just private inheritance shown in the C++ FAQ (linked in other's comments) you can use a combination of private and virtual inheritance to <em>seal</em> a class (in .NET terminology) or to make a class <em>final</em> (in Java terminology). This is not a common use, but anyway I found it interesting:</p> <pre><code>class ClassSealer { private: friend class Sealed; ClassSealer() {} }; class Sealed : private virtual ClassSealer { // ... }; class FailsToDerive : public Sealed { // Cannot be instantiated }; </code></pre> <p><em>Sealed</em> can be instantiated. It derives from <em>ClassSealer</em> and can call the private constructor directly as it is a friend.</p> <p><em>FailsToDerive</em> won't compile as it must call the <em>ClassSealer</em> constructor directly (virtual inheritance requirement), but it cannot as it is private in the <em>Sealed</em> class and in this case <em>FailsToDerive</em> is not a friend of <em>ClassSealer</em>.</p> <hr> <p><strong>EDIT</strong></p> <p>It was mentioned in the comments that this could not be made generic at the time using CRTP. The C++11 standard removes that limitation by providing a different syntax to befriend template arguments:</p> <pre><code>template &lt;typename T&gt; class Seal { friend T; // not: friend class T!!! Seal() {} }; class Sealed : private virtual Seal&lt;Sealed&gt; // ... </code></pre> <p>Of course this is all moot, since C++11 provides a <code>final</code> contextual keyword for exactly this purpose:</p> <pre><code>class Sealed final // ... </code></pre>
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    1. POWhen should I use C++ private inheritance?
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    1. USDavid Rodríguez - dribeas
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    1. POWhen should I use C++ private inheritance?
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    1. COThat's a great technique. I will write an blog entry on it.
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    2. COQuestion: if we didn't use virtual inheritance, then FailsToDerive would compile. Correct?
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    3. CO+1. @Sasha: Correct, virtual inheritance is needed since the most-derived class always calls the constructors of all virtually inherited class directly, which is not the case with plain inheritance.
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