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    <p>Let's consider a code example showing what would be allowed (or not) using different levels of inheritance:</p> <pre><code> class BaseClass {}; void freeStandingFunction(BaseClass* b); class DerivedProtected : protected BaseClass { DerivedProtected() { freeStandingFunction(this); // Allowed } }; </code></pre> <p><code>DerivedProtected</code> can pass itself to <code>freeStandingFunction</code> because it knows it derives from <code>BaseClass</code>.</p> <pre><code> void freeStandingFunctionUsingDerivedProtected() { DerivedProtected nonFriendOfProtected; freeStandingFunction(&amp;nonFriendOfProtected); // NOT Allowed! } </code></pre> <p>A non-friend (class, function, whatever) cannot pass a <code>DerivedProtected</code> to <code>freeStandingFunction</code>, because the inheritance is protected, so not visible outside derived classes. Same goes for private inheritance.</p> <pre><code> class DerivedFromDerivedProtected : public DerivedProtected { DerivedFromDerivedProtected() { freeStandingFunction(this); // Allowed } }; </code></pre> <p>A class derived from <code>DerivedProtected</code> can tell that it inherits from <code>BaseClass</code>, so can pass itself to <code>freeStandingFunction</code>.</p> <pre><code> class DerivedPrivate : private BaseClass { DerivedPrivate() { freeStandingFunction(this); // Allowed } }; </code></pre> <p>The <code>DerivedPrivate</code> class itself knows that it derives from <code>BaseClass</code>, so can pass itself to <code>freeStandingFunction</code>.</p> <pre><code>class DerivedFromDerivedPrivate : public DerivedPrivate { DerivedFromDerivedPrivate() { freeStandingFunction(this); // NOT allowed! } }; </code></pre> <p>Finally, a non-friend class further down the inheritance hierarchy cannot see that <code>DerivedPrivate</code> inherits from <code>BaseClass</code>, so cannot pass itself to <code>freeStandingFunction</code>.</p>
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    1. POWhat is difference between protected and private derivation in c++
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    1. USDaniel Gallagher
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    1. COSo there *are* "useful" applications of protected / private inheritence. Thanks for the nice examples.
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