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    <p>It is explicitly disallowed in the standard, even if some versions of VisualStudio do allow it.</p> <p>C++ Standard 7.1.5.3 Elaborated type specifiers, paragraph 2</p> <blockquote> <p>3.4.4 describes how name lookup proceeds for the identifier in an elaborated-type-specifier. If the identifier resolves to a class-name or enum-name, the elaborated-type-specifier introduces it into the declaration the same way a simple-type-specifier introduces its type-name. If the identifier resolves to a typedef-name or a template type-parameter, the elaborated-type-specifier is ill-formed. [Note: this implies that, within a class template with a template type-parameter T, the declaration <em>friend class T</em>; is ill-formed. ]</p> </blockquote> <p>I recognize the code above as a pattern to seal (disallow the extension of) a class. There is another solution, that does not really block the extension but that will flag unadvertidly extending from the class. As seen in <a href="http://stlab.adobe.com/group__adobe__final.html" rel="noreferrer">ADOBE Source Library</a>:</p> <pre><code>namespace adobe { namespace implementation { template &lt;class T&gt; class final { protected: final() {} }; }} #define ADOBE_FINAL( X ) private virtual adobe::implementation::final&lt;T&gt; </code></pre> <p>with the usage:</p> <pre><code>class Sealed : ADOBE_FINAL( Sealed ) {//... }; </code></pre> <p>While it allows extension if you really force it:</p> <pre><code>class SealBreaker : public Sealed, ADOBE_FINAL( Sealed ) { public: SealBreaker() : adobe::implementation::final&lt;Sealed&gt;(), Sealed() {} }; </code></pre> <p>It will restrict users from mistakenly do it.</p> <p><strong>EDIT</strong>: </p> <p>The upcoming C++11 standard does allow you to befriend a type argument with a slightly different syntax:</p> <pre><code>template &lt;typename T&gt; class A { // friend class T; // still incorrect: elaborate type specifier friend T; // correct: simple specifier, note lack of "class" }; </code></pre>
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