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POWhy does this derived class need to be declared as a friend?
 

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  1. POWhy does this derived class need to be declared as a friend?
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    <p>I'm learning C++ on my own, and I thought a good way to get my hands dirty would be to convert some Java projects into C++, see where I fall down. So I'm working on a polymorphic list implementation. It works fine, except for one strange thing.</p> <p>The way I print a list is to have the <code>EmptyList</code> class return "null" (a string, not a pointer), and <code>NonEmptyList</code> returns a string that's their data concatenated with the result of calling <code>tostring()</code> on everything else in the list.</p> <p>I put <code>tostring()</code> in a <code>protected</code> section (it seemed appropriate), and the compiler complains about this line (<code>s</code> is a <code>stringstream</code> I'm using to accumulate the string):</p> <pre><code>s &lt;&lt; tail-&gt;tostring(); </code></pre> <p>Here's the error from the compiler:</p> <pre> ../list.h: In member function 'std::string NonEmptyList::tostring() [with T = int]': ../list.h:95: instantiated from here ../list.h:41: error: 'std::string List::tostring() [with T = int]' is protected ../list.h:62: error: within this context </pre> <p>Here's most of <code>list.h</code>:</p> <pre><code>template &lt;class T&gt; class List; template &lt;class T&gt; class EmptyList; template &lt;class T&gt; class NonEmptyList; template &lt;typename T&gt; class List { public: friend std::ostream&amp; operator&lt;&lt; (std::ostream&amp; o, List&lt;T&gt;* l){ o &lt;&lt; l-&gt;tostring(); return o; } /* If I don't declare NonEmptyList&lt;T&gt; as a friend, the compiler complains * that "tostring" is protected when NonEmptyClass tries to call it * recursively. */ //friend class NonEmptyList&lt;T&gt;; virtual NonEmptyList&lt;T&gt;* insert(T) =0; virtual List&lt;T&gt;* remove(T) =0; virtual int size() = 0; virtual bool contains(T) = 0; virtual T max() = 0; virtual ~List&lt;T&gt;() {} protected: virtual std::string tostring() =0; }; template &lt;typename T&gt; class NonEmptyList: public List&lt;T&gt;{ friend class EmptyString; T data; List&lt;T&gt;* tail; public: NonEmptyList&lt;T&gt;(T elem); NonEmptyList&lt;T&gt;* insert(T elem); List&lt;T&gt;* remove(T elem); int size() { return 1 + tail-&gt;size(); } bool contains(T); T max(); protected: std::string tostring(){ std::stringstream s; s &lt;&lt; data &lt;&lt; ","; /* This fails if List doesn't declare NonEmptyLst a friend */ s &lt;&lt; tail-&gt;tostring(); return s.str(); } }; </code></pre> <p>So declaring <code>NonEmptyList</code> a friend of <code>List</code> makes the problem go away, but it seems really strange to have to declare a derived class as a friend of a base class.</p>
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