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    <p>(<strong>Update</strong>: Here's a fully tested version on <a href="http://ideone.com/3KGU4" rel="nofollow">http://ideone.com/3KGU4</a>. For the <em>Additional question</em>, see <a href="http://ideone.com/w0dLo" rel="nofollow">http://ideone.com/w0dLo</a>)</p> <p>There is a difference between ordinary overloaded functions and template functions. For example, without any reference to templates a developer can declare:</p> <pre><code>void f(int x); void f(char *x); </code></pre> <p>Alternatively, a developer could use templates,</p> <pre><code>template &lt;class T&gt; void f(T x); </code></pre> <p>A major difference between them is that with ordinary functions, you must decide on a fixed set of allowed parameters in advance, and you must provide an implementation for each one. With templates, you can be more flexible.</p> <p>Later in your program, it is clear that you want <code>a</code> to be a template function, not simply an (overloaded) ordinary function. But when the compiler first sees mention of <code>a</code> (around line 10), it looks like it is declaring an ordinary function. To resolve this, you must take two steps. You must declare as soon as possible that <code>a</code> is a template function, so your first line should be:</p> <pre><code>template &lt;typename T&gt; int a(T&amp; x); </code></pre> <p>Then you must declare the relevant friendship. If <code>T</code> is <code>int</code>, then <code>a</code> takes a parameter of <code>test&lt;int&gt;&amp;</code>, not <code>int&amp;</code>. Therefore the two friend lines should be replaced with:</p> <pre><code>friend int a&lt;test&lt;T&gt; &gt;( test&lt;T&gt; &amp; x); // around line 10 friend int a&lt;test&lt;int&gt; &gt;( test&lt;int&gt; &amp; x); // around line 27 </code></pre> <p>and the specialization of <code>a</code> should be:</p> <pre><code>template &lt;&gt; int a&lt; test&lt;int&gt; &gt;(test&lt;int&gt;&amp; ) // around line 30 </code></pre> <p><strong>The Additional Question</strong></p> <p>Use <code>ostream</code> instead of <code>ofstream</code> (or else include <code>#include &lt;fstream&gt;</code> if you will output only to files and not to <code>cout</code>). In my answer, <code>operator &lt;&lt;</code> is not a template, but is a normal overloaded function. I'm not sure it's possible to have <code>operator&lt;&lt;</code> as a template. Also, I defined the operator at the place where it is declared and declared as a friend. To be honest, I think there are other, maybe better, ways but this worked for me.</p>
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