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    copied!<p><strong>The essence is from the comment of WhozCraig</strong>:</p> <p>When you drop the T* in both templates the program will compile (with warnings -Wreorder) and give the expected output.</p> <pre><code>operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*,T...&gt; const &amp; x) operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*, T...&gt; const &amp; x) </code></pre> <p>After going through [14] Templates without any enlightenment, I think there is a compiler bug.</p> <p>A modified test:</p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; #include &lt;boost/type_traits/is_fundamental.hpp&gt; #include &lt;boost/utility/enable_if.hpp&gt; using namespace std; #define USE_VARIADIC_TEMPLATE 1 #define USE_AMBIGUOUS 1 template &lt;typename ... T&gt; struct X; template &lt;&gt; struct X &lt;&gt; { }; template &lt;typename H, typename ... T&gt; struct X&lt;H*,T*...&gt; : public X&lt;T*...&gt; { H* value; X(H* value, T*... args) : X&lt;T*...&gt;(args...), value(value) {} }; #if USE_VARIADIC_TEMPLATE template &lt;typename H, typename ... T&gt; #if USE_AMBIGUOUS std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*,T*...&gt; const &amp; x) #else std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*,T...&gt; const &amp; x) #endif #else template &lt;typename H, typename T&gt; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*, T*&gt; const &amp; x) #endif { return stream &lt;&lt; "specialized scalar pointer"; } #if USE_VARIADIC_TEMPLATE template &lt;typename V, typename ... T&gt; #if USE_AMBIGUOUS std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*,T*...&gt; const &amp; x) #else std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*,T...&gt; const &amp; x) #endif #else template &lt;typename V, typename T&gt; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*, T*&gt; const &amp; x) #endif { return stream &lt;&lt; "specialized vector pointer"; } int main() { double a,b; vector&lt;double *&gt; v; X&lt;double*,double*&gt; x (&amp;a,&amp;b); X&lt;vector&lt;double*&gt;*, double*&gt; y (&amp;v, &amp;b); cout &lt;&lt; x &lt;&lt; endl; cout &lt;&lt; y &lt;&lt; endl; // this line is ambiguous according to gcc 4.6 and later } </code></pre> <p>Even the case:</p> <pre><code>template &lt;typename H, typename T&gt; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;H*, T*&gt; const &amp; x) template &lt;typename V, typename T&gt; std::ostream&amp; operator&lt;&lt;(std::ostream&amp; stream, X&lt;vector&lt;V*&gt;*, T*&gt; const &amp; x) </code></pre> <p>compiles nicely.</p> <p><strong>Edit</strong>: If the variadic template hold just one T passed as T* it should be the simple case shown above.</p>
 

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