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    copied!<p>Starting with an <a href="http://sscce.org/" rel="nofollow">SSCCE</a>:</p> <pre><code>template&lt;typename T&gt; class Quaternion { }; template&lt;typename T&gt; class Vector3 { }; template&lt;typename T&gt; class Point3 { public: operator Vector3&lt;T&gt;() const { return Vector3&lt;T&gt;(); } friend Vector3&lt;T&gt; operator*( const Quaternion&lt;T&gt;&amp; lhs, const Vector3&lt;T&gt;&amp; rhs); }; template&lt;typename T&gt; Vector3&lt;T&gt; operator*(const Quaternion&lt;T&gt;&amp; lhs, const Vector3&lt;T&gt;&amp; rhs) { } int main() { Quaternion&lt;float&gt; orientation_; Point3&lt;float&gt; pt; Vector3&lt;float&gt; vec = orientation_*pt; return 0; } </code></pre> <p>Compiling that with gcc 4.7 I get the following:</p> <pre class="lang-none prettyprint-override"><code>x.cc:6:79: warning: friend declaration ‘Vector3&lt;T&gt; operator*(const Quaternion&lt;T&gt;&amp;, const Vector3&lt;T&gt;&amp;)’ declares a non-template function [-Wnon-template-friend] x.cc:6:79: note: (if this is not what you intended, make sure the function template has already been declared and add &lt;&gt; after the function name here) Undefined symbols for architecture x86_64: "operator*(Quaternion&lt;float&gt; const&amp;, Vector3&lt;float&gt; const&amp;)", referenced from: _main in ccKgI7Ru.o </code></pre> <p>This points out the problem: you are declaring a friend <em>function</em>, not a friend <em>template</em>. To fix this, you have to do two things: make sure the template declaration preceedes this friendship line, and add angle brackets:</p> <pre><code>template&lt;typename T&gt; Vector3&lt;T&gt; operator*(const Quaternion&lt;T&gt;&amp; lhs, const Vector3&lt;T&gt;&amp; rhs) { } template&lt;typename T&gt; class Point3 { public: operator Vector3&lt;T&gt;() const { return Vector3&lt;T&gt;(); } friend Vector3&lt;T&gt; operator*&lt;&gt;(const Quaternion&lt;T&gt;&amp; lhs, const Vector3&lt;T&gt;&amp; rhs); }; </code></pre> <p>This will turn the linker error into a compiler error:</p> <pre class="lang-none prettyprint-override"><code>error: no match for ‘operator*’ in ‘orientation_ * pt’ note: candidate is: note: template&lt;class T&gt; Vector3&lt;T&gt; operator*(const Quaternion&lt;T&gt;&amp;, const Vector3&lt;T&gt;&amp;) note: template argument deduction/substitution failed: note: ‘Point3&lt;float&gt;’ is not derived from ‘const Vector3&lt;T&gt;’ </code></pre> <p>Which makes sense: declaring a friend is a statement about who may access what data; it does not help a bit in automatic type conversions for template resolution. So I suggest the following instead:</p> <pre><code>template&lt;typename T&gt; class Point3 { public: operator Vector3&lt;T&gt;() const { return Vector3&lt;T&gt;(); } friend Vector3&lt;T&gt; operator*(const Quaternion&lt;T&gt;&amp; lhs, const Point3&amp; rhs) { return lhs * Vector3&lt;T&gt;(rhs); } }; </code></pre> <p>This declares a new inline friend operator, which will explicitely do the cast you had intended.</p>
 

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