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copied!<p>You need to move your implementation of <code>operator<<</code> into the same namespace as your class. It's looking for:</p> <pre><code>ostream& operator<< (ostream& os, const skg::Triplet<T>& p_t) </code></pre> <p>But won't find it because of a short-coming in argument-dependent look-up (ADL). ADL means that when you call a free function, it'll look for that function in the namespaces of it's arguments. This is the same reason we can do:</p> <pre><code>std::cout << "Hello" << std::endl; </code></pre> <p>Even though <code>operator<<(std::ostream&, const char*)</code> is in the <code>std</code> namespace. For your call, those namespaces are <code>std</code> and <code>skg</code>.</p> <p>It's going to look in both, not find one in <code>skg</code> (since yours is in the global scope), then look in <code>std</code>. It will see possibilities (all the normal <code>operator<<</code>'s), but none of those match. <em>Because the code running (the code in <code>ostream_iterator</code>) is in the namespace <code>std</code>, access to the global namespace is completely gone.</em></p> <p>By placing your operator in the same namespace, ADL works. This is discussed in an article by Herb Sutter: <em><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1792.pdf" rel="noreferrer">"A Modest Proposal: Fixing ADL."</a></em>. (PDF). In fact, here's a snippet from the article (demonstrating a shortcoming):</p> <pre><code>// Example 2.4 // // In some library header: // namespace N { class C {}; } int operator+( int i, N::C ) { return i+1; } // A mainline to exercise it: // #include <numeric> int main() { N::C a[10]; std::accumulate( a, a+10, 0 ); // legal? not specified by the standard } </code></pre> <p>Same situation you have.</p> <p>The book <em>"C++ Coding Standards"</em> by Sutter and & Alexandrescu has a useful guideline:</p> <blockquote> <ol start="57"> <li>Keep a type and its nonmember function interface in the same namespace.</li> </ol> </blockquote> <p>Follow it and you and ADL will be happy. I recommend this book, and even if you can't get one at least read the PDF I linked above; it contains the relevant information you should need.</p> <hr> <p>Note that after you move the operator, you'll need your friend directive (so you can access private variables):</p> <pre><code>template <typename U> friend ostream& operator<< (ostream& os, const Triplet<U>& p_t); </code></pre> <p>And ta-da! Fixed.</p>

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