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    <p>Bo provided the <em>reason</em> why this is happening (the type <code>T</code> is not deducible in the call to the nested <code>operator&lt;&lt;</code>. A simple workaround for this, and something that I recommend in general, not only here, is not befriending a template, but rather a single free function. For that you will need to <em>define</em> the function inline:</p> <pre><code>template&lt;typename T&gt; struct classA { struct classB { friend inline std::ostream&amp; operator&lt;&lt; (std::ostream &amp;out, const classB &amp;b) { // definition goes here } }; classB root; friend std::ostream&amp; operator&lt;&lt; (std::ostream &amp;out, const classA&lt;U&gt; &amp;tree) { // definition goes here } }; </code></pre> <p>There are a couple of differences among the two approaches. The most important one is that this approach will have the compiler define a non-templated overload for <code>operator&lt;&lt;</code> for each instantiation of the template, which because it is no longer a template, does not depend on deducing the arguments. Another side effects are that the approach is a little <em>tighter</em> (you are only befriending <em>one</em> function, while in your initial approach you befriended the template and all possible instantiations (which can be used as a loophole to gain access to your class internals). Finally the functions so defined will only be found through ADL, so there are less overloads of <code>operator&lt;&lt;</code> for the compiler to consider when the argument is not <code>ClassA&lt;T&gt;</code> or <code>ClassA&lt;T&gt;::ClassB</code>.</p> <hr> <p><em>How access can be gained with your approach</em></p> <pre><code>namespace { struct intruder { ClassA &amp; ref; intruder( ClassA&amp; r ) : ref(r) {} }; template &lt;&gt; std::ostream&amp; operator&lt;&lt; &lt;intruder&gt;( std::ostream&amp; _, ClassA&lt;intruder&gt; const&amp; i ) { std::cout &lt;&lt; i.ref.private_member &lt;&lt; std::endl; return _; } } </code></pre> <p><em>Alternative</em></p> <p>Alternatively you can befriend a particular specialization of a template. That will solve the <code>intruder</code> problem, as it will only be open to <code>operator&lt;&lt;</code> to <code>ClassA&lt;intruder&gt;</code>, which has a much lesser impact. But this will not solve your particular issue, as the type would still not be deducible.</p>
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    1. POOverloading operator<<: cannot bind lvalue to ‘std::basic_ostream<char>&&’
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    1. USDavid Rodríguez - dribeas
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    1. COThanks for providing a workaround.
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    2. COI see it like a *better design* rather than a *workaround*. It has it's downside (you cannot take the address of the friend function declared inside the template class), but on all other accounts it is a better fit to provide free function operators...
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      1. USDavid Rodríguez - dribeas
 

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