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POOverloading operator<<: cannot bind lvalue to ‘std::basic_ostream<char>&&’
 

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  1. POOverloading operator<<: cannot bind lvalue to ‘std::basic_ostream<char>&&’
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    <p>I have a class that uses a nested class, and want to use the nested class <code>operator&lt;&lt;</code> to define <code>operator&lt;&lt;</code> in the upper class. Here is how my code looks like:</p> <pre><code>#include &lt;memory&gt; #include &lt;iostream&gt; template&lt;typename T&gt; struct classA { struct classB { template&lt;typename U&gt; friend inline std::ostream&amp; operator&lt;&lt; (std::ostream &amp;out, const typename classA&lt;U&gt;::classB &amp;b); }; classB root; template&lt;typename U&gt; friend std::ostream&amp; operator&lt;&lt; (std::ostream &amp;out, const classA&lt;U&gt; &amp;tree); }; template&lt;typename T&gt; inline std::ostream&amp; operator&lt;&lt; (std::ostream &amp;out, const classA&lt;T&gt; &amp;tree) { out &lt;&lt; tree.root; return out; } template&lt;typename T&gt; inline std::ostream&amp; operator&lt;&lt; (std::ostream &amp;out, const typename classA&lt;T&gt;::classB &amp;b) { return out; } int main() { classA&lt;int&gt; a; std::cout &lt;&lt; a; } </code></pre> <ul> <li><p>When compiling without support for C++11, the definition of operator&lt;&lt; for the inner class seems not to be found by the compiler:</p> <pre><code>so.hpp:24:7: error: no match for ‘operator&lt;&lt;’ in ‘out &lt;&lt; tree.classA&lt;int&gt;::root’ so.hpp:24:7: note: candidates are: ... </code></pre></li> <li><p>With GCC 4.6 and 4.7 when compiling with std=c++0x:</p> <pre><code>so.hpp:21:3: error: cannot bind ‘std::ostream {aka std::basic_ostream&lt;char&gt;}’ lvalue to ‘std::basic_ostream&lt;char&gt;&amp;&amp;’ In file included from /usr/include/c++/4.7/iostream:40:0, from so.hpp:2: /usr/include/c++/4.7/ostream:600:5: error: initializing argument 1 of ‘std::basic_ostream&lt;_CharT, _Traits&gt;&amp; std::operator&lt;&lt;(std::basic_ostream&lt;_CharT, _Traits&gt;&amp;&amp;, const _Tp&amp;) [with _CharT = char; _Traits = std::char_traits&lt;char&gt;; _Tp = classA&lt;int&gt;::classB]’ </code></pre></li> </ul> <p>Can someone tell me why this code is not legal, and what's the best way to do what I want?</p>
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