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    <blockquote> <p><code>std::ostringstream()</code> is an rvalue, but <code>operator&lt;&lt;</code> takes an lvalue as first argument</p> </blockquote> <p>There's a generic inserter to take rvalue streams, but it returns a <code>basic_ostream&lt;charT, traits&gt;&amp;</code>:</p> <pre><code>template &lt;class charT, class traits, class T&gt; basic_ostream&lt;charT, traits&gt;&amp; operator&lt;&lt;(basic_ostream&lt;charT, traits&gt;&amp;&amp; os, const T&amp; x); </code></pre> <p>To work correctly for your example it would have to return the derived type of the stream (<code>std::ostringstram</code>).</p> <blockquote> <ol> <li>Is the output operator as I wrote it indeed working as expected (and fulfilling all the requirements I gave)? Or could it fail in unexpected ways or have other unexpected consequences? Especially: Is my use of <code>std::forward</code> correct here?</li> </ol> </blockquote> <p>Your code looks correct to me.</p> <blockquote> <ol> <li>Is there an easier way to meet the requirements?</li> </ol> </blockquote> <p> Your code looks similar to the code I wrote to solve this problem (see below).</p> <blockquote> <ol> <li>Assuming this is indeed correct and the simplest way to do it: Do you consider it a good idea to do that, or would you advice against it (both specifically for stream output as in my example, and as a more general scheme for passing objects through functions)?</li> </ol> </blockquote> <p>The idiom looks fine to me.</p> <p>In <a href="http://libcxx.llvm.org/" rel="nofollow">libc++</a> I defined the rvalue ostream inserter like the following (as an extension). I made an attempt to get it standardized but it was late and the committee was understandably not in the mood for more thrashing:</p> <pre><code>template &lt;class _Stream, class _Tp&gt; inline typename enable_if &lt; !is_lvalue_reference&lt;_Stream&gt;::value &amp;&amp; is_base_of&lt;ios_base, _Stream&gt;::value, _Stream&amp;&amp; &gt;::type operator&lt;&lt;(_Stream&amp;&amp; __os, const _Tp&amp; __x) { __os &lt;&lt; __x; return std::move(__os); } </code></pre> <p>Unlike yours, this <strong>only</strong> accepts rvalue streams. But like yours, it returns the concrete derived type, and so works with your example.</p>
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    1. COThanks for your answer. BTW, I think there's a flaw with the interaction of your two decisions in your library: Since your rvalue version of `operator<<` returns an lvalue, chained `operator<<` calls will use the lvalue version. And since the lvalue version of `operator<<` does not preserve the exact type, I think your library will allow `(std::ostringstream() << foo).str()` but *not* `(std::ostringstream() << foo << bar).str()`. Which I'd consider a rather strange inconsistency.
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    2. COThat's a good point, thanks.
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    3. COThe [Howard's answer to this similar question](http://stackoverflow.com/a/8829047/201725) mentions it was discussed as [LWG 1203](http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#1203), but not accepted in the standard.
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