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12280095
data
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0
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0
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8
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2012-09-05T11:07:42.957
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0
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2012-09-05T20:05:34.403
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2012-09-05T20:05:34.403
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500104
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500104
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12279601
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2
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22
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I'd probably opt for returning an <code>optional</code>, since the streaming could fail. To test if it did (in case you want to assign another value), use <code>get_value_or(default)</code>, as shown in the example. <pre><code>template<class T, class Stream> boost::optional<T> stream_get(Stream& s){ T x; if(s >> x) return std::move(x); // automatic move doesn't happen since // return type is different from T return boost::none; } </code></pre> <a href="http://liveworkspace.org/code/0feaa0fac77c5ec1a3f3057f44b6769d">Live example.</a> To further ensure that the user gets no wall-of-overloads presented when <code>T</code> is not input-streamable, you can write a trait class that checks if <code>stream >> T_lvalue</code> is valid and <code>static_assert</code> if it's not: <pre><code>namespace detail{ template<class T, class Stream> struct is_input_streamable_test{ template<class U> static auto f(U* u, Stream* s = 0) -> decltype((*s >> *u), int()); template<class> static void f(...); static constexpr bool value = !std::is_void<decltype(f<T>(0))>::value; }; template<class T, class Stream> struct is_input_streamable : std::integral_constant<bool, is_input_streamable_test<T, Stream>::value> { }; template<class T, class Stream> bool do_stream(T& v, Stream& s){ return s >> v; } } // detail:: template<class T, class Stream> boost::optional<T> stream_get(Stream& s){ using iis = detail::is_input_streamable<T, Stream>; static_assert(iis::value, "T must support 'stream >> value_of_T'"); T x; if(detail::do_stream(x, s)) return std::move(x); // automatic move doesn't happen since // return type is different from T return boost::none; } </code></pre> <a href="http://liveworkspace.org/code/f1f8bcca190d3736e5d2d68e76d6e7f6">Live example.</a> I'm using a <code>detail::do_stream</code> function, since otherwise <code>s >> x</code> would still be parsed inside <code>get_stream</code> and you'd still get the wall-of-overloads that we wanted to avoid when the <code>static_assert</code> fires. Delegating this operation to a different function makes this work.
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