Re: [boost] [operators] The Dangling Reference Polarization

On 5/4/2013 1:53 PM, Daniel Frey wrote:

On 04.05.2013, at 19:12, Michael Marcin wrote:

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On 5/4/2013 5:49 AM, Daniel Frey wrote:

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#ifdef PASS_BY_VALUE T operator-( T lhs, const T& rhs ) { lhs -= rhs; return lhs; } T operator-( const T& lhs, T&& rhs ) { T nrv( lhs ); nrv -= std::move( rhs ); return nrv; } T operator-( T&& lhs, T&& rhs ) { lhs -= std::move( rhs ); return std::move( lhs ); } #

One note of interest.

Do not replace: T operator-( T lhs, const T& rhs ) { lhs -= rhs; return lhs; } with T operator-( T lhs, const T& rhs ) { return lhs -= rhs; }

At least on msvc11.

Thanks for testing, the results for MSVC are similar to the results I got with GCC and Clang and are also what I expected, as the result from lhs-=rhs is T& and hence can only lead to a copy instead of a move. If anything, you could try "return std::move( lhs -= rhs );" but from my tests and what I got from Dave's article about pass-by-value, "lhs-=rhs; return lhs;" is the "correct" version which might allow future optimizations.

Here is another data point: http://ideone.com/GgaZZV And here are msvc11 results: http://codepad.org/XFT5qVyE based on: http://stackoverflow.com/questions/1693005/copy-elision-on-visual-c-2010-bet...