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

Subject: Re: [boost] [operators] The Dangling Reference Polarization
From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2013-05-04 13:12:47

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

PASS_BY_VALUE
0 copies, 1 moves, 0 lvalue ops, 1 rvalue ops, V r = V()-V()
0 copies, 1 moves, 1 lvalue ops, 0 rvalue ops, V r = V()-t
0 copies, 1 moves, 1 lvalue ops, 0 rvalue ops, V r = t+V()
1 copies, 0 moves, 0 lvalue ops, 1 rvalue ops, V r = t-V()
1 copies, 1 moves, 1 lvalue ops, 0 rvalue ops, V r = t-t
1         0        1             0             (expected)
1 copies, 2 moves, 2 lvalue ops, 0 rvalue ops, V r = t-t-t
1         1        2             0             (expected)
1 copies, 2 moves, 2 lvalue ops, 0 rvalue ops, V r = t+(t+t)
1         1        2             0             (expected)
2 copies, 1 moves, 1 lvalue ops, 1 rvalue ops, V r = t-(t-t)
2         0        1             1             (expected)
2 copies, 3 moves, 2 lvalue ops, 1 rvalue ops, V r = (t-t)-(t-t)
2         1        2             1             (expected)
1 copies, 3 moves, 3 lvalue ops, 0 rvalue ops, V r = t-t-t-t
1         2        3             0             (expected)
1 copies, 4 moves, 4 lvalue ops, 0 rvalue ops, V r = t-t-t-t-t
1         3        4             0             (expected)

becomes

PASS_BY_VALUE
0 copies, 1 moves, 0 lvalue ops, 1 rvalue ops, V r = V()-V()
1 copies, 0 moves, 1 lvalue ops, 0 rvalue ops, V r = V()-t
0         1        1             0             (expected)
0 copies, 1 moves, 1 lvalue ops, 0 rvalue ops, V r = t+V()
1 copies, 0 moves, 0 lvalue ops, 1 rvalue ops, V r = t-V()
2 copies, 0 moves, 1 lvalue ops, 0 rvalue ops, V r = t-t
1         0        1             0             (expected)
3 copies, 0 moves, 2 lvalue ops, 0 rvalue ops, V r = t-t-t
1         1        2             0             (expected)
1 copies, 2 moves, 2 lvalue ops, 0 rvalue ops, V r = t+(t+t)
1         1        2             0             (expected)
3 copies, 0 moves, 1 lvalue ops, 1 rvalue ops, V r = t-(t-t)
2         0        1             1             (expected)
4 copies, 1 moves, 2 lvalue ops, 1 rvalue ops, V r = (t-t)-(t-t)
2         1        2             1             (expected)
4 copies, 0 moves, 3 lvalue ops, 0 rvalue ops, V r = t-t-t-t
1         2        3             0             (expected)
5 copies, 0 moves, 4 lvalue ops, 0 rvalue ops, V r = t-t-t-t-t
1         3        4             0             (expected)