Re: [eigen] Clang issue on MacOS when the set variable is used in the given data

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On 19.04.2013 13:23, Christoph Hertzberg wrote:
On 19.04.2013 13:14, Gael Guennebaud wrote:
On Fri, Apr 19, 2013 at 12:53 PM, Christoph Hertzberg
<chtz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
I always assumed that
   foo << A, B, C;
is basically the same as
   foo.segment<dim_A>(0          ) = A;
   foo.segment<dim_B>(dim_A      ) = B;
   foo.segment<dim_C>(dim_A+dim_B) = C;

yes, in theory it's what happen in that order. but in that case
Arnaud's 1st example should fail regardless on the compiler. So there
might a c++ subtlety I'm missing.

I assume the compiler does some over-optimization here, i.e., loads
values of foo and starts evaluating the product before beginning to
write to foo. I don't know if that kind of optimization is allowed by
the C++ standard (it would not surprise me if it was).

I just found this:
However, an invocation of an overloaded comma operator is an ordinary function call; hence, *the evaluations of its argument expressions are unsequenced* relative to one another (see 1.9).
as an answer here:
So it seems there is no guarantee on the evaluation order for overloaded comma operators.


Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
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