On Monday 02 October 2006 20:53, Gabriel Dos Reis wrote:
[...]
>
> | the result of the "same" multiplication considered as complex *
> | complex (vs complex * real) has a different sign for the zero
> | imaginary component.
>
> Thanks for the example. I'm not sure this was anticipated by the C++
> specification. I suspect I have to raise this on LWG.
>
Note that this problem also makes the two common complex initialisation
strategies behave differently: in the second case below the sign of the
imaginary part of z has disappeared:
#include <complex.h>
#include <stdio.h>
int main() {
double complex z;
double x=1.0;
double y=-0.0;
__real__ z = x;
__imag__ z = y;
printf("%e %e\n", creal(z), cimag(z));
z = x + y*I;
printf("%e %e\n", creal(z), cimag(z));
return 0;
}
[EMAIL PROTECTED]:../bessel $ gcc t16.c -O0 && ./a.out
1.000000e+00 -0.000000e+00
1.000000e+00 0.000000e+00
Another effect is that: when z==(1,-0.0), we get: creal(z) + cimag(z)*I = (1,
+0.0). Since 0==-0, formally this is not in conflict with footnote 166 on
page 179 of ISO/IEC 9899:TC2, but well ... the standard is not too friendly
for users of complex functions with branch cuts.
Regards,
Jan.
