| Re: This topic is complex number portability, not Re: [eigen] FFT for Eigen |
[ Thread Index |
Date Index
| More lists.tuxfamily.org/eigen Archives
]
- To: eigen@xxxxxxxxxxxxxxxxxxx
- Subject: Re: This topic is complex number portability, not Re: [eigen] FFT for Eigen
- From: Gael Guennebaud <gael.guennebaud@xxxxxxxxx>
- Date: Wed, 20 May 2009 15:55:23 +0200
- Dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:mime-version:received:in-reply-to:references :date:message-id:subject:from:to:content-type :content-transfer-encoding; bh=0YudEPNdB5fGe4v7PTQEsbRPmlAEZtIMyBm+zx5qPtI=; b=hiawQMnCSEcZ88Fc+KCSjzNF5/mOeFJPPeRZcwRM+eYAPyuCKaAxUTGBo5fbnWyu9p jQxjAoqH7dWbdm2JIJk7oUoIKY7P/zLyAKm0L2vwkwC+g1HxqsPEwsSlt3FEmFpMa9ae nxPzJ2i2/o7EOM+l/QshFotfX6avkJR+6MWBg=
- Domainkey-signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type:content-transfer-encoding; b=QIfwxCdMUX5NvKilRHETl2NY08xC6su78Ip/5GNtDnzpGf7n+AOdr+sspr3Kc7cjyM +FESXKeOdK3xSJu2z+lAJ5HZI4m+Sbgv6NbSbzqsbBLywG2oWasmVVL1rv/NAUfARecn swy0j6ZUw+uRUEHPAA3eiZ6GGP3j23vbvTs6g=
note that here m1 and m2 are matrices of complex numbers, not
std::complex<>. Internally they use some reinterpret_cast assuming a
std::complex<T> as same storage than T array[2].
Gael.
On Wed, May 20, 2009 at 3:14 PM, Mark Borgerding <mark@xxxxxxxxxxxxxx> wrote:
> (Let me try one more time to change the email subject to reflect the
> offshoot conversation :)
>
> Gael Guennebaud wrote:
>>
>> according to other discussions, I committed this change, so, e.g.,
>>
>> m1.real() = m2.imag() + m3;
>>
>
> AFAIK this is not standard c++ and it fails on at least some older
> compilers: g++ (GCC) 3.2.3 20030502 "non-lvalue in assignment"
>
> According to
> http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#387 it may
> not become standard either since the committee seems to favor mutator
> functions over lvalues.
>
> The safe "standard" way to do it is something like:
> template <typename _Complex, typename _Scalar>
> void set_real( _Complex & a, const _Scalar & b) { a = _Complex( b ,
> a.imag() ); }
> (warning: above has not been compiled)
>
> If we tiptoe just a little off the standard path, lets' assume the array[2]
> layout, which is a fairly safe assumption, then the following should work:
> reinterpret_cast<_Scalar(&)[2]>(a)[0] = a_real_value
> reinterpret_cast<_Scalar(&)[2]>(a)[1] = an_imag_value
> and is probably easier for the compiler to optimize correctly.
>
>
> -- Mark
>
>
>