Subject: Re: [boost] How do I specify no padding bytes within a struct/class?
From: John Bytheway (jbytheway+boost_at_[hidden])
Date: 2013-06-17 18:34:53
On 2013-06-17 17:30, Daryle Walker wrote:
>> If you don't mind, why do you want to do pack structs in the
>> first place? Most people cringe at the thought of packing-specific
>> code.
>
> I've read about using an array segment of complex numbers as an
> array of real numbers (but twice as long) for stuff like Fourier transforms.
> At http://en.cppreference.com/w/cpp/numeric/complex#Non-static_data_members, the
> effect is documented for std::complex (at least for
> float/double/long-double). I want to simulate the effect.
>
> template < typename T, unsigned R >
> struct complex_it {
> T c[ 1ULL << R ];
> };
>
> The above class template will be standard-layout if "T" is; that
> mandates no starting padding. Array elements are packed. So I can do the
> array-segment translation only if there's no trailing padding. In contrast:
>
> template < typename T, unsigned R > struct complex_rt;
>
> template < typename T >
> struct complex_rt<T, 0u> { T r; };
>
> template < typename T, unsigned R >
> struct complex_rt { complex_rt<T, R - 1u> b[2]; };
>
> will have padding all over the place if there's any trailing padding at a lower level.
I wouldn't expect any padding in such structures, because at core
they're just a sequence of Ts.
Perhaps you could simply verify no padding by static_assert on the size,
and not worry about it unless that assertion ever fires?
John Bytheway