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Subject: Re: [boost] How do I specify no padding bytes within a struct/class?
From: Jonathan Wakely (jwakely.boost_at_[hidden])
Date: 2013-06-18 05:28:02

On 17 June 2013 22:30, 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, 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.

But why would it have padding at a lower level?

On a sane implementation complex_rt<T,0> will have the same layout as
T, so an array of them will have the same layout as an array of T, ...
*unless* you start dicking about with alignment and padding by
overriding the compiler's decisions, at which point you might break

I agree that it would be better to simply use a static_assert to check
the layout meets your requirements, and only force something different
if needed.

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