Re: [boost] How do I specify no padding bytes within a struct/class?

On 20 June 2013 06:36, Daryle Walker wrote:

-----Original Message----- From: Jonathan Wakely Sent: Tuesday, June 18, 2013 5:28 AM

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On 17 June 2013 22:30, Daryle Walker wrote:

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template < typename T, unsigned R > struct complex_rt;

template < typename T > struct complex_rt { T r; };

template < typename T, unsigned R > struct complex_rt { complex_rt 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 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 things.

I want the code to work on non-sane implementations too, when possible. A common case that might happen is when the element size is less than a word ("char," for example). The struct may be padding-filled to reach a word boundary.

Your original mail indicated you want to turn on packed structs for all compilers that support it, not just the insane ones :-)

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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.

I don't want a hard error. I've actually implemented a "constexpr" class constant to indicate if a particular class has padding. I was surprised that I could use an expression with "sizeof(MyClass)" within MyClass. I experimented and found out the compiler was smart about it; it won't complain when using the size of a class within that class if it's for something class-static or otherwise won't affect itself; using something like a non-static member "char x[sizeof(MyClass)]" does flag an error.

The hard error would let you find out which compilers need special handling, if any.