Re: [QT creator on "nix"] - "outer" padding within arrays of structures - was getting a strict 8 bit (1 byte) array with no padding

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From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c++
Subject	Re: [QT creator on "nix"] - "outer" padding within arrays of structures - was getting a strict 8 bit (1 byte) array with no padding
Date	Sat, 23 Nov 2019 12:35:22 -0800
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Soviet_Mario <SovietMario@CCCP.MIR> writes:
> I have realized I don't know an aspect of padding vs packing 
> management.
>
> In the other thread I have been explained of the aspects of 
> padding/packing INSIDE a given structure/class/union and 
> similar.
>
> But now I realize I know nothing for sure about packing in 
> between different element within arrays of such 
> structure/class/union and similar.

Packing for arrays is not a thing.

Every object type has a constant size, which you can query using sizeof.
Each element of an array is allocated the size of the element's type.

For example, given:

    some_type arr[N];

it's guaranteed that sizeof (arr) == N * sizeof (some_type).  (We can
use that to determine the number of elements in an array: sizeof arr /
sizeof arr[0]).

If a non-standard #pragma pack has been applied to some_type itself,
that might change the value of sizeof (some_type), but it does not
change the relationship between sizeof (some_type) and sizeof
(some_type[N]).

The size of any type is a multiple of its alignment, which is what
makes arrays possible.

If some_type is

    struct some_type {
        long big;
        char small;
    };

then there is likely to be padding after small, precisely so that in an
array of some_type elements each big sub-element is correctly aligned.
The padding is within the structure.  There is no padding between array
elements.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for Philips Healthcare
void Void(void) { Void(); } /* The recursive call of the void */

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[QT creator on "nix"] - "outer" padding within arrays of structures - was getting a strict 8 bit (1 byte) array with no padding Soviet_Mario <SovietMario@CCCP.MIR> - 2019-11-23 13:17 +0100
  Re: [QT creator on "nix"] - "outer" padding within arrays of structures - was getting a strict 8 bit (1 byte) array with no padding Keith Thompson <kst-u@mib.org> - 2019-11-23 12:35 -0800

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