Overlaying variables
lazarus / 21-General

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mtanner · 2021-02-18 11:12

I am using a record structire that containns an array, call it
A:array[1..3] of integer;

Some parts of my program use all the values in the arry. Other parts
use only a single value. Now it's easy enough to just code a[1], but
it would improve code readability, and possibly help to avoid bugs, if
I could define something like

B:integer "equivalent to" A[1]

I should perhap emphasise that I obviously could just code B as a
separate field in the record, but I may need to contain memory
requirements agressively, and this record may be repeated millions of
times.

Does it make any difference to the above whther the fields are in a
record or object/clas.

Related question. I could use either records or objects for storing
the data. What is the overhead for a minimal class (just simple
constructor/destructor methods) compared to a record?

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Mr.Madguy · 2021-02-18 11:27

There is analog of C++ union. It's syntax is little bit weird, but it
works.
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i TMyAandB =
record  case Boolean of    False:(A:array[1..3] of Integer);   
True:(B:Integer);end;

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MarkMLl · 2021-02-18 11:36

Quote from: Mr.Madguy on February 18, 2021, 12:27:37 pmThere is analog
of C++ union. It's syntax is little bit weird, but it works.

Often referred to as an "untagged variant record", as distinct from

Code: [Select]TMyAandB = record
  case fullArray: Boolean of
    False:(A:array[1..3] of Integer);
    True:(B:Integer);
end;

which has an explicit tag which can be checked by code at runtime.

However TBH I'd stick to the classic A[1] notation and investigate
different levels of optimisation etc. The reason is that if at some
point you replaced your fixed-size array with a dynamically-sized one
you'd be likely to find that your B overlaid the data structure
providing access to the array storage which would be on the heap,
rather than to A[1] itself. That change would also break attempts to
cache a pointer to A[1] etc.

I'm not saying that you /would/ make that change, just that it's
something to bear in mind and in the general case is an argument
against trying to be clever.

MarkMLl

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Kays · 2021-02-18 12:02

Quote from: mtanner on February 18, 2021, 12:12:57 pm[…]
B:integer "equivalent to" A[1]
[…]
Inside a routine you can write:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i var  b:
int64 absolute parameter.value[1];This will not reserve any additional
space, but just “alias” the memory location specified after absolute.
It can, of course, become repetitive if you have to type that out in
every routine, so let’s make it part of your data structure:

Properties typically redirect read/write access, but this requires an
object or at least {$modeSwitch advancedRecords+}:Code: Pascal
 [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type 
triple = array[1..3] of int64;  data = object      value: triple;    
property firstValue: int64 read value[1] write value[1];    end;
Quote from: mtanner on February 18, 2021, 12:12:57 pmDoes it make any
difference to the above whther the fields are in a record or
object/clas.As soon as an object type contains virtual methods it
differs from a record. The nice thing about objects is inheritance.

Quote from: mtanner on February 18, 2021, 12:12:57 pm[…] What is the
overhead for a minimal class (just simple constructor/destructor
methods) compared to a record?In short: If you do not use polymorphism
or other OOP features, the overhead doesn’t pay off. I say KISS.

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speter · 2021-02-18 22:56

Another way of "skinning the cat" would be to use a pointer.
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type 
triple = array [1..3] of integer;var  foo : triple = (6,5,4);  p :
^integer;begin  p := @foo[1];  writeln(p^);end;
If using "mode delphi" you could maybe leave off the ^ in the
writeln(p^).

cheers
S.

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avra · 2021-02-19 08:54

Yet another way would be to use https://wiki.freepascal.org/Absolute.
I would prefer variant record (union) as the safest of all shown
methods.

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CCRDude · 2021-02-19 10:18

Depending on the complexity, §MODESWITCH ADVANCEDRECORDS and
Getter/Setter record functions might do some good.

https://wiki.freepascal.org/Record#Advanced_record

Or simply record helpers.

Or if you've got an array, defined as your own type, a type helper.

https://wiki.freepascal.org/Helper_types