Open array parameters and subranges

[Stefan Glienke 2009]

How to use open array parameters to handle array subranges:

https://delphisorcery.blogspot.com/2020/09/open-array-parameters-and-subranges.html

[Mahdi Safsafi 225]

Very nice article Stefan and the way you used Slice to avoid the copy is amazing ! 

Speaking of bugs, I found two drawback related to using open array param:

First, I already wrote on this forum a topic about unnamed type and rtti. Compiler do not generate rtti for an unnamed type ! this is easy to avoid with most types because all what you need to do is to declare the type : type x = xxx. But for open array type that's impossible because the syntax conflicts with dynamic array syntax ! So I suppose there is no official way to have a correct rtti for a function that uses open array param.

Second, On my D10.3, the TArray<T> example caused an "IDE/Compiler not responding" issue that always ends up by IDE restart:

 

I was able to isolate the line that causes the issue : 

MergeSort(Slice(TOpenArray<T>((@values[mid])^), len - mid)); // if I comment this line everything works great !

I tried with different build mode (debug,release) and on different project and the result is always the same 

[Stefan Glienke 2009]

Ugh, nasty sh*t - can repro in earlier versions than 10.4 as well. The compiler trips over the type inference - need to write MergeSort<T>(...)

I would guess it spirals down into some endless loop trying to infer whatever comes back from that tricked Slice call.

 

Updated the blog post - thanks

Edited September 8, 2020 by Stefan Glienke

[Mahdi Safsafi 225]

2 hours ago, Stefan Glienke said:

Updated the blog post - thanks

There is another way without specifying explicitly the generic type param and it generates the same output. Internally slice requires dst(open array type), src(pointer type) and of course a count(integer type). It uses src to get the pointer and dst info to compute the offset meaning you can provide a fake declaration for the source:

 type TOpenArray = array[0 .. 0] of Byte;
 POpenArray  = ^TOpenArray ;
 MergeSort(Slice(POpenArray(@values[mid])^, len - mid));

 

Edited September 8, 2020 by Mahdi Safsafi

[David Heffernan 2347]

1 hour ago, Mahdi Safsafi said:

There is another way without specifying explicitly the generic type param and it generates the same output. Internally slice requires dst(open array type), src(pointer type) and of course a count(integer type). It uses src to get the pointer and dst info to compute the offset meaning you can provide a fake declaration for the source:

 type TOpenArray = array[0 .. 0] of Byte;
 POpenArray  = ^TOpenArray ;
 MergeSort(Slice(POpenArray(@values[mid])^, len - mid));

 

What does this offer over what Stefan posted in his article? 

[Mahdi Safsafi 225]

16 minutes ago, David Heffernan said:

What does this offer over what Stefan posted in his article? 

Typically both produce the same output. Stefan code is based on template. Mine is not. I found my code more readable and friendly for typing (you don't need to worry about <>). But its just a taste of manner.

[David Heffernan 2347]

20 minutes ago, Mahdi Safsafi said:

Typically both produce the same output. Stefan code is based on template. Mine is not. I found my code more readable and friendly for typing (you don't need to worry about <>). But its just a taste of manner.

But your code can only be used for arrays of byte. Stefan is demonstrating generic code.