How to pass in reference a fixed array in parameter

[evilrat]

On Tuesday, 4 June 2024 at 12:22:23 UTC, Eric P626 wrote:
> I am currently trying to learn how to program in D. I thought 
> that I could start by trying some maze generation algorithms. I 
> have a maze stored as 2D array of structure defined as follow 
> which keep tracks of wall positions:
>
> ~~~
> struct s_cell
> {
>    bool north = true;
>    bool east = true;
>    bool south = true;
>    bool west = true;
> }
> ~~~
>
> I try to create a 2D array of fixed length and pass it in 
> parameter as a reference. Normally, in C, I would have used a 
> pointer as parameter, and pass the address of the array. Here, 
> I thought it would have been easier just to pass a slice of the 
> array, since a slice is a reference to the original array. So I 
> wrote the signature like this:
>
> ~~~
> void main()
> {  writeln("Maze generation demo");
>
>    s_cell [5][5] maze;
>    print_maze (maze);
>
> }
>
> void print_maze ( s_cell [][] maze )
> {
> }
> ~~~
>
> My idea is that print_maze use a slice of what ever is sent in 
> parameter. Unfortunately, I get the following error message:
>
> ~~~
> Error: function `mprmaze.print_maze(s_cell[][] maze)` is not 
> callable using argument types `(s_cell[5][5])`
>   cannot pass argument `maze` of type `s_cell[5][5]` to 
> parameter `s_cell[][] maze`
> ~~~
>
> I tried to find a solution on the internet, but could not find 
> anything, I stumble a lot on threads about Go or Rust language 
> even if I specify "d language" in my search.

You have declared static array here, they cannot be implicitly 
converted to dynamic arrays.

It is not very obvious but it is a part of language design to 
avoid unnecessary GC allocations and for C compatibility reasons 
in some cases (e.g. strings known at compile implicitly has null 
appended to it to be able to pass pointer as is to C functions).

IIRC you can explicitly cast it to s_cell[][] to make it work but 
it will allocate new array when you append to it.

> Else is there other ways to pass an array as reference using 
> parameter modifiers like: ref,in,out ...

`ref` is exactly for that.

> Else, can it be done the C way using pointers?

absolutely, even ref behind the scenes will basically do the same 
thing anyway.