I have global static array and I want it to initialize in module constructor, but I get error. I there way to do it without using enum.

immutable int[5][5] arr;

static this () {
   arr = new int[5][5]; // Error: slice arr[] is not mutable
}

On 31/10/2010 17:10, Michal Minich wrote:
> I have global static array and I want it to initialize in module
> constructor, but I get error. I there way to do it without using enum.

So you want to initialise it with data acquired at runtime, but make it immutable once initialised?

> immutable int[5][5] arr;
>
> static this () {
>     arr = new int[5][5]; // Error: slice arr[] is not mutable
> }

You have a more fundamental problem here.  arr is a static array, so you can't reference-assign it.

Best you can do is to either:

- initialise it statically, using CTFE or template metaprogramming to do the calculations, if they don't depend on getting external data

- use a dynamic array instead, and use .idup to populate it

- create a mutable pointer to it in the module constructor
    int[5][5]* marr = cast(int[5][5]*) arr;
and use that to populate the array.  But I'm not sure whether this leads to UB, and I still wish the means of casting away const or immutable were explicit.

Stewart.

On Sun, 31 Oct 2010 17:46:29 +0000, Stewart Gordon wrote:

> On 31/10/2010 17:10, Michal Minich wrote:
>> I have global static array and I want it to initialize in module constructor, but I get error. I there way to do it without using enum.
> 
> So you want to initialise it with data acquired at runtime, but make it immutable once initialised?
> 
>> immutable int[5][5] arr;
>>
>> static this () {
>>     arr = new int[5][5]; // Error: slice arr[] is not mutable
>> }
> 
> You have a more fundamental problem here.  arr is a static array, so you can't reference-assign it.
> 
> Best you can do is to either:
> 
> - initialise it statically, using CTFE or template metaprogramming to do the calculations, if they don't depend on getting external data
> 
> - use a dynamic array instead, and use .idup to populate it
> 
> - create a mutable pointer to it in the module constructor
>      int[5][5]* marr = cast(int[5][5]*) arr;
> and use that to populate the array.  But I'm not sure whether this leads
> to UB, and I still wish the means of casting away const or immutable
> were explicit.
> 
> Stewart.

I would rather use CTFE, but seems it isn't possible to work with 2 dim arrays at compile time. Also I need it to be static array, otherwise i would need to rewrite and test more code. I like the simple workaround with using mutable pointer, but I'm getting access violation error even when arr is mutable ... and really don't understand why... :(

int[256][256] arr;

static this () {

    int[256][256]* pArr = &arr;

    for (int y = 0; y <= 255; y++)
        for (int x = 0; x <= 255; x++)
            pArr[x][y] = x * y; // Access violation when x = 2 and y = 0
}

Michal Minich:

> I like the simple workaround with using mutable pointer, but I'm getting access violation error even when arr is mutable ... and really don't understand why... :(
> 
> int[256][256] arr;
> 
> static this () {
> 
>     int[256][256]* pArr = &arr;
> 
>     for (int y = 0; y <= 255; y++)
>         for (int x = 0; x <= 255; x++)
>             pArr[x][y] = x * y; // Access violation when x = 2 and y = 0
> }

You need to put a bit more thought on your code:

import std.stdio: writeln;

int[256][256] arr;

static this () {
    auto pArr = &arr;
    foreach (y; 0 .. (*pArr)[0].length)
        foreach (x; 0 .. (*pArr).length)
            (*pArr)[x][y] = x * y;
}

void main() {
    writeln(arr);
}

(I have just fixed your code locally, I don't know what you are doing and why you are using a pointer to a fixed sized array, it's an unusual idiom in D.)

Bye,
bearophile

On Sun, 31 Oct 2010 14:53:42 -0400, bearophile wrote:

> import std.stdio: writeln;
> 
> int[256][256] arr;
> 
> static this () {
>     auto pArr = &arr;
>     foreach (y; 0 .. (*pArr)[0].length)
>         foreach (x; 0 .. (*pArr).length)
>             (*pArr)[x][y] = x * y;
> }
> 
> void main() {
>     writeln(arr);
> }
> 
> (I have just fixed your code locally, I don't know what you are doing and why you are using a pointer to a fixed sized array, it's an unusual idiom in D.)
> 
> Bye,
> bearophile

(*pArr)[x][y] = .. fixes it!

Thank you!

I need to use this workaround because global array I want to initialize in module constructor is immutable.

On Sun, 31 Oct 2010 19:13:33 +0000, Michal Minich wrote:

>>     foreach (y; 0 .. (*pArr)[0].length)

also good tip on the foreach syntax, it is much cooler :)

is there some performance overhead compared to using for (int x = 0; x <= 255; x++) ?

Michal Minich:

> I need to use this workaround because global array I want to initialize in module constructor is immutable.

A pointer to a immutable array is immutable. So you aren't improving the code, just making it more obfuscated. So there is zero need to use a pointer.

Immutable data is not meant to be modified, a compiler may move your empty array in read only memory. This is bad.

Some people have recently discussed about this problem and a bug report was proposed. Until some language-supported solution is found, the simpler solution for this problem may be to use a static mutable array...


> is there some performance overhead compared to using for (int x = 0; x <= 255; x++) ?

In theory there is no difference. In practice there is no difference with dmd only if you use the -O compiler switch. So it's a compiler implementation thing, and you need to look at the produced asm if you want to be sure with a different compiler.

Bye,
bearophile

> Some people have recently discussed about this problem and a bug report was proposed. Until some language-supported solution is found, the simpler solution for this problem may be to use a static mutable array...

http://d.puremagic.com/issues/show_bug.cgi?id=5147

On Sun, 31 Oct 2010 17:21:50 -0400, bearophile wrote:

>> Some people have recently discussed about this problem and a bug report was proposed. Until some language-supported solution is found, the simpler solution for this problem may be to use a static mutable array...
> 
> http://d.puremagic.com/issues/show_bug.cgi?id=5147

Thank you for the bug report. Voted.