• Clinton (21/21) Feb 19 2018 Hi all, I need advice from better developers on this concern.
• Clinton (6/9) Feb 19 2018 Sorry, on second look my explanation isn't very clear. I want to
• Rene Zwanenburg (4/15) Feb 19 2018 It's a pointer. In D, string is an alias to immutable(char)[]
• ketmar (15/25) Feb 19 2018 there is absolutely no reason to copy `string` ever, as it is `immutable...
• Clinton (4/29) Feb 20 2018 Thanks. I actually did a similar test a little while ago and

Clinton <clinton.d.skakun gmail.com> writes:

Hi all, I need advice from better developers on this concern.

I'm using an AA to reference another array for quicker access:

[code]
alias contactId = string;
bool[contactId][] matches;
ulong[contactId] idsToMatches;

bool[string] matchesForId(string id) {
   return matches.get(idsToMatches[id], bool[string].init);
}
[/code]

Just wondering, how do I set the keys to avoid copying the id 
string? So, let's say ids come from another array of structs(e.g. 
Contact[]). I want to avoid having two copies of the string id 
value in both of these AAs above (to avoid using extra memory 
usage).

The reason this is a potential issue is because these arrays can 
get extremely large. This is to match duplicate contacts and 
group them in matches. The reason I use bool is because it's the 
smallest size type and I don't think I can use void.

I guess my question is: does dmd already create pointers to the 
id from the AA, or is each new key a new allocation?

Clinton <clinton.d.skakun gmail.com> writes:

On Monday, 19 February 2018 at 14:55:01 UTC, Clinton wrote:
 Hi all, I need advice from better developers on this concern.

 I'm using an AA to reference another array for quicker access:

 [...]

Sorry, on second look my explanation isn't very clear. I want to 
know if:

bool[string] myAA;

myAA[contact.id] = true; // Does this copy contact.id or is this 
a pointer to contact.id?

Rene Zwanenburg <renezwanenburg gmail.com> writes:

On Monday, 19 February 2018 at 14:57:47 UTC, Clinton wrote:
 On Monday, 19 February 2018 at 14:55:01 UTC, Clinton wrote:
 Hi all, I need advice from better developers on this concern.

 I'm using an AA to reference another array for quicker access:

 [...]

 Sorry, on second look my explanation isn't very clear. I want 
 to know if:

 bool[string] myAA;

 myAA[contact.id] = true; // Does this copy contact.id or is 
 this a pointer to contact.id?

It's a pointer. In D, string is an alias to immutable(char)[] 
(Slice of immutable characters). A slice is a combination of 
pointer and length.

ketmar <ketmar ketmar.no-ip.org> writes:

Clinton wrote:

 On Monday, 19 February 2018 at 14:55:01 UTC, Clinton wrote:
 Hi all, I need advice from better developers on this concern.

 I'm using an AA to reference another array for quicker access:

 [...]

 Sorry, on second look my explanation isn't very clear. I want to know if:

 bool[string] myAA;

 myAA[contact.id] = true; // Does this copy contact.id or is this a 
 pointer to contact.id?

there is absolutely no reason to copy `string` ever, as it is `immutable`. 
and compiler knows that. anyway, why don't you just check it by writing the 
code first?

	import std.stdio;
	void main () {
		int[string] a;
		string s = "test";
		writefln("%08x", s.ptr);
		a[s] = 666;
		s = "test1";
		writefln("%08x", s.ptr);
		a[s] = 42;
		foreach (string k; a.byKey) writefln("%08x", k.ptr);
	}

Clinton <clinton.d.skakun gmail.com> writes:

On Monday, 19 February 2018 at 15:02:29 UTC, ketmar wrote:
 Clinton wrote:

 On Monday, 19 February 2018 at 14:55:01 UTC, Clinton wrote:
 [...]

 Sorry, on second look my explanation isn't very clear. I want 
 to know if:

 bool[string] myAA;

 myAA[contact.id] = true; // Does this copy contact.id or is 
 this a pointer to contact.id?

 there is absolutely no reason to copy `string` ever, as it is 
 `immutable`. and compiler knows that. anyway, why don't you 
 just check it by writing the code first?

 	import std.stdio;
 	void main () {
 		int[string] a;
 		string s = "test";
 		writefln("%08x", s.ptr);
 		a[s] = 666;
 		s = "test1";
 		writefln("%08x", s.ptr);
 		a[s] = 42;
 		foreach (string k; a.byKey) writefln("%08x", k.ptr);
 	}

Thanks. I actually did a similar test a little while ago and 
found it out. Thanks for confirming. I still struggle a bit with 
these basic things.