How do you allocate an associative array on the heap?
----
void main(){
	alias A=int[string];
	auto b=new A;
}
----
$ rdmd test
test.d(4): Error: new can only create structs, dynamic arrays or class objects, not int[string]'s
Failed: ["dmd", "-v", "-o-", "test.d", "-I."]

On Monday, 18 May 2015 at 23:55:40 UTC, Freddy wrote:
> How do you allocate an associative array on the heap?
> ----
> void main(){
> 	alias A=int[string];
> 	auto b=new A;
> }
> ----
> $ rdmd test
> test.d(4): Error: new can only create structs, dynamic arrays or class objects, not int[string]'s
> Failed: ["dmd", "-v", "-o-", "test.d", "-I."]

They are allocated on the heap implicitly; there's no need for `new`. You actually *can't* use new with an AA, which is what the compiler is telling you.

void main()
{
    alias A = int[string];
    A b = []; //No allocation yet, b is null
    b["test"] = 1; //b is now non-null
}

On Tuesday, 19 May 2015 at 00:00:30 UTC, Meta wrote:
>     A b = []; //No allocation yet, b is null

Whoops, you actually can't assign the empty array literal to an AA. This line should be:

A b;

Which has the exact same effects.

On Mon, 18 May 2015 23:55:38 +0000, Freddy wrote:

> How do you allocate an associative array on the heap?
> ----
> void main(){
> 	alias A=int[string];
> 	auto b=new A;
> }
> ----
> $ rdmd test test.d(4): Error: new can only create structs, dynamic arrays or class objects, not int[string]'s Failed: ["dmd", "-v", "-o-", "test.d", "-I."]

AAs are always allocated on heap, you don't need to do anything special.

On Tuesday, 19 May 2015 at 00:00:30 UTC, Meta wrote:
> On Monday, 18 May 2015 at 23:55:40 UTC, Freddy wrote:
>> How do you allocate an associative array on the heap?
>> ----
>> void main(){
>> 	alias A=int[string];
>> 	auto b=new A;
>> }
>> ----
>> $ rdmd test
>> test.d(4): Error: new can only create structs, dynamic arrays or class objects, not int[string]'s
>> Failed: ["dmd", "-v", "-o-", "test.d", "-I."]
>
> They are allocated on the heap implicitly; there's no need for `new`. You actually *can't* use new with an AA, which is what the compiler is telling you.
>
> void main()
> {
>     alias A = int[string];
>     A b = []; //No allocation yet, b is null
>     b["test"] = 1; //b is now non-null
> }

Sorry mis-phrased my question,
 Who do you allocate a pointer to an associative array(int[string]*).

On Tuesday, 19 May 2015 at 00:31:50 UTC, Freddy wrote:
> Sorry mis-phrased my question,
>  Who do you allocate a pointer to an associative array(int[string]*).

Ignoring the why for a moment, one trick is to place it in an array literal so it's heap allocated. This requires writing an associative array literal with a single key-element pair though.

int[string]* a = [["zero":0]].ptr;


Another trick is to initially define the associative array in a class. Since classes are heap allocated, you can allocate an instance of the class and grab a pointer to the associative array.

class HeapAA
{
    int[string] a;
}

int[string]*b = &(new HeapAA).a;

On 5/18/15 7:55 PM, Freddy wrote:
> How do you allocate an associative array on the heap?
> ----
> void main(){
>      alias A=int[string];
>      auto b=new A;
> }
> ----
> $ rdmd test
> test.d(4): Error: new can only create structs, dynamic arrays or class
> objects, not int[string]'s
> Failed: ["dmd", "-v", "-o-", "test.d", "-I."]

As others have said, I don't know why you would want to do this, since AA is already simply a wrapper for a pointer to a heap-allocated AA.

But if you wanted to, you could put it in a struct:

struct AA
{
   int[string] x;
}

void main()
{
   auto b = &((new AA).x);
}

-Steve

On Tuesday, 19 May 2015 at 12:21:48 UTC, Steven Schveighoffer wrote:
> On 5/18/15 7:55 PM, Freddy wrote:
>> How do you allocate an associative array on the heap?
>> ----
>> void main(){
>>      alias A=int[string];
>>      auto b=new A;
>> }
>> ----
>> $ rdmd test
>> test.d(4): Error: new can only create structs, dynamic arrays or class
>> objects, not int[string]'s
>> Failed: ["dmd", "-v", "-o-", "test.d", "-I."]
>
> As others have said, I don't know why you would want to do this, since AA is already simply a wrapper for a pointer to a

AA is a wrapper for a pointer (e.g a struct with some extra info beyond the plain pointer), or AA is just the plain pointer (nothing extra)?

I tried this:

  class Foo {}
  Foo[string] foos;
  writeln(foos.sizeof);  // print 8

looks like it's just a plain pointer?


The usage pattern to have AA on the heap is, e.g:

class Class {
  StudentInfo[string] students;  // dict-by-name
  // many other fields
}

suppose in a multi-threaded app, the Class object is shared, and one thread will perform a lengthy updates on all the students. To ensure data consistency among all the students object, instead of updating each student's info of the original AA in a loop (with lengthy locking period), it can be achieved by heap-alloc a new AA, update the new AA, and atomic-set:

  new_students = new StudentInfo[string];  // heap-alloc a new AA
  // length update on each of new_students
  atomicStore(theClass.students, new_students);

On 7/7/20 3:08 AM, mw wrote:
> On Tuesday, 19 May 2015 at 12:21:48 UTC, Steven Schveighoffer wrote:
>> On 5/18/15 7:55 PM, Freddy wrote:
>>> How do you allocate an associative array on the heap?
>>> ----
>>> void main(){
>>>      alias A=int[string];
>>>      auto b=new A;
>>> }
>>> ----
>>> $ rdmd test
>>> test.d(4): Error: new can only create structs, dynamic arrays or class
>>> objects, not int[string]'s
>>> Failed: ["dmd", "-v", "-o-", "test.d", "-I."]
>>
>> As others have said, I don't know why you would want to do this, since AA is already simply a wrapper for a pointer to a
> 
> AA is a wrapper for a pointer (e.g a struct with some extra info beyond the plain pointer), or AA is just the plain pointer (nothing extra)?

AA is a pImpl type wrapper. Yes, it's just a pointer inside.

But it's not simply a pointer because things like indexing can change the pointer (i.e. if the pointer is null, it will allocate a new AA impl).

If it were just a pointer, then using it without initializing would be a segfault.

-Steve