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digitalmars.D.learn - Associative array on the heap

reply "Freddy" <Hexagonalstar64 gmail.com> writes:
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."]
May 18 2015
next sibling parent reply "Meta" <jared771 gmail.com> writes:
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
}
May 18 2015
next sibling parent "Meta" <jared771 gmail.com> writes:
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.
May 18 2015
prev sibling parent reply "Freddy" <Hexagonalstar64 gmail.com> writes:
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]*).
May 18 2015
parent "Xinok" <xinok live.com> writes:
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;
May 18 2015
prev sibling next sibling parent ketmar <ketmar ketmar.no-ip.org> writes:
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=3Dint[string];
 	auto b=3Dnew 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.=
May 18 2015
prev sibling parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
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
May 19 2015
parent reply mw <mingwu gmail.com> writes:
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);
Jul 07 2020
parent Steven Schveighoffer <schveiguy gmail.com> writes:
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
Jul 07 2020