Creating a reference counted type?

[ketmar via Digitalmars-d-learn]

On Sunday, 12 June 2016 at 14:49:18 UTC, Gary Willoughby wrote:
> Hmmm. I thought it looked *too* simple. Have you any idea if 
> there is a simple solution to this?

yes. you have to turn your refcount to pointer. ;-)

the cause of "misbehave" is the fact that there can exist several 
copies of the struct made in different program points, and they 
all have *independent* refcounter. but all those copies should 
have the *same* refcounter. the easiest way to solve this is to 
allocate refcounter separately, and only share *pointer* to it, 
like this:

struct Foo {
   int* refcount;

   void allocResources () {
     import core.stdc.stdlib : malloc;
     assert(refcount is null);
     refcount = cast(int*)malloc(int.size);
     *refcount = 1;
     // init other resources
   }

   this (this) { if (refcount !is null) *refcount += 1; }

   // we need to do this in opAssign() and in dtor, hence the 
function
   private void decRef () {
     if (refcount !is null) {
       if ((*refcount -= 1) == 0) {
         import core.stdc.stdlib : free;
         free(refcount);
         // free resources
       }
     }
   }

   ~this (this) { decRef(); }

   // yes, we need this too
   void opAssign (Foo src) {
     *src.refcount += 1; // it is important to increase it first, 
in case `src` and `this` are sharing refcount
     decRef(); // release our resources
     *refcount = *src.refcount;
     // copy other handles and so on
   }
}


this is basically how refcounted structs are done. note that i 
just typed the code into reply box, so it may not compile or 
contain some small bugs, but i think you got the idea.