GC for noobs

[Adam D. Ruppe]

On Thursday, 27 February 2014 at 14:37:14 UTC, Szymon Gatner 
wrote:
> They actually don't have all the necessary features in D afaiu. 
> They do have value semantics but can't represent uniqueness 
> because of missing move d-tor.

struct MovingStruct {
     @disable this(this);
     typeof(this) release() {
         auto p = this.payload;
         this.payload = null;
         return typeof(this)(p);
     }
}


That does moving via an explicit release call which is statically 
forced upon you by the disabled copy.

MovingStruct s = some_payload;
// MovingStruct s2 = s; // compile error
MovingStruct s2 = s.release;
assert(s.payload is null); // passes
assert(s2.payload is some_payload); // we good


You can also easily do structs with reference semantics:

struct RefStruct {
     private struct Impl {
       // implementation here
     }
     Impl* payload;
     alias payload this;
}

RefStruct is now just a thin wrapper over a pointer and thus 
inherits the pointer's semantics.

You can add a destructor to RefStruct to free it 
deterministically with RAII as well. To avoid double free, you 
can use the move technique or refcounting:


struct RefCountingStruct {
     private struct Impl {
       // implementation here

       int refcount;
     }
     Impl* payload;
     alias payload this;

     @disable this(); // disable default constructor to force 
factory
     static RefCountingStruct create() {
          RefCountingStruct t = void;
          t.payload = new Impl();
          t.payload.refcount = 1;
          return t;
     }
     this(this) { if(payload !is null) payload.refcount++; }
     ~this() {
           if(payload !is null) {
               payload.refcount--;
               if(payload.refcount == 0)
                    whatever_free(payload);
           }
     }
}



i wish my book was out but it is still only half done... but i 
wrote a chapter with like eight different struct tricks like 
this. structs rock.