Why aren't _AddRef and _Release called on my D

问题:

I'm really confused.

// initial class
type
    TTestClass = 
        class( TInterfacedObject)
        end;

{...}

// test procedure
procedure testMF();
var c1, c2 : TTestClass;
begin
    c1 := TTestClass.Create(); // create, addref
    c2 := c1; // addref

    c1 := nil; // refcount - 1

    MessageBox( 0, pchar( inttostr( c2.refcount)), '', 0); // just to see the value
end;

It should show 1, but it shows 0. No matter how many assignments we'll perform, the value would not change! Why not?

回答1:

Refcount is only modified when you assign to an interface variable, not to an object variable.

procedure testMF(); 
var c1, c2 : TTestClass; 
    Intf1, Intf2 : IUnknown;
begin 
    c1 := TTestClass.Create(); // create, does NOT addref
    c2 := c1; // does NOT addref 

    Intf1 := C2;  //Here it does addref
    Intf2 := C1;  //Here, it does AddRef again

    c1 := nil; // Does NOT refcount - 1 
    Intf2 := nil; //Does refcount -1

    MessageBox( 0, pchar( inttostr( c2.refcount)), '', 0); // just to see the value 
    //Now it DOES show Refcount = 1
end; 

回答2:

The compiler doesn't add in any ref-counting code if you assign it to a class type variable. The refcount was never even set to 1, much less 2.

You'll see the expected behavior if you declare c1 and c2 as IInterface instead of TTestClass.