Synchronized Classes and Struct Members

[Timon Gehr]

On 10/25/2011 08:36 AM, Andrew Wiley wrote:
> On Tue, Oct 25, 2011 at 1:26 AM, Benjamin Thaut <code at benjamin-thaut.de
> <mailto:code at benjamin-thaut.de>> wrote:
>
>     Am 25.10.2011 08:06, schrieb Andrew Wiley:
>
>         Geez, I try to write some multithreaded code and I just keep
>         hitting these:
>         --------
>         module test3;
>
>         struct SomeData {
>         int i;
>         int iPlus2() {
>         return i + 2;
>         }
>         }
>
>         synchronized class Bob {
>         private:
>         SomeData _dat;
>         public:
>         this() {
>         _dat.i = 3;
>         }
>         @property
>         int i() {
>         return _dat.iPlus2(); // test3.d(22): Error: function
>         test3.SomeData.iPlus2 () is not callable using argument types ()
>         shared
>         }
>         }
>         --------
>
>         This seems like it should be legal because SomeData is a value type.
>         Accessing _dat.i directly is legal, and _dat can't possibly be
>         shared.
>         If I'm understanding things correctly, transitive shared
>         shouldn't apply
>         to value types like this, so the type of "this" when calling iPlus2
>         should just be SomeData.
>
>
>     It can't be garantueed that a pointer to _dat is not given away at
>     some point in the program. For example you could have a clas Foo
>     that inherits from Bob and has a getter for _dat. Then it wouldn't
>     be correct anymore to sasume _dat is unshared. Because of that it
>     has been decided that trying to figure out if _dat has to be shared
>     or not will not happen, thus its just shared always. I've already
>     reported this some time ago and essentially it makes synchronized
>     classes useless for me. I usually just write a normal class and but
>     the synchronized blocks in myself. Also I use __gshared ;-)
>
>
> Alright, but why can I access _dat.i without synchronization?

_dat.i is shared by transitivity. You can access shared variables 
without synchronization (according to TDPL, memory barriers are inserted 
automatically, but I don't know if DMD implements this). 
SomeData.iPlus2's implicit 'this' parameter is not shared.