Re: 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 <c...@benjamin-thaut.de
<mailto:c...@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.