Comments on: Use of the pure attribute for GObject convenience getters

By: Philip Withnall

Philip Withnall — Sat, 03 Apr 2010 23:40:09 +0000

That's my conclusion, yes. Of course, objects using pure getters could still be used in a multi-threaded app, as long as all changes to them were marshalled through the main thread.

By: fscan

fscan — Sat, 03 Apr 2010 21:35:58 +0000

this only works if my_object_get_property_foo() is only called from frobnicate() (or you have to somehow guarantee that it is called between locks, assuming my_object_set_property_foo() locks too). locking outside of my_object would not work, because there is nothing to tell the compiler the value could have changed.

so, the only _save_ use is the single threaded case (modified and read from 1 thread)...

By: Philip Withnall

Philip Withnall — Sat, 03 Apr 2010 20:53:28 +0000

In my example, the my_object_get_property_foo() would be the pure one, while frobnicate() (which is calling it) has the locking. I expect that would be the normal solution, since there'd be a potential race in frobnicate() otherwise.

You are correct though: functions with locking can never be pure, because the locks use volatile memory.

My argument is that pure could be used for all trivial getters of an object, under the assumption that the object is only modified from one thread.

By: fscan

fscan — Sat, 03 Apr 2010 15:49:56 +0000

as i understand it, methods with locking can never be __pure__, because a lock is a system resource. and how should the compiler know at compile time if an other thread has changed the value, even with locking?
the only methods i can think of using pure are getters for variables that do not change or methods that do not access object members.

By: Philip Withnall

Philip Withnall — Sat, 03 Apr 2010 07:40:05 +0000

There are more situations than that which could be optimised, but you're right that it probably wouldn't give many gains. Still, it's only 12 characters per function.

By: db

db — Sat, 03 Apr 2010 02:59:09 +0000

Declaring these functions pure only saves if two calls to the function occur in succession (or if the return value isn't used -- but why call "get" then?) -- so the gain is extremely marginal. not worth the typing.

By: Philip Withnall

Philip Withnall — Fri, 02 Apr 2010 22:52:40 +0000

Yeah; I just merged the declaration into the definition in the example to make it shorter. I think I'll split them up, since it isn't clear enough this way.

By: Christian

Christian — Fri, 02 Apr 2010 22:30:29 +0000

I take it the header is where the attribute needs to be applied so that calling/linking code can be optimized appropriately?