D const enables multi-reader synchronization

[Sean Kelly]

Tomek Sowiñski Wrote:

> This is a continuation of a recent thread "Synchronized const methods" on  
> D.learn.
> 
> Currently only one thread at a time can execute a synchronized method. But  
> think about D's const -- it's deep, so if a method is const, it can't  
> possibly mutate its object. So many synchronized const method calls could  
> safely look-but-not-touch the same object at the same time.

It's an interesting idea but I'd recommend against it for a few reasons:

1. Acquiring, using, and releasing a reader lock is actually more expensive than a plain old mutex so it isn't a good idea to use one just because you can.  A ReadWriteMutex is really for addressing convoying problems on containers.

2. The typical implementation of a ReadWriteMutex doesn't permit recursive up/downgrades from reader to writer and such.  It's technically possible to do this, but doing so requires a lot more machinery and consequently trades away even more performance.

That said, if you're inclined to experiment there's a ReadWriteMutex in core.sync.rwmutex (which you already may know).