What exactly shared means?

[via Digitalmars-d-learn]

On Friday, 2 January 2015 at 21:06:03 UTC, John Colvin wrote:
> Hmm. I went in to writing that thinking "shared isn't so bad". 
> Now I've thought about it, it is pretty damn useless. What's 
> the point of knowing that data is shared without knowing how to 
> safely use it? I guess it protects against completely naive 
> usage.

The real issue with "shared" is that objects may change status 
during runtime based on the state of the program.

What you really want to know is when a parameter is "local", that 
is, guaranteed to not be accessed by another thread during the 
execution of the function. If so you open up for optimizations.