Confusion over what types have value vs reference semantics

[Neurone via Digitalmars-d-learn]

Hi,

Are there universal rules that I can apply to determine what 
types have reference or value semantics? I know that for the 
basic primitive C types (int, bool, etc) has value semantics.

In particular, I'm still trying to understand  stack vs 
GC-managed arrays, and slices.

Finally, I have an associative array, and I want to pass it into 
a function. The function is only reading data. Would putting ref 
on in function parameter pass it by reference?