Question: Could someone explain to a C++ programmer most important differences between Java (and C# as well) references and shared\_ptr (from Boost or from C++0x).

I more or less aware how shared\_ptr is implemented. I am curious about differences in the following ares:

1) Performance.
2) Cycling. shared\_ptr can be cycled (A and B hold pointers to each other). Is cycling possible in Java?
3) Anything else?

Thank you.

Answer: 
Nobody pointed the possibility of moving the object by the memory manager in managed memory. So in C# there are no simple references/pointers, they work like IDs describing object which is returned by the manager.  

In C++ you can't achieve this with shared\_ptr, because the object stays in the same location after it has been created.
Cyclical references with C++ reference-counted pointers will not be disposed. You can use weak pointers to work around this. Cyclical references in Java or C# may be disposed, when the garbage collector feels like it.

When the count in a C++ reference-counted pointer drops to zero, the destructor is called. When a Java object is no longer reachable, its finalizer may not be called promptly or ever. Therefore, for objects which require explicit disposal of external resources, some form of explicit call is required.