Tuesday, October 18, 2011

rooted vs re-bindable variables - c#, c++, java, python

Q: What kinds of variables can re-bind (reseated) to a different object at run-time and what kinds can't? This understanding is not academic but helps programmers remember ground rules.

----Python moves further towards rebinding. Even a simple myInt variable can rebind. I feel the fundamental distinction in python world is between immutable vs mutable "Objects" (defined as storage-locations).
* Python Immutables are reference-counted, probably copy-on-write. Therefore variables bound to immutable Objects are reseat-able.
* What python variables are rooted? Well I believe the first element (other elements too) in a tuple is, though the tuple variable itself can rebind.

----In java, all primitive variables are "rooted". All reference variables are reseat-able.
+ Assigning to a primitive variable writes into "the ultimate" memory location;
+ Assigning to a reference variable reseats the pointer, without cloning any object.
- There's a Separate 32-bit storage for every reference variable, distinct from pointee's storage.[1]
- There's no separate storage for a primitive variable. Variable name is a nickname of the storage address. Compiler translates variable name into storage address. Run-time access to variable is one-hit. In contrast, Reference variables' access is 2-hit - following the pointer.

[1] Evidence? See memory layout of any MyClass having a non-primitive field. How much memory (like sizeof(MyClass)) is allocated by new MyClass()?
----In c#, all Value variables (including structs) are rooted. Assignment clones, including pass-by-value into a method.
All reference variables can be rebound.
----C is simple and clean
All pointer variables can be re-seated but non-pointer variables are rooted. When a variable is on the LHS, it either rebinds or the Object is "edited". See post on "Immutable, initialize.." to see the difference between Object vs Variable.
----C++ feels more complicated.
In C++, all nonref and reference variables are rooted. Assignment writes directly into the object's "stomach". Pointers are reseat-able.

However, a C++ reference variable (like pointer variable) has a separate 32-bit storage (address hidden) distinct from pointee/referent storage. Some writers say "referent" but I find "pointee" more distinct and less ambiguous.

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