Saturday, May 4, 2013

%%jargon - readonly ^ permanent handles

If we refer to pointer variables and reference variables collectively as "handles" on objects (on heap/stack), then know the difference between a permanent handle and a readonly handle. A permanent handle can't (compiler enforcement) rebind/reseat to another object. A RO handle can't (compiler enforcement) be used to edit the target object.

Tip: a stack object can have 0, 1 or multiple handles attached. However, a heap object must have some handle, otherwise it's unreachable and a memory leak.
Tip: there are detached handles -- null pointer and uninitialized pointer
Tip: the pointee/referent of a handle can be another handle

Now Let's focus on permanent vs readonly handles. There are 4 basic forms -- perm, RO, permRO or a plain old handle. Now since there are pointer and reference, we have up to 8 flavors, some illegal.

RO ptr? ptr to const
RO ref? ref to const, widely known as "const-ref"
perm ptr? const ptr
perm ref? the plain vanilla reference
permRO ptr? const ptr to const
permRO ref? same thing as RO ref
plain old ptr? non-const ptr to non-const
plain old ref? rebindable ref is illegal

That's c++. How about java? pointer variables are illegal. All handles are references.
RO ref? no language support
perm ref? final variable
permRO ref? no language support
plain old ref? yes by default.

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