On Sat, 13 Feb 2010 23:45:47 -0800, Steve Howell wrote: > The term "pointer" is very abstract. Please give me a concrete > definition of a pointer.
A programming language data type whose value directly specifies (or "points to") another value which is stored elsewhere in the computer memory. I quote from Wikipedia: http://en.wikipedia.org/wiki/Pointer_(computing) [quote] A pointer is a simple, less abstracted implementation of the more abstracted reference data type [end quote] And later: [quote] While "pointer" has been used to refer to references in general, it more properly applies to data structures whose interface explicitly allows the pointer to be manipulated (arithmetically via pointer arithmetic) as a memory address... [end quote] And again: [quote] A memory pointer (or just pointer) is a primitive, the value of which is intended to be used as a memory address; it is said that a pointer points to a memory address. It is also said that a pointer points to a datum [in memory] when the pointer's value is the datum's memory address. More generally, a pointer is a kind of reference, and it is said that a pointer references a datum stored somewhere in memory; to obtain that datum is to dereference the pointer. The feature that separates pointers from other kinds of reference is that a pointer's value is meant to be interpreted as a memory address, which is a rather 'low-level' concept. [end quote] > A curly brace is one of these: { } > > Pretty concrete, I hope. But { and } are glyphs in some typeface. Chances are that what you see, and what I see, are quite different, and whatever pixels we see, the compiler sees something radically different: two abstract characters implemented in some concrete fashion, but that concrete fashion is a mere implementation detail. They could be implemented as bytes x7b and x7d, or as four-byte sequences x0000007b and x0000007d for UTF-32, or who knows what in some other system. So the *concrete* representation of the curly brace varies according to the system. >From that, it's not a difficult leap to say that Pascal's BEGIN and END key words are mere alternate spellings of the abstract "open curly brace" and "close curly brace" with different concrete representations, and from that it's a small step to say that the INDENT and DEDENT tokens seen by the Python compiler (but absent from Python source code!) are too. >> But reference also has a concrete meaning: C++ has a type explicitly >> called "reference": >> >> http://en.wikipedia.org/wiki/Reference_(C++) >> >> > Of course, "reference" has concrete meanings in specific contexts. But I > can refer you to much more general and abstract uses of the term > "reference." Do you want references? I will be happy to refer you to > appropriate references. I know that reference can also be used in the abstract. I'm just warning that it can also be used in the concrete, and so we need to be wary of misunderstandings and confusions. >> And of course call-by-reference (or pass-by-reference) has a specific, >> technical meaning. >> >> > Which is what? http://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_reference -- Steven -- http://mail.python.org/mailman/listinfo/python-list
