Subject: Re: C++ briar patch (Was: Object IDs are bad) From: Erik Naggum <firstname.lastname@example.org> Date: 1997/05/31 Newsgroups: comp.lang.scheme,comp.lang.lisp,comp.lang.misc,comp.lang.functional,comp.lang.c++ Message-ID: <email@example.com> * Peter da Silva | For example, people keep telling me that other objects in Lisp also act | as pointers. I've never denied it, I just brought up CONS cells because | they're the best example of a data structure that's uniformly available | in lisps that can be used as "an opaque reference" (avoiding using the P | word here). dragging in the cons cell is like dragging a red herring across your path to confuse anybody who might be trying to follow your point. the cons cell holds two Lisp objects. any Lisp variable can hold one Lisp object. what's the big deal with cons cells that isn't true of variables? the general machine representation of a Lisp object is a machine word (in all implementations I know and have heard about), which, if it can hold the value, holds the value, otherwise it holds the machine address of the value plus some administrative information like type tags. the typical values that fit in a machine word are small integers (fixnums) and characters. other values have to be "boxed", i.e., put in some other memory and a reference (pointer!) to that memory passed around as the value. `eq' compares the machine representation of these objects. assignment copies the machine representation of the Lisp object, which is this pointer or value-that-fits-in-a-machine-word. with the exception of the values that fit in a machine word, this has the traditional C pointer semantics. (I have already granted Peter this point several times over. please get it!) it's worth noting that (cons 1 2) yields a pointer (!) to a cons cell that contains two fixnums, not two pointers. where this pointer concept differs from C's pointer concept is that you can't make a new pointer to a different part of memory by doing arithmetic on an existing pointer, you can't obtain the address of an object that doesn't already have a pointer to it, and you can't treat the pointed-to object and the object as two distinct types or values. somebody requested a definition. the defining property of a pointer qua language construct is that the language supports a type that is a pointer to another of its supported types. Common Lisp does not have this property for any of its types. C does have it for all of its types. (however, some Lisps support locatives, which have a type that is a pointer to another type.) as far as language semantics goes, Lisp does _not_ have pointers. #\Erik -- if we work harder, will obsolescence be farther ahead or closer?