Subject: Re: C++ briar patch (Was: Object IDs are bad) From: Erik Naggum <email@example.com> Date: 1997/05/29 Newsgroups: comp.lang.scheme,comp.lang.lisp,comp.lang.misc,comp.lang.functional,comp.lang.c++ Message-ID: <firstname.lastname@example.org> * Johann Hibschman | I just want to inject a few examples to make sure I know what you guys | are driving at. your examples are good. | I don't know Common Lisp, but a pointer-like action in Scheme would be: | | (define a (vector 1 2 3 4 5)) | (define b a) | (vector-set! b 2 10) | a | => #(1 2 10 4 5) | | I suppose this is "copying an existing pointer", right? right. you can't take the address of an existing object to create a new pointer. an object and the pointer to it are inseperable. in C, you can view them as distinct. | As someone else pointed out, in C you could make a pointer to the third | item in an array, but you can't in Scheme, AFAIK. Can you do this in | Common Lisp? in Lisp, you can't make a pointer into an array just like that. in C you can, since you can have an object and a pointer to it as distinct types. Lisp vectors are frequently optimized for certain types, such that you can store 32-bit integers in vector slots even though the rest of the type system allows only 30-bit fixnums, or you can store floating-point values directly without boxing. also, a Lisp array knows its dimensions and their sizes. various other features are available, too. it would be necessary to keep track of such things in a "pointer" into an array. Common Lisp does offer a way to "point" into an array, through arrays displaced to other arrays. (setq foo (make-array 10 :initial-contents '(0 1 2 3 4 5 6 7 8 9))) => #(0 1 2 3 4 5 6 7 8 9) (setq bar (make-array 1 :displaced-to foo :displaced-index-offset 5)) => #(5) (setf (aref bar 0) nil) => nil bar => #(nil) foo => #(0 1 2 3 4 nil 6 7 8 9) clearly, this is not the trivial thing it is in C, where vectors are just blocks of contiguous memory and a pointer to a particular object can just be incremented to point to the next few bytes of the memory. I don't think it would be recommended practice to use displaced arrays like a "pointer", but one could use `adjust-array' to modify an existing "pointer" object. | So in Scheme at least, it seems like the only big problem is vectors, | since most other data structures are built up out of cons-cells which | can be used like pointers. actually, very few of the other data structures in a Lisp system are list-like. most are indeed vector-like, since it is much more efficient for structures of fixed size to be represented as contiguous blocks of memory with named slots with direct accessors than the pointer-chasing in lists. in fact, the cons cell itself is just such a vector-like structure. #\Erik -- a software manager is a person who, upon reports of the existence of icebergs, launches a major operation to remove the tip, yet refuses to listen to experts who say that when the tip is removed, some more of the iceberg will float up. such a manager was in charge on the Titanic.