On Dec 31, 2005, at 10:07 PM, Johannes Friestad wrote: > Fixnums and a few other special types (symbols, true/false/nil, > floats?) are assigned as immediate values: Instead of storing a > pointer (or reference) to the value object, the variable stores the > value directly. So in > a=4 > 'a' does not hold a reference, technically speaking, but rather the > immediate value 4. > This is an implementation issue, and is done for efficiency. I understand that this is the standard explanation for this sort of thing but does anyone else feel that it doesn't quite fit? If Fixnums can have instance variables then doesn't it make more sense to think of a as containing a reference to the Fixnum object known as 4? In what way does a contain the value 4? $ irb irb(main):001:0> a = 4 => 4 irb(main):002:0> a.object_id => 9 irb(main):003:0> 4.object_id => 9 I'm pretty sure that it is the object ids that are stored in variables not the associated values. Ruby of course doesn't actually allocate space for Fixnums but instead encodes the state of the referenced Fixnum in the object_id itself. It can do this because Fixnum state is completely encoded by its identity (i.e., its object_id). I know I'm being pedantic (again) but I'd rather think of assignment as *always* copying references. It is simpler that way. The fancy bit-twiddling/implementation issues really come into play when the variable is dereferenced not when assignment occurs. At least that is the way I've come to think about it. Does anyone else think about it that way? Gary Wright
on 2006-01-01 08:08
on 2006-01-01 08:54
<firstname.lastname@example.org> wrote in message news:8C1EA245-472B-4C94-9750-064CB61494BA@mac.com... > I know I'm being pedantic (again) but I'd rather think of assignment > as *always* copying references. It is simpler that way. The > fancy bit-twiddling/implementation issues really come into play > when the variable is dereferenced not when assignment occurs. At least > that is the way I've come to think about it. Does anyone else think > about > it that way? > > Gary Wright Some do (including me), but others don't find it useful. e.g. See http://groups.google.com/group/comp.lang.ruby/brow...
on 2006-01-01 14:22
On 1/1/06, email@example.com <firstname.lastname@example.org> wrote: > On Dec 31, 2005, at 10:07 PM, Johannes Friestad wrote: > > So in > > a=4 > > 'a' does not hold a reference, technically speaking, but rather the > > immediate value 4. > > I know I'm being pedantic (again) but I'd rather think of assignment > as *always* copying references. It is simpler that way. You're describing the user model, I was talking about the implementation. Immediate values are objects for (almost) all practical purposes, so whether you think of fixnums as references to immutable singleton objects or as immediate values doesn't make much difference. And as you say, it's simpler to use the same model for everything, which is why many languages go to quite some lengths to hide the difference. I tend to think of programming languages (Ruby, C, assembler) as user intefaces for the computer hardware. That makes 'usability' an important factor for programming langauges, and supporting simple (but still powerful) user models for how the language works is one aspect of usability. Implementation details are IMO mostly relevant for explaining why there are two classes for integers (fixnum and bignum) in the first place, or for that matter why there are 'integers' and 'floats' instead of simply 'numbers'. In day-to-day programming, they rarely matter. jf