I know that the Fixnum
class inherits from the Integer
class. But what is the actual difference between them? Are there any use cases where we sometimes use Fixnum
, and sometimes use Integer
instead?
A Fixnum holds Integer values that can be represented in a native machine word (minus 1 bit). If any operation on a Fixnum exceeds this range, the value is automatically converted to a Bignum . Fixnum objects have immediate value.
Bignum objects hold integers outside the range of Fixnum. Bignum objects are created automatically when integer calculations would otherwise overflow a Fixnum. When a calculation involving Bignum objects returns a result that will fit in a Fixnum, the result is automatically converted.
UPDATE: As of Ruby 2.4, the Fixnum
and Bignum
classes are gone, there is only Integer
. The exact same optimizations still exist, but they are treated as "proper" compiler optimizations, i.e. behind the scenes, invisible to the programmer.
This is somewhat confusing. Integer
is the real class that you should think about. Fixnum
is basically a performance optimization that should never have been made visible to the programmer in the first place. (Compare this with flonums in YARV, which are implemented entirely as an optimization inside the VM, and never exposed to the programmer.)
Basically, Fixnum
s are fast and Bignum
s are slow(er), and the implementation automatically switches back and forth between them. You never ask for one of those directly, you will just get one or the other, depending on whether your integer fits into the restricted size of a Fixnum
or not.
You never "use" Integer
. It is an abstract class whose job is to endow its children (Fixnum
and Bignum
) with methods. Under effectively no circumstances will you ever ask for an object's class and be told that it is an Integer
.
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