个常量由大写字母开头.它应最多被赋值次.在Ruby当前版本中,常量再赋值只会产生警告而不是(non-ANSI版eval.rb不会报告这警告)

ruby>fluid=30
30
ruby>fluid=31
31
ruby>Solid=32
32
ruby>Solid=33
(eval):1: warning: already initialized constant Solid
33

　　常量可以定义在类里,但不像实变量,它们可以在类外部访问.

ruby> ConstClass
| C1=101
| C2=102
| C3=103
| def show
| pr C1," ",C2," ",C3,"n"
| end
| end
nil
ruby> C1
ERR: (eval):1: uninitialized constant C1
ruby> ConstClass::C1
101
ruby> ConstClass..show
101 102 103
nil


　　常量也可以定义在模块里.

ruby> module ConstModule
| C1=101
| C2=102
| C3=103
| def showConstants
| pr C1," ",C2," ",C3,"n"
| end
| end
nil
ruby> C1
ERR: (eval):1: uninitialized constant C1
ruby> ConstModule
Object
ruby> C1
101
ruby> showConstants
101 102 103
nil
ruby> C1=99 # not really a good idea
99
ruby> C1
99
ruby> ConstModule::C1 # the module's constant is undisturbed ...
101
ruby> ConstModule::C1=99
ERR: (eval):1: compile error
(eval):1: parse error
ConstModule::C1=99
^
ruby> ConstModule::C1 # .. regardless of how we tamper with it.
101