A constant has a name starting with an uppercase character. It should be assigned a value at most once. In the current implementation of ruby, reassignment of a constant generates a warning but not an error (the non-ANSI version of eval.rb does not report the warning):
ruby>fluid=30
30
ruby>fluid=31
31
ruby>Solid=32
32
ruby>Solid=33
(eval):1: warning: already initialized constant Solid
33
30
ruby>fluid=31
31
ruby>Solid=32
32
ruby>Solid=33
(eval):1: warning: already initialized constant Solid
33
Constants may be defined within classes, but unlike instance variables, they are accessible outside the class.
ruby> class ConstClass
| C1=101
| C2=102
| C3=103
| def show
| puts "#{C1} #{C2} #{C3}"
| end
| end
nil
ruby> C1
ERR: (eval):1: uninitialized constant C1
ruby> ConstClass::C1
101
ruby> ConstClass.new.show
101 102 103
nil
| C1=101
| C2=102
| C3=103
| def show
| puts "#{C1} #{C2} #{C3}"
| end
| end
nil
ruby> C1
ERR: (eval):1: uninitialized constant C1
ruby> ConstClass::C1
101
ruby> ConstClass.new.show
101 102 103
nil
Constants can also be defined in modules.
ruby> module ConstModule
| C1=101
| C2=102
| C3=103
| def showConstants
| puts "#{C1} #{C2} #{C3}"
| end
| end
nil
ruby> C1
ERR: (eval):1: uninitialized constant C1
ruby> include 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
101
ruby> ConstModule::C1=99 # .. this was not allowed in earlier versions
(eval):1: warning: already initialized constant C1
99
ruby> ConstModule::C1 # "enough rope to shoot yourself in the foot"
99
| C1=101
| C2=102
| C3=103
| def showConstants
| puts "#{C1} #{C2} #{C3}"
| end
| end
nil
ruby> C1
ERR: (eval):1: uninitialized constant C1
ruby> include 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
101
ruby> ConstModule::C1=99 # .. this was not allowed in earlier versions
(eval):1: warning: already initialized constant C1
99
ruby> ConstModule::C1 # "enough rope to shoot yourself in the foot"
99