Ruby `when' keyword does not use == in case st

问题:

x == User returns true, but case x statement does not run the block associated with User. What's happening here?

u = User.new
# => #<User:0x00000100a1e948>

x = u.class
# => User

x == User
# => true

case x
when User
  puts "constant"
when "User"
  puts "string"
else
  puts "nothing?"
end
# => nothing?

回答1:

Case comparisons use === rather than ==. For many objects the behaviour of === and == is the same, see Numeric and String:

5 == 5 #=> true
5 === 5 #=> true

"hello" == "hello" #=> true
"hello" === "hello" #=> true

But for other kinds of object === can mean many things, entirely depending on the receiver.

For the case of classes, === tests whether an object is an instance of that class:

Class === Class.new #=> true. 

For Range it checks whether an object falls in that range:

(5..10) === 6 #=> true

For Procs, === actually invokes that Proc:

multiple_of_10 = proc { |n| (n % 10) == 0 }
multiple_of_10 === 20 #=> true (equivalent to multiple_of_10.call(20))

For other objects, check their definition of === to uncover their behaviour. It's not always obvious, but they usually make some kind of sense..

Here is an example putting it all together:

case number
when 1
    puts "One"
when 2..9
    puts "Between two and nine"
when multiple_of_10
    puts "A multiple of ten"
when String
    puts "Not a number"
end  

See this link for more info: http://www.aimred.com/news/developers/2008/08/14/unlocking_the_power_of_case_equality_proc/

回答2:

In case statement , the comparison is done using === operator.

So your code is translated to following:

case x
when User === x 
    puts "Constant"
when "User" === x
    puts "string"
else 
    puts "nothing"
end

Different class define === in different way:

The Class class define === so that it tests whether the righthand operand (x)is an instance of the class named by the lefthand operand (User). So , It is not surprise that User === x will be evaluated as false. Instead, User === u (u = User.new) is true.

irb(main):001:0> class User
irb(main):002:1> end
=> nil
irb(main):003:0> u = User.new
=> #<User:0xb7a90cd8>
irb(main):004:0> User === u.class
=> false
irb(main):005:0> User === u
=> true