Overriding to_s

[kimersen]

Hi,

I'm playing around with ruby trying to understand whats going on, and
here is something I don't understand.

class ZNum
def initialize(n)
@n=n
end
def to_s
self.class
end
end

n = ZNum.new(1234)

puts n # prints #<ZNum:0xb75dfcc8>
puts n.to_s # prints ZNum

In both cases it uses ZNum#to_s, but the results are different. Why?

/kim

 

[Robert Klemme]

Hi,

I'm playing around with ruby trying to understand whats going on, and
here is something I don't understand.

class ZNum
def initialize(n)
@n=n
end
def to_s
self.class
end
end

n = ZNum.new(1234)

puts n # prints #<ZNum:0xb75dfcc8>
puts n.to_s # prints ZNum

In both cases it uses ZNum#to_s, but the results are different. Why?

Because puts will revert to something else (likely #inspect) if the
result of to_s is not String.

irb(main):001:0> class Foo
irb(main):002:1> def to_s; self.class.to_s end
irb(main):003:1> end
=> nil
irb(main):004:0> puts Foo.new
Foo
=> nil

Kind regards

robert

 

[Rohan Dey]

Hi,

I'm playing around with ruby trying to understand whats going on, and
here is something I don't understand.

class ZNum
def initialize(n)
@n=n
end
def to_s
self.class
end
end

n = ZNum.new(1234)

puts n # prints #<ZNum:0xb75dfcc8>
puts n.to_s # prints ZNum

In both cases it uses ZNum#to_s, but the results are different. Why?

/kim

Not able to understand what you are actually asking. Can you put little
detail on this.

 

[kimersen]

Because puts will revert to something else (likely #inspect) if the
result of to_s is not String.

irb(main):001:0> class Foo
irb(main):002:1> def to_s; self.class.to_s end
irb(main):003:1> end
=> nil
irb(main):004:0> puts Foo.new
Foo
=> nil

Kind regards

robert

My understanding of puts is that it puts the result of to_s.
puts n and puts n.to_s are the same then and should in my example
print the same.
But they don't and that is for me a bit annoying because I thought I
understood what was going on.

/kim

 

[Robert Klemme]

My understanding of puts is that it puts the result of to_s.
puts n and puts n.to_s are the same then and should in my example
print the same.

No, they are not the same because to_s returns the class. You really
have "puts foo" and "puts Foo" ("puts ZNum" in your case).

But they don't and that is for me a bit annoying because I thought I
understood what was going on.

Please carefully reread my comment. Since you chose to make to_s return
something that is *not a String* you get the behavior that you see.

robert

 

[Brian Candler]

My understanding of puts is that it puts the result of to_s.
puts n and puts n.to_s are the same then and should in my example
print the same.
But they don't and that is for me a bit annoying because I thought I
understood what was going on.

Yes, but what do you expect "puts" to do when it calls to_s on an object,
but the result is not a string? Raise an exception perhaps? It's more
friendly for puts to have a fallback behaviour.

Note: there's one other special case I'm aware of. If you do
puts nil
then you get the string "nil" printed (plus newline). However, nil.to_s is
the empty string.

Regards,

Brian.

 

[kimersen]

Not able to understand what you are actually asking. Can you put little
detail on this.

I'm working on an interface for some legacy data.
The legacy data will be stored in objects as ruby numbers (fixnum,
bignum and float), but I need to override ruby's number formating. My
example above has no practical value, it's just some odd behavior that
I would like to understand before I design my legacy interface.

/kim