What is the use case for delegate?

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2

The main use case is allowing the user to use some methods on an internal ivar within a class, without exposing the actual ivar to the user.

Example

class Validator
  def valid?(value : String)
    value == "foo"
  end
end

class WithoutDelegate
  getter validator : Validator = Validator.new
end

# Notice in this case you have to make a getter
# to the validator class
pp WithoutDelegate.new.validator.valid? "foo"

class WithDelegate
  delegate :valid?, to: @validator
  
  @validator : Validator = Validator.new
end

# Notice in this case we can keep the validator
# instance private and just expose a select few 
# methods on it.  Also notice the UX is nicer
# since you dont have to have that extra
# .validator in there
pp WithDelegate.new.valid? "foo"

In the example you provided they’re similar, but the StringWrapper is only allowing a subset of String's methods to be used. If you tried to do wrapper.tr("l","") it wouldn’t compile since StringWrapper doesn’t have a tr method defined, and you’re not delegating it to the internal string.