Replacing the to_s method in Ruby. Not printing ou

问题:

So, I am just beginning to learn Ruby and I included a to_s method in my Class so that I can simply pass the Object to a puts method and have it return more than just the Object ID. I made a mistake and defined it as such:

def to_s
    puts "I'm #{@name} with a health of #{@health}."
end

instead of:

def to_s
    "I'm #{@name} with a health of #{@health}."
end

So, when I do this while using the first code block:

player1 = Player.new("larry")
puts player1

I get an object ID and a string when I execute the above two lines of code and not just the string. Why is this? I get this output:

I'm Larry with a health of 90.
#<Player:0x007fca1c08b270>

I am trying to think about why the first version of the program doesn't just print out the string to console, but instead returns the object ID and the string. I thought that when I pass the object to puts, all that is happening is that puts turns around and calls the to_s method to get the player's string representation. Right?

回答1:

When given arguments that are not strings or arrays puts calls rb_obj_as_string to turn its arguments into strings (see rb_io_puts)

If you search for rb_obj_as_string through the ruby codebase (I find http://rxr.whitequark.org useful for this) you can see it's defined as

VALUE rb_obj_as_string(VALUE obj)
{
  VALUE str;

  if (RB_TYPE_P(obj, T_STRING)) {
    return obj;
  }
  str = rb_funcall(obj, id_to_s, 0);
  if (!RB_TYPE_P(str, T_STRING))
    return rb_any_to_s(obj);
  if (OBJ_TAINTED(obj)) OBJ_TAINT(str);
  return str;
}

In brief this:

• returns straightaway if the argument is already a string
• calls to_s
• if the result is not a string, call rb_any_to_s and return that.

rb_any_to_s is what implements the default "class name and id" result that you're seeing: for any object it returns a string of the form #<ClassName: 0x1234567890abcdef>

Returning to your code, when you run puts player1 it calls rb_obj_as_string to convert your player to a string.

This first calls your to_s method, which uses puts to output your message. Your method then returns nil (because that's what puts always returns) so ruby calls rb_any_to_s, and that is what the outermost puts ends up using.

回答2:

That's because the puts returns nil, so does that version of to_s:

def to_s
  puts "I'm #{@name} with a health of #{@health}."
end

With puts player1, player1.to_s method is called, which prints the String "I'm ...", but the return value is that of the puts call inside to_s, which is nil.

So player1 is an object of which to_s returns nil, thus puts player1 in the end prints the result of the inherited to_s method.

回答3:

Experiential Rule: If the result of to_s is not a String, then ruby returns the default.

Application of Rule: puts() returns nil, which means your to_s method returns nil, and nil is not a String, so ruby returns the default.

Another example:

class Object
  def inspect
    'obj-inspect'
  end

  def to_s
    'obj-to_s'
  end
end

class Dog 
  def inspect
    'dog-inspect'
  end

  def to_s
    nil
  end
end

puts Dog.new

--output:--
#<Dog:0x1001b6218>

Once to_s fails to return a String, ruby does not continue along the method lookup path to call another to_s method. That makes some sense: the method was found, so there is no need to look up the method in a parent class. Nor does ruby alternatively call inspect() to get a result.

Where does the default come from? I think ruby must directly call the Object#to_s method which is a method written in C--thereby bypassing ruby's method overriding mechanism.

回答4:

The first example using puts will write to stdout and return nil. It does not actually return a String.

The second example returns a String.

If you want to write to the console you can, but you will need to also return the value.

#or put it in a variable first and return that after you print it
def to_s
  puts "I'm #{@name} with a health of #{@health}."
  "I'm #{@name} with a health of #{@health}."
end