What happens to the object I assign to $stdout in Ruby? [closed] - ruby

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EDIT: Don't bother reading this question, I just can't delete it. It's based on broken code and there's (almost) nothing to learn here.
I am redirecting console output in my Ruby program and although it works perfectly there is one thing I'm curious about:
Here's my code
capture = StringIO.new
$stdout = capture
puts "Hello World"
It looks like even though I'm assigning my capture object to $stdout, $stdout contains a new and different object after the assignment, but at least the type is correct.
In other words:
$stdout.to_s # => #<IO:0x2584b30>
capture = StringIO.new
$stdout = capture
$stdout.to_s # => #<StringIO:0x4fda948>
capture.to_s # => #<StringIO:0x4e3b220>
Subsequently $stdout.string contains "Hello World", but capture.string is empty.
Is there something happening behind the scenes or am I missing something here?
EDIT: This might be specific to certain versions only. I'm using Ruby 2.0.0-p247 on Windows 8.1

Answer 1

It works as expected.
>> capture = StringIO.new
=> #<StringIO:0x00000001ea8c00>
>> $stdout = capture
>> $stdout.to_s
>> capture.to_s
Above two line does not print anything because $stdout is now disconnected from terminal.
So I used $stderr.puts in following lines (can also use STDOUT.puts as Stefan commented):
>> $stderr.puts $stdout.to_s
#<StringIO:0x00000001ea8c00>
>> $stderr.puts capture.to_s
#<StringIO:0x00000001ea8c00>
$stdout.to_s, capture.to_s give me same result.
I used ruby 1.9.3. (Same for 2.0.0)

Answer 2

Are you sure there is no other manipulation of $stdout or capturehappening in between?
For me, output looks different. Both capture and $stdout are the same object and subsequently answer to string with the same response (ruby 1.9.2):
require 'stringio'
$stdout.to_s # => #<IO:0x2584b30>
capture = StringIO.new
$stdout = capture
puts $stdout.to_s # => #<StringIO:0x89a38c0>
puts capture.to_s # => #<StringIO:0x89a38c0>
puts "redirected"
$stderr.puts $stdout.string # => '#<StringIO:0x89a38c0>\n#<StringIO:0x89a38c0>\nredirected'
$stderr.puts capture.string # => '#<StringIO:0x89a38c0>\n#<StringIO:0x89a38c0>\nredirected'

Answer 3

Although this question was the result of overlooking a change to the value of $stdout, Ruby does have the ability to override assignment to global vars in this way, at least in the C api, using hooked variables.
$stdout actually does make use of this to check whether the new value is appropriate (it checks whether the new value responds to write) and raises an exception if it doesn’t.
If you really wanted (you don’t) you could create an extension that defines a global variable that automatically stores a different object than the value assigned, perhaps by called dup on it and using that instead:
#include "ruby.h"
VALUE foo;
static void foo_setter(VALUE val, ID id, VALUE *var){
VALUE dup_val = rb_funcall(val, rb_intern("dup"), 0);
*var = dup_val;
}
void Init_hooked() {
rb_define_hooked_variable("$foo", &foo, 0, foo_setter);
}
You could then use it like:
2.0.0-p247 :001 > require './ext/hooked'
=> true
2.0.0-p247 :002 > s = Object.new
=> #<Object:0x00000100b20560>
2.0.0-p247 :003 > $foo = s
=> #<Object:0x00000100b20560>
2.0.0-p247 :004 > s.to_s
=> "#<Object:0x00000100b20560>"
2.0.0-p247 :005 > $foo.to_s
=> "#<Object:0x00000100b3bea0>"
2.0.0-p247 :006 > s == $foo
=> false
Of course this is very similar to simply creating a setter method in a class that dups the vale and stores that, which you can do in plain Ruby:
def foo=(new_foo)
#foo = new_foo.dup
end
Since using global variables is generally bad design, it seems reasonable that this isn’t possible in Ruby for globals.