Question

In Ruby, what is the difference between reassigning an IO stream and using the IO#reopen method? In other words, what is the difference between

$stdout = newfile

and:

$stdout.reopen(newfile)

Answer 1

So basically reopen will associates $stdout with the newfile stream => $stdout and newfile will be two File instances associated to the same stream.

= will actually assign the newfile File instance to $stdout => $stdout and newfile will be two variables pointing to the same File instance.

Consequences:

When using reopen any change on the instance (that does not affect the stream itself) won’t be reflected in the other instance.

= Example:

a = File.new('name')
b = File.new('name')

a.lineno #=> 0
b.lineno #=> 0

a.lineno = 3

a.lineno #=> 3
b.lineno #=> 0

b = a 

a.lineno #=> 3
b.lineno #=> 3

a.lineno = 0

a.lineno #=> 0
b.lineno #=> 0

reopen example:

a = File.new('name')
b = File.new('name')

a.lineno #=> 0
b.lineno #=> 0

a.lineno = 3

a.lineno #=> 3
b.lineno #=> 0

b.reopen(a)

a.lineno #=> 3
b.lineno #=> 3

a.lineno = 0

a.lineno #=> 0
b.lineno #=> 3